How this calculator works
This tool estimates how many calories to eat each day to lose, maintain, or gain weight on an intermittent fasting schedule, then shows your macros and a week-by-week projection. Here is exactly what happens behind the button.
1. Your calorie burn (BMR and TDEE)
First it estimates your Basal Metabolic Rate (BMR), the energy you would burn at complete rest, using the Mifflin-St Jeor equation. It is the most accurate standard formula:
Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161
Then it multiplies BMR by an activity factor to get your Total Daily Energy Expenditure (TDEE), which is your maintenance level:
- Sedentary ×1.2, Light ×1.375, Moderate ×1.55, Very active ×1.725, Extra active ×1.9.
- You can enter imperial (lb, ft/in) or metric (kg, cm). Values are converted to metric before the math, so both give the same result.
2. Your goal (the slider)
The goal slider sets your calorie target relative to maintenance: target = TDEE × (1 + slider%). A −20% setting eats 20% below maintenance to lose fat, 0% maintains, and +10% adds a surplus to support muscle gain. For safety the target is never set below your BMR.
- −15% to −20% is the sustainable fat-loss range. Steeper than −25% is aggressive and risks muscle loss.
- 5:2 and alternate-day fasting are weight-loss methods only. They create their deficit through dedicated fasting days, so the slider does not apply to them and they switch off when your goal is maintenance or gain.
3. Macros
The calorie target is split into protein, carbs, and fat. Protein is set per body weight to protect muscle: about 2.0 g/kg when cutting, 1.6 g/kg at maintenance, and 1.9 g/kg in a surplus. Fat is set to 25% of calories and the rest goes to carbs for training energy.
4. The projection
Roughly 7,700 kcal is about 1 kg of body mass (3,500 kcal is about 1 lb). The calculator multiplies your daily calorie gap by 7 and divides by that figure to estimate weekly change, then charts it over 12 weeks.
One honest caveat: this assumes a steady rate, but real weight change is not linear. It slows as your body adapts and your maintenance calories fall along with your weight. Treat the projection as a direction of travel, not a promise.
The fasting methods
- 14/10, 16/8, 18/6, 20/4, OMAD are time-restricted eating. You eat all your calories inside a daily window, from 10 hours down to 1 hour. The shorter the window, the more advanced.
- 5:2 means eat normally five days and cap calories at about 500 to 600 on two non-consecutive days.
- Alternate-day fasting alternates normal days with very low-calorie days at about 25% of maintenance.
Common questions
Is intermittent fasting effective for weight loss? Yes, when it puts you in a calorie deficit. Studies find it produces similar fat loss to standard calorie restriction, and many people find a daily eating window easier to stick to. Results come from the deficit, not from fasting magic.
What can I drink while fasting? Water, black coffee, and plain tea have effectively no calories and are fine during a fast. Anything with sugar, milk, or sweeteners can break the fast or add calories.
Will I lose muscle? Not if you keep protein high (the target above is set for this) and do some resistance training two or three times a week. Those two habits are what preserve muscle in a deficit.
Which method should a beginner pick? Start with 14/10 or 16/8. They fit most schedules, and you can tighten the window later if you want a longer daily fast.
Does the time of day matter? Adherence matters far more than the exact hours. Pick the window you can keep consistently. The schedule table above gives a few options to choose from.
Important
These figures are estimates for general information only and are not medical or nutritional advice. Calorie equations are population averages, so your real needs will vary. Intermittent fasting is not suitable for everyone, including people who are pregnant or breastfeeding, under 18, or have a history of disordered eating or certain medical conditions. Talk to a qualified healthcare professional before starting a fasting or calorie-restricted plan.