Intermittent Fasting Calculator
By Rick Campbell · Updated · Sourced to primary literature · Not medical advice
Intermittent fasting is a timetable for eating, not a second metabolism. This page lays out 14:10, 16:8, 18:6 and 5:2 against your estimated TDEE so you can see the clock and the weekly calories in one place.
Time-restricted eating does not add a fat-loss bonus in the arithmetic. Liu and colleagues, in a 2022 randomised trial, found that calorie restriction with a daily eating window did not beat calorie restriction alone for weight loss. Weekly energy still has to sit below TDEE if fat is the goal.
Enter vitals, activity, a protocol and when the eating window starts. You get the clock, eating-day and low-day calories, and the weekly total versus seven days at TDEE. Protein still has to fit inside the window.
In brief
- 14:10, 16:8 and 18:6 are eating clocks. Weekly calories stay at TDEE if you eat maintenance inside the window.
- 5:2 uses two low days (classic 500 to 600 kcal, lifted toward 40% of this site's floor) and five days at TDEE.
- Trials that match calories between groups do not show extra fat loss from the clock alone.
- Pack protein into the hours you eat. A skipped breakfast that also skips protein is just a low-protein day.
Calculator
What you'll see here
The eating-window clock, weekly calories from TDEE (unchanged for 14:10, 16:8 and 18:6), and the 5:2 weekly total with two low days.
Windows versus weekly energy
14:10 means 14 hours fasting and a 10-hour eating window. 16:8 and 18:6 tighten the same idea. If you eat your TDEE inside those hours, the week is maintenance. The clock did not burn extra fat. It only moved lunch.
People lose weight on fasting plans when the window cuts snacks, late drinks and a fourth meal. That is a deficit by another name. Log a week. If intake matches TDEE, the scale will behave like maintenance, window or not.
The TDEE page is the number underneath. This page only schedules it.
What the trials actually showed
Liu et al. randomised adults with obesity to calorie restriction with or without a time-restricted eating window. Weight loss did not differ once calories were matched. That is the result this site will not talk around.
Reviews of intermittent fasting find that adherence and weekly energy explain most of the fat-loss variance. Early time-restricted eating can improve some metabolic markers in short studies even without weight loss, which is interesting and not the same as 'fasting melts fat'.
If a video promises autophagy as a reason to skip protein, it is selling a word. This form will not print an autophagy score.
14:10, 16:8 and 18:6
The form places your eating start (late morning through late afternoon) and adds the eating hours. 16:8 starting at 12:00 runs to 20:00. 18:6 starting at 12:00 runs to 18:00. Pick the start you can keep on workdays.
Eating-day calories are estimated TDEE. Weekly calories are seven times that figure. The weekly-versus-maintenance row is then zero, on purpose. To lose fat, use the deficit calculator and eat that lower target inside the same window.
Shorter windows make protein and fibre harder. If 18:6 turns into two huge, low-protein meals, you will lose lean mass. The protein calculator still applies.
How 5:2 is handled here
Classic 5:2 is five days at maintenance and two non-consecutive days at about 500 kcal (women) or 600 kcal (men). Those low days sit well below this site's usual 1,200 / 1,500 kcal floors, so they are shown as a published pattern, not as a recommended everyday target.
The form lifts the low day to at least 40% of the sex-specific floor so the printed number is not a 200 kcal crash. Weekly calories are five times TDEE plus two low days, which is below seven times TDEE. That weekly gap is the deficit, not the fasting hours.
Low days still need protein and fluid. Two 500 kcal days of juice are a lean-mass plan. Two days of lean meat, yoghurt and vegetables are closer to the point.
Who should not use these clocks
This site will not run adult energy maths under 18. Pregnancy, a history of eating disorders, insulin-treated diabetes, and underweight BMI are reasons to ignore a fasting window and speak to a clinician. A web form cannot sort those.
Shift workers should pick a window that follows sleep, not the clock on this page. A 16:8 that lands in the middle of a night shift is just sleep loss.
If fasting raises binge evenings, it has failed as a tool. Eat on a regular timetable and use a modest deficit instead.
What to do with the result
If the goal is fat loss, take the TDEE into the deficit calculator, then eat that target inside the window. If the goal is a hold, eat TDEE inside the window and use the check-in every two to four weeks.
Carry protein grams into the macro calculator. Fibre still has a daily target; stuffing it into six hours is how people get uncomfortable. Raise water with fibre.
The TDEE explained and calories guides are the longer reading. The clock is the short reading.
How it's calculated
Time-restricted week
Weekly kcal = TDEE × 7 (if you eat at TDEE inside the window)
The clock does not subtract extra fat. Use a deficit target if fat loss is the goal.
5:2 week
Weekly kcal = (TDEE × 5) + (low-day kcal × 2)
Classic low days are 500 kcal (women) or 600 kcal (men), lifted toward 40% of this site's floor.
Window end
Eat end hour = (start hour + eating hours) mod 24
14:10, 16:8 and 18:6 use 10, 8 and 6 eating hours.
Worked example: 16:8 starting at 12:00, TDEE 2,100 kcal
- Eating window is 12:00 to 20:00, 8 hours eating, 16 hours fasting.
- Eating-day calories = 2,100. Weekly calories = 2,100 × 7 = 14,700 if you eat at TDEE.
- Weekly versus maintenance = 0 kcal. The window alone does not create a deficit.
- A 400 kcal daily deficit inside the same window is 1,700 kcal between 12:00 and 20:00, or 11,900 kcal a week.
- Protein at 2.2 g/kg for an 80 kg adult is 176 g, which still has to fit in those eight hours.
Where this number is used in the real world
- Picking 14:10 versus 16:8 for a workday lunch-to-evening window.
- Seeing that 16:8 at TDEE is maintenance, not a cut.
- Laying out 5:2 weekly calories versus seven maintenance days.
- Checking that protein still fits inside a short window.
- Handing TDEE into the deficit calculator for a real gap.
Frequently asked questions
Does fasting burn extra fat even if I eat the same calories?
Not in the trials that match weekly energy between groups. Liu and colleagues compared calorie restriction with and without a daily eating window and did not find extra weight loss from the window. Reviews reach the same practical conclusion: the clock helps when it cuts intake, not when it is stacked on top of the same calories. This form will not add a fasting bonus to TDEE. If fat loss is the goal, eat below TDEE inside the hours you chose.
Is 16:8 better than 14:10?
Only if you can keep it and still hit protein, fibre and a calorie target. A 14:10 window that you hold for months beats an 18:6 window you break every Friday. Shorter windows make social meals and training snacks harder. Start at 14:10 if you currently eat across 16 hours. Move to 16:8 if late-night energy is the thing you need to stop. Do not skip to 18:6 to impress a feed.
Are 5:2 low days safe?
Classic 500 to 600 kcal days sit well below the 1,200 and 1,500 kcal floors this site uses for daily planning. They are shown because people search 5:2, and the weekly total can still be a moderate deficit. They are not a everyday recommendation here, and they are a bad idea if you have a history of disordered eating, are pregnant, or take insulin. Put protein on the low days. If the two days become a binge-restrict cycle, stop and use a steady daily deficit.
Should I train fasted?
Some people prefer it. Performance and high-load lifting usually suffer without carbohydrate nearby. This page does not require fasted training. Place the window so a meal sits before the session you care about. The calorie and protein targets do not change because the session was fasted. Do not add 'fasted cardio burn' on top of TDEE.
Can I drink coffee in the fasting hours?
Black coffee and unsweetened tea are the usual practise and do not carry meaningful energy. Milk, sugar, juice and 'fasting' creamers do. If the goal is a true energy fast, keep the hours at near-zero kcal. If the goal is just to stop evening snacking, a splash of milk is not the thing that will make or break the week. Weekly calories still will.
Will fasting raise or wreck my metabolism?
A short daily window at maintenance does not 'boost metabolism' and does not damage it. Long, severe restriction lowers expenditure the same way any large deficit does, through less mass and often less movement. That is Hall and the adaptation literature, not a fasting-specific curse. Reverse toward TDEE when the cut ends, window or not. The reverse-diet calculator is the exit schedule.
Keep going
A single number rarely tells the whole story. Alongside the fasting window result, the TDEE calculator, the calorie deficit calculator, the protein calculator and the reverse diet calculator each add a different angle on the same measurements. For the reasoning behind the numbers, read Fasting and calories and How many calories to eat.
Sources
- Liu D, Huang Y, Huang C, et al. Calorie restriction with or without time-restricted eating in weight loss. N Engl J Med 2022;386:1495-504. doi.org/10.1056/NEJMoa2114835
- Hall KD, Sacks G, Chandramohan D, et al. Quantification of the effect of energy imbalance on bodyweight. Lancet 2011;378:826-37. doi.org/10.1016/S0140-6736(11)60812-X
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr 1990;51:241-7. doi.org/10.1093/ajcn/51.2.241
Cite this page
Quoting a figure from here in an article, a report or a piece of coursework? Use whichever of these your style guide asks for.
- APA
- Campbell, R. (2026). Intermittent Fasting Calculator. Body Stats. https://bodystats.co/app/intermittent-fasting-calculator
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- “Intermittent Fasting Calculator”, Body Stats, last updated 15 September 2026, https://bodystats.co/app/intermittent-fasting-calculator
Every formula and threshold on this page is written out with its primary source on our methodology page. These results are informational and educational, not a diagnosis or a substitute for professional advice. See the medical disclaimer.
Last updated . Written by Rick Campbell; not medically reviewed. See review status.