Fasting windows and calories
By Rick Campbell · Updated · Sourced to primary literature · Not medical advice
Intermittent fasting is a schedule for when you eat. It is not a separate law of fat loss. The scale still answers to weekly energy in versus weekly energy out. A 16:8 window that holds 2,400 kilocalories against a 2,200 TDEE is a surplus with a fashionable clock. A regular meal pattern at 1,900 against the same TDEE is a deficit with a fork.
This site's fasting calculator prints the clock for 14:10, 16:8 and 18:6, and the two low-calorie days for 5:2. On the time-restricted protocols, weekly calories equal seven times TDEE unless you choose a lower daily target. There is no hidden burn for the hours you do not eat. This guide separates the protocols, says what the trials found when calories were matched, and names who should not fast at all.
In brief
- Time-restricted eating times the meals. It does not add a fat-loss bonus once weekly calories are equal.
- 16:8, 14:10 and 18:6 only produce loss if intake inside the window stays below TDEE. The window can help some people eat less. It can also help others eat more.
- Classic 5:2 uses two low days (this site will not go below 40% of the 1,200 or 1,500 floor) and five days at TDEE. The weekly total is what matters.
- Trials that match calories between fasting and regular meal patterns find similar weight change. The schedule is a tool for adherence, not a metabolic shortcut.
Windows versus weekly energy
Fat loss is a weekly energy gap. A window can create that gap by removing evening snacks, late alcohol and the second dinner. It can also destroy the gap by turning the eating hours into a compressed feast. The clock does not know which one you are doing. The log does.
On this site, a TRE protocol (14:10, 16:8, 18:6) sets an eating start hour and an eating length. Default start is 12:00. A 16:8 window then ends at 20:00. Weekly calories default to maintenance: seven times TDEE. If you want a deficit, you still set one, the same way you would without a clock. The calculator will not invent a 300 kilocalorie bonus for the sixteen fasting hours.
People confuse hunger timing with energy cost. Being hungry at 10:00 does not mean you are burning extra fat at 10:00. Overnight and morning hours were already mostly fasting for anyone who sleeps. Extending that stretch changes when the first meal arrives. It does not rewrite TDEE.
14:10, 16:8 and 18:6
These are time-restricted eating patterns. The first number is fasting hours, the second is eating hours. 14:10 is the gentlest: last calorie at 20:00, first at 10:00, or any shift of that ten-hour window. 16:8 is the popular one. 18:6 is two meals or a meal and a snack for most people.
Shorter windows are not automatically better. They are easier to overeat in. Three takeaways and a pint inside six hours can outrun a day of ordinary meals. They are also harder to protein-load if you train in the morning and do not eat until the afternoon. Protein still sits near 1.6 to 2.2 grams per kilogram if you care about lean mass. The window does not lower the gram target.
Shift the window to fit the life. A 16:8 that starts at 08:00 and ends at 16:00 is a valid 16:8. So is noon to 20:00. The protocol is the length, not a moral attachment to skipping breakfast. If breakfast is the meal you can eat in peace and dinner is the meal you overeat, skip dinner, not breakfast.
| Protocol | Fast hours | Eat hours | Window if start is noon | Weekly calories at maintenance |
|---|---|---|---|---|
| 14:10 | 14 | 10 | 12:00 to 22:00 | 7 x TDEE |
| 16:8 | 16 | 8 | 12:00 to 20:00 | 7 x TDEE |
| 18:6 | 18 | 6 | 12:00 to 18:00 | 7 x TDEE |
| 5:2 | n/a | n/a | Two low days, five TDEE days | 5 x TDEE + 2 x low day |
5:2 and the low-calorie days
5:2 is a weekly pattern, not a daily clock. Five days at about maintenance, two days at a much lower intake. The original popular figures were about 500 kilocalories for women and 600 for men on the low days. This site will not place those low days below 40 per cent of the 1,200 or 1,500 absolute floor, because a web form has no business planning 480 kilocalories for a woman without a clinician in the room.
The weekly total is five times TDEE plus two times the low day. If TDEE is 2,200 and the low day is 600, the week is 12,200 kilocalories against a 15,400 maintenance week, a gap of 3,200, or about 0.4 kilograms at 7,700 kilocalories per kilogram. That is a real deficit. It is also easy to erase by eating 400 extra on each of the five 'normal' days. 5:2 fails in the same place every other diet fails: the days you decided did not count.
Do not stack 5:2 on top of a 20 per cent daily deficit. That is a very low energy week with a fashionable name. Pick a daily deficit or pick 5:2. The floors still bind.
What the trials actually found
Trepanowski and colleagues randomised adults with obesity to daily calorie restriction or to alternate-day fasting for a year. Weight loss was similar when the energy gap was similar, and the fasting arm did not show a metabolic advantage. Dropout was higher on the fasting schedule. That is the pattern most later trials repeat: when calories are matched, the clock is not the active ingredient.
Liu and colleagues compared time-restricted eating with daily calorie restriction in adults with obesity. Again, the weight change followed the calorie gap. Early-window TRE has some insulin-timing papers behind it. Those papers do not license a claim that 16:8 burns fat you did not leave uneaten.
The useful reading is about adherence. If a window stops the evening graze and you can hold protein and training, it is a good tool. If a window turns lunch into a binge and you skip the lift because you have not eaten, it is a bad tool. The trial average cannot tell you which person you are. A four-week log can.
Protein, training and who should not fast
Training in a long fast is a preference, not a requirement. Some people lift well before the first meal. Some people do not. Place the window so the session has fuel if your performance drops. Protein still needs to land in the eating hours. Two meals of 80 grams is workable. One meal of 160 grams is a poorer use of the same grams for most people.
Who should not use these protocols without clinical advice: anyone who is pregnant or breastfeeding, anyone with a history of an eating disorder, children and adolescents, people with type 1 diabetes or insulin-treated type 2 diabetes unless a clinician has planned the insulin, and anyone already below the calorie floors. A fasting window is a bad place to hide a very low energy diet.
Medicines that require food, a history of fainting, and jobs that are unsafe when attention dips (driving, heights, machinery) are also reasons to pick a regular meal pattern. The calculator will still print a clock. Printing a clock is not a recommendation that you use it.
How to use the calculator without fooling yourself
Enter TDEE from the TDEE calculator, pick a protocol, pick a start hour, and read the weekly total. If you want loss, set a daily target below TDEE on the TRE protocols, or accept that 5:2's gap is the two low days. Then log the week. If weekly intake is at or above weekly TDEE, the window did not fail. The intake did.
Pair the schedule with the deficit and check-in tools. Four weeks of a seven-day weight average will tell you whether the clock is helping. One hungry Wednesday will not. If the average is flat and the log is honest, you are at maintenance in a window. Eat less or move the job to a named deficit.
Alcohol still counts. A 16:8 that starts at noon and includes three pints is a calorie schedule with beer in it. See the alcohol guide. The window does not metabolise ethanol into a free food.
Frequently asked questions
Does 16:8 fasting burn more fat than eating across the day?
Not once weekly calories are the same. Trials that match energy intake find similar weight change on time-restricted eating and on ordinary meal patterns. 16:8 can help you eat less by closing the kitchen at 20:00. It can also help you eat more by packing the day's food plus extras into eight hours. The clock is a habit tool. The weekly gap versus TDEE is the fat-loss tool. If someone promises a metabolic bonus for the sixteen hours, ask for a calorie-matched trial that showed one.
Can I drink coffee during the fasting window?
Black coffee and plain tea are the usual practical answer for people using TRE as an appetite schedule. They are not zero, but they are small. Milk, sugar, juice, diet soda debates and bone broth turn the fast into a theology problem. This site cares about weekly kilocalories. If the 'fasting' drink is 200 kilocalories, it is breakfast. If you are fasting for a medical protocol or a lab test, follow the clinician's rules, not a forum.
Will fasting make me lose muscle?
A large weekly deficit plus low protein plus no lifting will lose lean mass, with or without a window. Fasting does not add a special muscle-wasting clause on top of that. It can make protein harder to spread if you only eat twice. Keep protein near 1.6 to 2.2 grams per kilogram, lift, and keep the weekly gap moderate. If strength is falling and the average is dropping faster than one per cent of weight a week, the problem is the rate, not the clock.
Is 5:2 better than 16:8?
They are different tools. 16:8 is a daily clock. 5:2 is two low days and five ordinary days. 5:2 builds a weekly gap even if the five days are at TDEE. 16:8 builds a gap only if the eight hours stay below TDEE. Neither is better in a calorie-matched trial. Pick the one you can repeat for months without bingeing the next meal. If you cannot hold the low days without overeating the next morning, 5:2 is a bad fit.
Who should not try intermittent fasting?
Pregnant or breastfeeding people, children and teens, anyone with a current or past eating disorder, and anyone on insulin or other medicines that make long gaps unsafe unless a clinician has planned it. Also skip it if your job is unsafe when you are light-headed, or if you are already under the 1,200 or 1,500 kilocalorie floors. A website schedule is not a treatment plan. If a clinician has you on a specific meal timing for blood sugar or a procedure, that plan wins.
Put it into practice
Run your own numbers through the fasting window calculator, the calorie deficit calculator and the TDEE calculator. Related reading: How many calories to eat, TDEE explained and Alcohol calories.
Sources
- Trepanowski JF, Kroeger CM, Barnosky A, et al. Effect of alternate-day fasting on weight loss, weight maintenance, and cardioprotection among metabolically healthy obese adults. JAMA Intern Med 2017;177:930-8. doi.org/10.1001/jamainternmed.2017.0936
- 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/NEJMoa2114833
- 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
- NICE. Overweight and obesity management. NICE guideline NG246, published 14 January 2025. www.nice.org.uk/guidance/ng246
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- “Fasting windows and calories”, Body Stats, last updated 15 September 2026, https://bodystats.co/app/guides/fasting-windows-and-calories
This guide is informational and educational, not medical advice. Formula details live on the methodology page; see also the medical disclaimer.
Last updated . Written by Rick Campbell; not medically reviewed. See review status.