An intermittent fasting calculator maps a clock — 16:8, 18:6, 20:4, or a longer water fast — onto estimated energy use and the split between fat and water on the scale. The fasting window does not burn a special calorie. It only removes eating hours. If the meals inside the window still add up to your full daily energy need, the week is maintenance, not fat loss. People get confused because the first two days often drop a kilogram that returns as soon as they eat carbohydrates again. That swing is glycogen and the water bound to it, not a kilogram of fat.
Use this page to pick a window you can repeat, to see a rough calorie hole for the hours you skip, and to stop treating overnight scale drops as fat. It is not a medical fast, not a detox, and not appropriate as a DIY plan if you are pregnant, underweight, have a history of disordered eating, or take insulin or other glucose-lowering drugs. Those cases need a clinician, not a timer.
What the fasting hours are estimating
Energy missed ≈ (TDEE / 24) × fast hours; fat-tissue change ≈ energy deficit / 7,700; early scale drop also includes glycogen water
Daily energy uses the same Mifflin-St Jeor BMR as the TDEE calculator (Mifflin et al., American Journal of Clinical Nutrition, 1990) times an activity factor. The 7,700 kcal per kilogram of fat tissue is Wishnofsky’s rule of thumb, not a measured constant for every body. Glycogen binds roughly 3–4 grams of water per gram of carbohydrate stored, which is why the first day of a fast moves the scale more than the fat math allows.
Mifflin-St Jeor for a man is 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5. For a woman the constant is −161. The fasting tool then multiplies by an activity factor (about 1.2 sedentary, 1.35 light, 1.5 moderate) and divides by 24 to get an hourly burn. A 16-hour fast does not multiply fat loss by 16. You still eat inside the 8-hour window. The hourly figure only estimates energy you did not take in during the skipped hours, and only if those meals are not piled into dinner.
One kilogram of mixed body fat is often treated as about 7,700 kcal. A 500 kcal daily deficit is about 0.45 kg of fat per week. A single 16-hour fast that merely delays breakfast does not create a 500 kcal deficit if lunch and dinner grow to fill it. Write the day’s food down once. The clock is a compliance tool. The grams and calories are the deficit.
Liver glycogen is largely used across the first day of not eating, depending on how full stores were after the last high-carbohydrate meal. Muscle glycogen is harder to empty without exercise. As glycogen falls, bound water falls with it, often 0.5 to 2 kg on the scale in the first 24 to 48 hours. The calculator separates that water estimate from the fat estimate so a “2 kg loss” after a weekend fast is not read as 2 kg of fat. Most of the water returns with the next normal meals.
Worked examples
Example 1 — 16:8, maintenance eater
- Take a 30-year-old man, 80 kg, 180 cm. Mifflin BMR = 10 × 80 + 6.25 × 180 − 5 × 30 + 5 = 1,780 kcal. Moderate activity × 1.55 is about 2,760 kcal per day. The fasting tool’s moderate factor is a bit lower (1.5); use the TDEE calculator for the meal-plan number.
- Hourly burn ≈ 2,760 / 24 = 115 kcal. Sixteen skipped hours would be about 1,840 kcal if he ate nothing else to compensate.
- If dinner simply absorbs the missed breakfast, the deficit is near zero. The scale may still dip 0.3 to 0.6 kg from a shorter glycogen window and less food in the gut.
- 16:8 is useful here as a way to stop night grazing, not as a multiplier on fat loss. Keep protein in the two or three meals that remain.
Example 2 — 36-hour fast versus the fat math
- Same 115 kcal per hour × 36 hours ≈ 4,140 kcal of missed intake, if the surrounding days are not larger.
- Fat-tissue estimate = 4,140 / 7,700 ≈ 0.54 kg. That is the fat math, not the scale math.
- Glycogen water over a day and a half can be another 1 to 2 kg. A 2.5 kg scale drop is mostly water plus a half kilogram of fat, if the deficit was real.
- Refeeding carbohydrates brings a large share of the water back within a day or two. Judge the fast by weekly average weight and waist, not by the morning after the fast.
Example 3 — 20:4 with too little protein
- A 65 kg woman with a TDEE near 2,000 kcal who eats one large plate and a snack in a 4-hour window can still overshoot 2,000 kcal. The window does not cap energy.
- The same window often undershoots protein. At 1.6 g/kg she needs about 104 g. A single restaurant dinner may land at 40 g unless it is planned.
- Split the four hours into two protein-heavy meals, or the fasting schedule fights the meal plan. Use the protein calculator, then fit those grams into the eating window before you tighten the clock.
Which window to pick
16:8 means 16 hours without calories and an 8-hour eating window, often noon to 8 pm, or 10 am to 6 pm if evenings are the problem. Black coffee, plain tea, and water do not break the fast in the way this calculator counts calories. Milk, cream, and sugar do. 18:6 and 20:4 shrink the window further. They help some people who overeat at night. They hurt training if the only meal is after the workout and protein never gets spread out.
OMAD — one meal a day — is a 22- to 24-hour cycle. It is easy to under-eat protein and fiber and easy to overeat energy in one sitting. It is a poor default for anyone lifting three days a week. A 5:2 pattern is different again: five normal days and two very low-calorie days, not a daily clock. Do not stack 5:2 on top of 20:4 unless a clinician is involved. Longer water fasts (36 to 72 hours) drop electrolytes, not just glycogen. Sodium, potassium, and magnesium are the practical risks, plus dizziness on standing.
A window that you keep six days a week beats a heroic 48-hour fast you quit on Monday with a binge. Match the window to the training clock: eat protein within a few hours after lifting if you can. Fasting through every workout and then calling the missed reps “discipline” is how strength stalls.
What the scale is lying about
Gut contents, sodium, and glycogen move daily weight by more than a single day’s fat loss. A 7,700 kcal rule means even a hard 1,000 kcal deficit removes about 0.13 kg of fat in a day. Anything faster than that on the scale is water or food volume. The calculator’s water term exists so you can subtract that story from the fat story.
If the goal is fat loss, set the week’s calorie deficit in the TDEE calculator, then use fasting only as the schedule that makes the deficit easier. A high-protein meal plan inside the eating window protects fat-free mass better than skipping protein because the clock ran out. Reintroduce normal meals after a long fast with food and salt, not with a gallon of plain water.
Frequently asked questions
Does 16:8 fasting burn more fat than eating the same calories earlier?
For fat loss, the week’s calorie deficit matters more than the clock. 16:8 helps some people eat less because breakfast is gone and night snacking has a hard stop. If you move the same food into a shorter window, weight change should be similar. Early scale drops of several pounds are mostly glycogen, water, and less food in the gut. Compare weekly averages, not the first morning.
How much weight do you lose on a 48-hour fast?
Fat loss is roughly the missed calories divided by about 7,700. At a 2,400 kcal daily need, 48 hours is on the order of 0.6 kg of fat tissue if you do not overeat before and after. The scale often drops 1.5 to 2.5 kg because glycogen water leaves as well. Most of that water returns when you eat normally. Electrolytes, not the scale, are the safety limit. Do not repeat multi-day fasts without medical advice.
When does ketosis start during a fast?
Ketone production rises as liver glycogen falls, often somewhere after 12 to 24 hours, sooner if the last meals were low in carbohydrate and you exercised. It is not a switch at hour 16, and it is not required for fat loss. You can lose fat in a deficit without being in ketosis, and you can be in ketosis without losing fat if calories are high. A breath meter is optional. It does not change the meal math.
What can you drink while intermittent fasting?
Water, black coffee, and plain tea are the usual zero-calorie choices and do not change the energy estimate on this page. A splash of milk, a latte, bone broth with calories, or a diet soda habit that triggers a snack all change the plan even if the timer is still running. If you want the deficit the calculator shows, the window has to be actually low-calorie, not just “no solid food.”
Will intermittent fasting make me lose muscle?
A large deficit plus low protein plus no lifting loses muscle, with or without a timer. Inside 16:8 or 18:6 you can hold muscle if you hit about 1.6 to 2.2 g of protein per kilogram and keep training. One meal a day makes those grams harder. Spread protein across two or three meals in the eating window. Use the protein calculator for the gram target before you shorten the window further.
Is intermittent fasting safe every day?
A daily 16:8 window is tolerated by many healthy adults. It is a bad idea during pregnancy, while breastfeeding, with a history of disordered eating, for adolescents, and for anyone on insulin or sulfonylureas unless their clinician redesigned the doses. Dizziness, insomnia, or bingeing when the window opens are reasons to widen the window, not to “push through.” Longer fasts need electrolytes and a reason, not a streak.
How do I combine fasting with a workout?
Lifting fasted is possible for short sessions if you feel normal, but protein after training still matters. A practical pattern is a 16:8 window that opens soon after the workout, with 30 to 45 g of protein in the first meal. Long Zone 2 work fasted is common; all-out intervals fasted are where people get lightheaded. Hydrate either way. A fast does not remove sweat losses.
Why did I gain it all back the day I stopped?
You regained water, glycogen, and gut contents, which is what left in the first 24 hours. Fat that was actually lost does not return in one refeed unless that refeed was thousands of calories above maintenance for days. Weigh the weekly average a week after normal eating. If that average is back to the start, the fast never created a lasting deficit because the meals on either side grew.
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