This calculator is provided for informational and educational purposes only. It is not intended to replace professional medical advice, diagnosis, or treatment. Always consult with a qualified healthcare professional before making any medical decisions or starting any dietary protocol, including intermittent fasting. The results from this calculator should be used as a reference guide only and not as the sole basis for clinical or dietary decisions.
5:2 Diet Calculator
Calculate your personalized fasting day calorie target, total daily energy expenditure (TDEE), and weekly calorie deficit for the 5:2 intermittent fasting protocol. Uses the Mifflin-St Jeor BMR equation with activity-adjusted TDEE, AHA-aligned BMI classification, and estimated weight loss projection based on your body measurements and activity level.
Estimated weight at the end of each month assuming consistent 5:2 fasting with no compensatory overeating on normal days. Weight loss naturally slows as body weight decreases and TDEE reduces.
| Month | Projected Weight | Total Lost | BMI | Est. TDEE |
|---|
How your TDEE, fasting day target, and weekly deficit change at different activity levels for your current body measurements.
| Activity Level | TDEE | Fasting Day Target | Weekly Deficit | Est. Weekly Loss |
|---|
Sample foods and calorie counts optimised for the 5:2 fasting day budget. Prioritise protein and fiber for maximum satiety within your calorie allowance of 500 calories.
| Food Item | Serving | Calories | Protein (g) |
|---|---|---|---|
| HIGH-PROTEIN OPTIONS (Best satiety per calorie) | |||
| Chicken breast (grilled) | 150g | 165 | 31g |
| Eggs (large, boiled or poached) | 2 eggs | 140 | 12g |
| Salmon fillet (baked) | 120g | 180 | 25g |
| White fish (cod, grilled) | 180g | 140 | 30g |
| Prawns / Shrimp | 150g | 120 | 23g |
| Greek yogurt (0% fat) | 200g | 110 | 20g |
| Tofu (firm, plain) | 150g | 120 | 13g |
| LOW-CALORIE VEGETABLES (Fill volume cheaply) | |||
| Mixed salad leaves | 100g | 20 | 1g |
| Broccoli (steamed) | 200g | 70 | 6g |
| Cucumber | 200g | 30 | 1g |
| Zucchini (Courgette) | 200g | 36 | 3g |
| Cauliflower (steamed) | 200g | 50 | 4g |
| Spinach (raw) | 100g | 23 | 3g |
| MODERATE CALORIE ADDITIONS | |||
| Apple (medium) | 1 apple | 80 | 0g |
| Lentils (cooked) | 100g | 116 | 9g |
| Olive oil (dressing) | 1 tbsp | 120 | 0g |
| Black coffee (no milk) | 1 cup | 2 | 0g |
Average BMR values by age and sex at 70 kg body weight and 170 cm height (Mifflin-St Jeor). Your personal BMR is shown highlighted.
| Age Range | Male BMR | Female BMR | Activity 1.55 (TDEE) |
|---|---|---|---|
| 20 – 29 years | 1,772 cal | 1,606 cal | 2,749 / 2,489 cal |
| 30 – 39 years | 1,722 cal | 1,556 cal | 2,669 / 2,412 cal |
| 40 – 49 years | 1,672 cal | 1,506 cal | 2,592 / 2,334 cal |
| 50 – 59 years | 1,622 cal | 1,456 cal | 2,514 / 2,257 cal |
| 60 – 69 years | 1,572 cal | 1,406 cal | 2,437 / 2,179 cal |
| 70+ years | 1,522 cal | 1,356 cal | 2,359 / 2,102 cal |
Note: BMR decreases approximately 50 calories per decade after age 30. This is why fasting day targets should be recalculated periodically as both body weight and age change over time.
About This 5:2 Diet Calculator
This free 5:2 diet calculator is designed for adults worldwide who are considering or following the 5:2 intermittent fasting protocol and need personalized calorie targets for fasting days and normal days. It computes your Basal Metabolic Rate (BMR) using the Mifflin-St Jeor equation - the formula recommended by the Academy of Nutrition and Dietetics - then multiplies by your selected activity factor to calculate Total Daily Energy Expenditure (TDEE). Your fasting day calorie target is derived from either the standard fixed values (500 calories for women, 600 for men) or a personalized 25% of TDEE approach that scales to your individual body size and energy needs.
The underlying calculation methodology follows the Mifflin-St Jeor BMR formula published in the Journal of the American Dietetic Association (1990) and validated in multiple global populations. Activity factors are drawn from standard metabolic equivalents used in clinical nutrition practice. BMI classification follows World Health Organisation criteria (underweight below 18.5, healthy weight 18.5-24.9, overweight 25-29.9, obese 30 and above). Weekly calorie deficit is calculated as the sum of two fasting day deficits against TDEE, with estimated weight loss derived from the 7,700 calorie per kilogram fat energy equivalent.
Beyond the core calorie targets, this calculator provides a yearly weight loss projection, a fasting day macro breakdown showing recommended calorie allocation across protein, vegetables, fats, and carbohydrates, and a comparison of how your targets change across activity levels. The fasting day meal guide tab provides a reference calorie and protein table for common foods well-suited to the 500-600 calorie budget. All calculations update in real time as you adjust your inputs. Recalculate every 4-6 weeks as your body weight changes to keep your targets accurate throughout your 5:2 journey. Always consult a registered dietitian or physician before beginning any intermittent fasting protocol.
This 5:2 diet calculator is provided for informational and educational purposes only. It is not intended to replace professional medical advice, diagnosis, or treatment. The calorie targets and projections generated are estimates based on population-level equations and may not reflect individual metabolic variation. Always consult with a qualified healthcare professional, registered dietitian, or physician before starting any intermittent fasting protocol, particularly if you have diabetes, cardiovascular disease, a history of eating disorders, or take prescription medications. Do not use this calculator as the sole basis for making dietary or clinical decisions.
The 5:2 Diet - A Complete Guide to Intermittent Fasting for Weight Management and Metabolic Health
The 5:2 diet is one of the most studied forms of intermittent fasting, built around a simple principle: eat normally for five days each week, then restrict calorie intake to around 500-600 calories on the remaining two days. Unlike continuous calorie restriction, this approach allows substantial flexibility while still delivering measurable metabolic benefits. Research published in journals such as the International Journal of Obesity and Cell Metabolism has consistently shown that intermittent fasting protocols like 5:2 can support weight loss, improve insulin sensitivity, and reduce markers of cardiovascular risk.
This calculator helps you determine your personalized calorie targets for both normal eating days and fasting days, based on your body measurements, activity level, and health goals. Understanding your numbers is the foundation of any effective dietary strategy.
Women: BMR = (10 x weight kg) + (6.25 x height cm) - (5 x age) - 161
Fasting Day Calories = TDEE x 0.25 (approximately 25% of TDEE)
Standard Fasting Target: 500 cal (women) / 600 cal (men)
What Is the 5:2 Diet?
The 5:2 diet, sometimes called the Fast Diet, was popularised by British journalist and physician Dr. Michael Mosley following the 2012 BBC documentary Eat, Fast and Live Longer. The core structure is two non-consecutive fasting days per week, with normal (unrestricted) eating on the remaining five days. Fasting days are not complete fasts - they allow a small caloric intake, typically 500-600 calories, distributed across one or two small meals.
The appeal of 5:2 over daily calorie restriction lies in compliance. Many people find it psychologically easier to restrict severely for two days knowing they can eat freely the next day, compared to moderate restriction every single day. This psychological flexibility has been associated with better long-term adherence in several studies.
The Science Behind Intermittent Fasting
Intermittent fasting works through several overlapping mechanisms that go beyond simple calorie reduction. When you fast, insulin levels drop, which facilitates the breakdown of stored body fat for energy. After 12-16 hours of fasting, the body begins producing ketone bodies from fat stores - an alternative fuel source that may have beneficial effects on brain function and inflammation.
A state called autophagy - the cellular "self-cleaning" process - is also upregulated during fasting periods. Autophagy involves the degradation and recycling of damaged cellular components, and some researchers believe this process is linked to longevity and reduced risk of certain diseases. The evidence base for autophagy's health benefits in humans is still developing, but it represents one of the more intriguing areas of fasting research.
Studies have also documented improvements in several cardiometabolic biomarkers during intermittent fasting protocols, including reductions in triglycerides, LDL cholesterol, blood pressure, and inflammatory markers such as C-reactive protein. These effects appear to be partly independent of weight loss, suggesting that the timing of eating itself has metabolic consequences.
Calculating Your Calorie Needs
Accurate calorie targets depend on several individual variables. Body weight is the most obvious factor, but height, age, and biological sex all influence how many calories your body burns at rest. The Mifflin-St Jeor equation accounts for all of these and remains the recommended formula for clinical use.
Activity level is the multiplier applied to your BMR to estimate real-world energy expenditure. The standard activity factors are:
- Sedentary (1.2): Desk job, little or no intentional exercise
- Lightly active (1.375): Light exercise or sport 1-3 days per week
- Moderately active (1.55): Moderate exercise 3-5 days per week
- Very active (1.725): Hard exercise or sport 6-7 days per week
- Extra active (1.9): Very hard exercise, physical job, or twice-daily training
Most people overestimate their activity level. If in doubt, select one category lower than your initial instinct. Studies consistently show that self-reported physical activity tends to be overestimated.
Normal Day Eating - What Does "Unrestricted" Actually Mean?
A common misconception about the 5:2 diet is that normal days mean truly unrestricted eating - including overeating to compensate for fasting days. Research, however, suggests that most people naturally consume only slightly more than their maintenance calories on normal days, and this slight increase does not negate the weekly calorie deficit created by the two fasting days.
For effective weight loss on the 5:2 protocol, normal day eating should be mindful rather than compensatory. The goal is to eat at or near your TDEE on normal days - not to restrict severely, but not to binge either. If weight loss is not progressing, the most common reason is significant calorie compensation on normal days following fasting.
On a standard 5:2 protocol, two fasting days at 500 calories creates a deficit of roughly 1,000-1,500 calories compared to eating at maintenance. Spread across the week, this translates to approximately 150-215 calories per day - consistent with evidence-based weight loss recommendations of 0.5-1 kg per week for sustainable loss.
Choosing Your Fasting Days
Most practitioners recommend choosing two non-consecutive fasting days to avoid back-to-back restriction, which can increase fatigue and hunger. Common patterns include Monday and Thursday, Tuesday and Friday, or Wednesday and Sunday. Choosing days that align with your social calendar helps - scheduling fasting days on days without social eating commitments reduces temptation and social friction.
Some people prefer consecutive fasting days (Monday and Tuesday, for example) and tolerate this well. This is acceptable, though research specifically examining consecutive versus non-consecutive days in the 5:2 format is limited. Individual preference and adherence should guide this choice.
What to Eat on Fasting Days
With only 500-600 calories available, food choices on fasting days require some planning to maximise satiety and maintain nutritional adequacy. Research on fasting day food choices consistently points toward high-protein, high-fiber foods as most effective for hunger management.
Protein is the most satiating macronutrient per calorie. Prioritising protein on fasting days - through lean meats, eggs, fish, legumes, or low-fat dairy - helps preserve muscle mass while also reducing subjective hunger. A meta-analysis in Obesity Reviews found that higher dietary protein during calorie restriction is associated with greater fat loss and better lean mass retention.
Fiber from non-starchy vegetables adds volume without significant calories, contributing to fullness. A large bowl of leafy greens, cucumber, and tomato can add substantial meal volume for under 50 calories. Cruciferous vegetables such as cauliflower, broccoli, and cabbage are particularly filling relative to their calorie content.
Breakfast (150 cal): 2 large eggs (scrambled or poached), spinach, small tomato - approximately 140-160 calories, high in protein and micronutrients.
Dinner (350 cal): 150g grilled chicken breast (165 cal) + large mixed salad with cucumber, peppers, and rocket (50 cal) + 1 tablespoon olive oil-based dressing (100 cal) + small apple (50 cal).
This pattern delivers approximately 35-40g protein, substantial fiber, and excellent micronutrient coverage within the 500-calorie target.
Hydration on Fasting Days
Adequate hydration is especially important on fasting days. Hunger and thirst signals can be difficult to distinguish, and staying well-hydrated throughout the day significantly reduces subjective hunger. Water, plain sparkling water, black coffee, green tea, and herbal teas are all calorie-free and can be consumed freely on fasting days.
A common strategy is to drink a large glass of water first thing in the morning and with any hunger sensation before deciding whether to eat. Black coffee in the morning is widely used to suppress appetite during morning fasting hours, and evidence from several studies supports caffeine's modest appetite-suppressing effect.
Exercise and the 5:2 Protocol
Combining exercise with the 5:2 diet is effective and generally well-tolerated. Most people can continue their regular exercise routine on both fasting and normal days. However, high-intensity exercise on fasting days may be challenging due to reduced glycogen availability, particularly for activities requiring sustained power output.
For most people, lighter to moderate exercise (walking, yoga, steady-state cardio) is well-suited to fasting days. Strength training or high-intensity interval training (HIIT) is generally better scheduled on normal eating days when glycogen stores are replenished. Scheduling exercise according to eating days can enhance performance and recovery while maintaining the fasting protocol.
Expected Weight Loss and Timeline
Evidence from randomised controlled trials suggests that the 5:2 diet produces weight loss broadly comparable to daily calorie restriction over periods of 6-24 weeks. A notable study by Harvie et al. (2013) in the British Journal of Nutrition found that women following a 5:2 protocol lost a similar amount of weight to those following continuous restriction, but showed greater improvements in insulin sensitivity.
Realistic expectations matter. Most individuals following the 5:2 diet can expect to lose approximately 0.5-1 kg per week during the active weight loss phase, assuming normal days are not used for compensatory overeating. Weight loss will naturally slow as body weight decreases because BMR falls with weight, reducing the calorie deficit from the same fasting protocol.
If weight loss stalls after several weeks of consistent 5:2 fasting, recalculate your TDEE based on your new lower body weight. Your fasting day target may need to be slightly reduced, or normal day calories may need to be modestly reduced. Recalculating every 4-6 weeks during active weight loss is good practice.
Who Should Avoid or Modify the 5:2 Diet
The 5:2 diet is not appropriate for everyone. Certain medical conditions and circumstances require medical supervision or make this protocol unsuitable:
- Diabetes (type 1 or type 2): Fasting significantly affects blood glucose and insulin requirements. People with diabetes should only undertake intermittent fasting under medical supervision with regular glucose monitoring.
- History of eating disorders: Restrictive dietary patterns can trigger or worsen disordered eating behaviors. Anyone with a history of anorexia, bulimia, or binge eating disorder should consult a healthcare provider before beginning any calorie restriction protocol.
- Pregnancy and breastfeeding: Calorie restriction during pregnancy or lactation is contraindicated. Nutritional needs are significantly elevated during these periods.
- Underweight individuals (BMI below 18.5): Further calorie restriction in underweight individuals carries risk of nutritional deficiency and muscle loss.
- Children and adolescents: Growing bodies require consistent nutrition. Fasting protocols are not recommended for those under 18 years of age without paediatric medical guidance.
- Individuals taking certain medications: Some medications must be taken with food or at specific times. Fasting can affect drug metabolism and absorption. Always discuss dietary changes with a prescribing physician.
Common Side Effects and How to Manage Them
Most side effects of the 5:2 diet are transient and diminish as the body adapts, typically within 2-4 weeks. Common initial experiences include:
Hunger: Expected and normal on fasting days. Management strategies include high-protein and high-fiber food choices, adequate hydration, staying busy, and delaying the first meal until mid-morning to compress the eating window.
Headaches: Often related to dehydration or caffeine withdrawal if coffee intake is reduced on fasting days. Increasing water intake and maintaining regular caffeine consumption typically resolves this.
Irritability: Blood glucose fluctuations during fasting can affect mood, particularly early in the adaptation phase. This generally improves substantially after 2-3 weeks as metabolic flexibility increases.
Difficulty concentrating: Some people experience reduced cognitive performance early in fasting days. Many report the opposite effect after adaptation - enhanced mental clarity. Splitting fasting day calories into two small meals rather than one may help cognitive performance during adaptation.
Sleep disruption: Hunger on fasting evenings can interfere with sleep. Scheduling a larger portion of fasting day calories in the evening can address this, though some protocols recommend earlier eating windows to align with circadian rhythms.
5:2 vs Other Intermittent Fasting Protocols
The 5:2 diet is one of several intermittent fasting approaches, each with different practical structures and evidence bases:
16:8 (time-restricted eating): Daily fasting window of 16 hours with eating confined to an 8-hour window. No specific calorie restriction on eating days. Evidence suggests benefits for weight management and cardiometabolic health. Easier to maintain long-term for many people as it requires no counting.
5:2 (two-day modified fast): Two days of severe restriction with five normal eating days. Largest evidence base of the intermittent fasting protocols for direct comparison with continuous restriction. Greater flexibility on non-fasting days.
Alternate Day Fasting (ADF): Alternating between fasting days and normal days throughout the week. More severe than 5:2, with a larger weekly calorie deficit. More challenging to maintain and associated with greater hunger burden.
One Meal A Day (OMAD): All daily calories consumed in a single meal. Very aggressive restriction. Limited long-term evidence and challenging for most people to sustain.
Metabolic Adaptations and Long-Term Considerations
A concern with any calorie restriction protocol is adaptive thermogenesis - the body's tendency to reduce metabolic rate in response to sustained energy deficit. Research specifically examining 5:2 suggests that intermittent fasting may cause less severe metabolic adaptation than equivalent continuous restriction, potentially because five normal eating days prevent the persistent low-energy state that drives downregulation of metabolic rate.
A study by Byrne et al. (2018) in the International Journal of Obesity demonstrated that intermittent approaches to weight loss, including periods of maintenance eating, preserved resting metabolic rate better than uninterrupted restriction. This has practical implications: the 5:2 pattern may preserve long-term metabolic rate better than equivalent daily restriction, potentially aiding weight maintenance after goal weight is reached.
Tracking Progress Beyond the Scale
Body weight alone is an incomplete measure of progress. Weight fluctuates daily by 1-3 kg due to water retention, food volume in the digestive tract, and hormonal fluctuations - particularly in women. Tracking weight trends over weeks rather than days provides a more accurate picture.
Additional metrics worth monitoring include waist circumference (a proxy for visceral fat), energy levels, sleep quality, and blood pressure if elevated. Many people following the 5:2 protocol report improvements in these metrics even before significant scale weight loss occurs, which can be motivating to track.
If access to body composition measurement is available (DEXA scan, bioelectrical impedance analysis), tracking fat mass and lean mass separately is more informative than scale weight alone. Effective weight loss involves predominantly fat loss with minimal lean mass loss. Adequate protein intake and resistance exercise both protect lean mass during calorie restriction.
The Role of Protein on the 5:2 Diet
Protein has three properties that make it especially important during a 5:2 protocol: it is the most satiating macronutrient per calorie, it has the highest thermic effect of food (meaning more calories are burned digesting it), and it is the primary nutritional signal for muscle protein synthesis. Ensuring adequate protein intake - both on fasting and normal days - protects lean mass and supports recovery from exercise.
General guidance from sports nutrition bodies suggests approximately 1.6-2.2 g of protein per kilogram of body weight per day for individuals exercising regularly. For a sedentary individual focused primarily on weight loss, 1.2-1.6 g/kg is a reasonable target. On fasting days, prioritising protein within the 500-600 calorie budget has consistent support in the literature for both satiety and lean mass preservation.
Psychological Aspects of the 5:2 Diet
The psychological experience of the 5:2 diet is distinct from daily calorie restriction. Knowing that restriction is temporary - that tomorrow is a normal eating day - makes fasting days more manageable for many people. This psychological relief mechanism is one of the reasons adherence rates for 5:2 tend to be comparable to or better than daily restriction in many trials.
However, some individuals experience heightened food preoccupation on fasting days, and for a minority, this can develop into unhealthy patterns. If fasting days consistently trigger obsessive thinking about food, strong urges to binge after fasting, or significant anxiety around eating, this warrants consultation with a healthcare professional or registered dietitian who can suggest alternative approaches.
Nutrient Density on Fasting Days
With only 500-600 calories available, every calorie on fasting days should be nutritionally dense. Foods that provide the most vitamins, minerals, and fiber per calorie include:
- Non-starchy vegetables: Leafy greens, broccoli, zucchini, peppers, mushrooms, cucumber
- Lean protein sources: Chicken breast, white fish, eggs, prawns, tofu, Greek yogurt
- Berries: High in antioxidants and fiber with modest calorie counts
- Legumes (small portions): Lentils and chickpeas offer protein, fiber, and micronutrients
- Eggs: One of the most nutritionally complete foods per calorie
Highly processed foods, refined carbohydrates, and calorie-dense items with low nutritional value (biscuits, sweets, crisps) are particularly unsuitable for fasting days - they consume a large share of the calorie budget while providing little satiety or nutritional benefit.
Frequently Asked Questions
Conclusion
The 5:2 diet represents a well-evidenced, practical approach to weight management and metabolic health improvement that suits many people better than continuous calorie restriction. Its flexibility - five normal days interspersed with two days of modest restriction - makes it more sustainable for many individuals over months and years. The evidence base supports its efficacy for weight loss, insulin sensitivity improvement, and cardiometabolic risk reduction when followed consistently.
Success with 5:2 depends on accurate understanding of your individual calorie needs, sensible food choices on fasting days, and avoiding compensatory overeating on normal days. This calculator provides your personalized TDEE and fasting targets as a starting point. These figures should be recalculated as body weight changes to ensure targets remain appropriate throughout your journey.
As with any dietary intervention, particularly if you have existing medical conditions or take regular medications, consulting a registered dietitian or physician before beginning is advisable. The information provided here is educational and should complement, not replace, professional healthcare guidance.