Intermittent Fasting Calculator
A free fasting calculator: pick a method and the time you want to start eating, and it works out your exact fasting window, eating window and total fasting hours.
Intermittent-fasting numbers split a 24-hour day — 16:8 means 16 hours fasting with an 8-hour eating window, and beginners usually start gentler at 12:12 or 14:10.
How the schedule works
Intermittent fasting splits your day into an eating window and a fasting window. The shorthand — 16:8, 18:6, 20:4 — is simply the fasting hours to eating hours, and the two always add up to 24. This scheduler takes the method you choose and the time you want to start eating, then works out exactly when your eating window closes and how long your fast runs.
There is no metabolic magic in the clock itself. Fasting windows mostly help by trimming the hours in which it is easy to overeat, which can make a modest calorie deficit easier to hold. If you eat the same calories in a shorter window, weight will not necessarily change — pair this with the TDEE and macro tools to know your numbers.
Your fasting window, eating window and fasting hours
Three numbers come out of the same choice, and it helps to keep them straight. Your eating window is the block of hours in which you eat — the "8" in 16:8. Your fasting window is everything else, and the fasting hours figure is simply its length. Because the two windows tile a 24-hour day, fixing one fixes the other — which is why a single tool serves as a fasting window calculator, a fasting time calculator and a fasting hours calculator at once. Choose a method and a first-meal time above and it returns all three, plus the clock times they start and end.
A worked example. Pick 16:8 and a first meal at 12:00, and your eating window runs 12:00 to 20:00; the fasting window is 20:00 to 12:00 the next day, which is 16 fasting hours. Shift the first meal to 10:00 and everything moves with it — the window closes at 18:00 instead. The split is what the method fixes; where you place it in the day is yours.
The calculator is free, needs no sign-up and stores nothing — the arithmetic runs in your browser and the schedule is shown on the page, not saved to an account.
What the 16 hours actually bought
The clock arithmetic above is exact to the minute. The evidence behind it is thinner than that. The one study in the sources below that tested this exact schedule — Gabel and colleagues, 2018 — put 23 obese adults on ad libitum eating between 10:00 and 18:00, with water fasting from 18:00 to 10:00, for 12 weeks. It compared them against a matched historical control group rather than a randomised one, which is the paper's own wording and a real limit on what the result can carry. Three things moved: energy intake fell 341 ± 53 kcal a day, body weight fell 2.6% ± 0.5, and systolic blood pressure fell 7 ± 2 mm Hg. Eleven things did not.
Run the trial's own 341 kcal a day through the constant this site already applies in its calorie deficit and weight loss timeline tools — 3,500 kcal per pound, 7,700 per kilogram — and the schedule is worth 2,387 kcal a week, or 0.68 lb (0.31 kg). The FAQ below calls 0.5–1 kg (1–2 lb) a week the usual target. 0.68 lb does not reach the bottom of it. Carry the trial's own ±53 kcal error bar through the same arithmetic and the band runs 0.58 to 0.79 lb a week (0.26–0.36 kg) — wider, but never once crossing into the recommended range at either end.
What changed in the one trial that tested this window
| Outcome | Change vs controls | Read as |
|---|---|---|
| Energy intake | −341 ± 53 kcal/day | The whole measured effect — 0.68 lb (0.31 kg) a week |
| Body weight | −2.6% ± 0.5 over 12 weeks | Real, but under the 1–2 lb per week band |
| Systolic blood pressure | −7 ± 2 mm Hg | Reported as measured; this site has no clinical reviewer to price it |
| Eleven other outcomes | No difference from controls | Fat mass, lean mass, visceral fat, diastolic BP, LDL, HDL, triglycerides, fasting glucose, fasting insulin, HOMA-IR, homocysteine |
Gabel et al. (2018), n = 23, 12 weeks, against a matched historical control group (n = 23). Weekly figures use the 3,500 kcal/lb (7,700 kcal/kg) rule this site's deficit and timeline calculators already apply.
That rule run out to the full 12 weeks predicts 28,644 kcal, or 3.72 kg. Which equals 2.6% of body weight only for someone starting at about 143 kg — a sign the straight-line rule is running ahead of the scale, not that the trial was wrong. The weight loss timeline calculator carries that caveat and its source.
Which is the point, and it is worth saying without decoration: the window is a calorie cap with a clock attached. The measured effect was the 341 kcal a day. Whether the hour it started mattered was not tested. Eat those calories back inside the window and there is nothing left for the schedule to have done — and this calculator would not notice, because it never asks what you eat.
There is a second thing it cannot see. The script takes your start time, adds the eating hours, and does nothing else with the clock. So 16:8 beginning at 10:00 AM — the window the trial actually ran — and 16:8 beginning at 10:00 PM come back identical: an 8-hour window and a 16-hour fast. The tool will schedule 10:00 PM to 6:00 AM (+1 day) with the same confidence and the same label, and none of the sources below tested that placement. The same gap runs through the menu: six methods sit in the dropdown, and exactly one of them, 16:8, has a dedicated trial in this page's source list. 5:2 has one (Harvie, 2011) and the tool does not schedule it at all.
What this calculator does not cover
This is a scheduler for time-restricted eating — fasting windows measured in hours within a single day. It is not built for multi-day or water fasts, and deliberately so. Extended fasting carries risks that a calculator cannot assess for you, including electrolyte disturbance and refeeding syndrome when the fast is broken, and the appropriate length differs by age, medication and medical history. If you are considering a fast lasting longer than a day, that is a conversation to have with a doctor or registered dietitian rather than a web form — this site has no clinical reviewer and will not estimate one for you.
Choosing a method
Beginners usually do best starting gentle — 12:12 or 14:10 — before tightening to the popular, well-studied 16:8. Longer protocols like 20:4 or OMAD are far more restrictive and suit experienced fasters; they also make it harder to get enough protein and micronutrients. During the fast, water, black coffee and unsweetened tea are fine; anything with calories opens the eating window. Staying hydrated helps — our water intake calculator gives a daily baseline.
This is general lifestyle guidance, not medical advice. Intermittent fasting is not suitable for everyone — including people who are pregnant or breastfeeding, under 18, have a history of disordered eating, or take medication for diabetes — so check with a healthcare professional first.
Common fasting protocols
| Method | Fasting | Eating | Typically suits |
|---|---|---|---|
| 12:12 (starter) | 12 h | 12 h | Absolute beginners; easiest to sustain |
| 14:10 | 14 h | 10 h | Easing into time-restricted eating; low-stress |
| 16:8 | 16 h | 8 h | Most popular and most-studied daily routine |
| 18:6 | 18 h | 6 h | Comfortable with 16:8 and wanting a tighter window |
| 20:4 (Warrior) | 20 h | 4 h | Experienced fasters; one main meal plus snacks |
| OMAD (23:1) | 23 h | 1 h | Advanced only; higher risk of nutrient gaps |
| 5:2 (not a window) | 2 low-cal days/wk | Normal 5 days | ~500 kcal (women) / 600 kcal (men) on 2 days |
Hours are the fasting-to-eating split of a 24-hour day. 5:2 is calorie-based rather than a daily window. Lifestyle guidance only.
Trial figures: Gabel et al., Nutrition & Healthy Aging (2018), PMID 29951594 — n = 23, 12 weeks, matched historical controls. Weekly loss uses the 3,500 kcal/lb (7,700 kcal/kg) rule already applied by this site's deficit and timeline calculators. Background: de Cabo & Mattson, New England Journal of Medicine (2019); NIH, Johns Hopkins Medicine and NHS. Not medical advice.
Frequently asked questions
What do the numbers like 16:8 mean?
They describe how a 24-hour day is split between fasting and eating. In 16:8 you fast for 16 hours and eat only during an 8-hour window; the two numbers always add up to 24. So 18:6 is 18 hours fasting / 6 hours eating, and OMAD (23:1) is 23 hours fasting with a roughly 1-hour eating window. The calculator takes the method and the time you want to start eating, then works out when your eating window closes and your fast begins.
Which method should a beginner start with?
Gentler windows such as 12:12 or 14:10 are the usual starting points because they cause the least disruption and are easiest to keep up. Many people then progress to 16:8, which is the most common and most-studied daily approach. Longer windows like 20:4 or OMAD are far more restrictive and are generally better suited to experienced fasters. This is general guidance, not a recommendation for your specific situation.
Does drinking water, black coffee or tea break the fast?
For a time-restricted eating window, plain water is fine and staying hydrated is encouraged. Black coffee and unsweetened tea are essentially calorie-free and are commonly consumed during the fasting window. Anything with calories — milk, sugar, sweetened drinks, or snacks — is treated as eating and would fall inside the eating window.
How is 5:2 different from a 16:8-style window?
5:2 is not a daily clock. Instead you eat normally on five days and cap intake at roughly 500 kcal (women) or 600 kcal (men) on two non-consecutive days of the week. Because there is no fixed daily eating window, the start-time calculation used for 16:8 or 18:6 does not apply to it — it is a calorie-based schedule rather than a time-based one.
How do I work out my fasting window?
Take the two numbers of your method and place them on the clock. The eating window is the smaller number in hours, starting from your first meal; the fasting window is the remaining hours. For 16:8 with a first meal at 12:00, the eating window is 12:00–20:00 and the fasting window runs 20:00 to 12:00 the next day — 16 fasting hours. The calculator above does this for any method and start time and shows the clock times for both windows.
How many hours should I fast?
There is no single correct figure. 12 to 14 hours is the usual starting range because it mostly means not eating after dinner, and 16 hours is the most common and most-studied daily pattern. Beyond about 18 hours the eating window gets short enough that hitting protein and micronutrient targets becomes genuinely harder, which is the practical limit for most people rather than any metabolic threshold. This is general guidance, not a recommendation for your circumstances.
Is this fasting calculator free?
Yes — it is free to use, there is no sign-up, no paywall and no account. The calculation runs in your browser, so your schedule is not sent anywhere or stored. The method behind it is shown on the page along with its sources.
Can I use this for a water fast?
No. This tool schedules time-restricted eating — fasting windows measured in hours inside a single day. It does not cover multi-day or water fasts, which carry risks a calculator cannot assess, including electrolyte disturbance and refeeding problems when the fast is broken. Safe duration depends on age, medication and medical history. If you are considering a fast longer than a day, speak to a doctor or registered dietitian rather than relying on any online calculator.
Will intermittent fasting on its own make me lose weight?
Not automatically. Fasting windows tend to help mainly by making it easier to eat fewer total calories, since there is less time to eat. If you consume the same or more calories inside the window, weight may not change. A steady, sustainable rate of about 0.5–1 kg (1–2 lb) per week is the widely recommended target for healthy adults. To see your daily calorie needs, use the TDEE or macro calculator alongside this scheduler.
How much weight does 16:8 actually cause?
In the one trial on this page that tested exactly that schedule — Gabel et al. (2018), 23 obese adults eating between 10:00 and 18:00 for 12 weeks — body weight fell 2.6% and energy intake fell 341 kcal a day relative to controls. Put through the 3,500 kcal-per-pound (7,700 kcal-per-kg) rule, 341 kcal a day comes to roughly 0.68 lb (0.31 kg) a week, which sits below the 1–2 lb per week usually recommended; the trial reported no change in fat mass, lean mass, visceral fat, blood lipids, fasting glucose or insulin. The measured effect was the smaller calorie intake rather than the clock, so eating the same amount inside a shorter window would not be expected to do much.
Sources
- de Cabo & Mattson — Effects of Intermittent Fasting on Health, Aging, and Disease (NEJM, 2019) — Foundational review of fasting regimens
- Gabel et al. — 8-hour time-restricted feeding pilot (Nutrition & Healthy Aging, 2018) — 16:8 time-restricted eating trial
- Harvie et al. — Intermittent vs continuous energy restriction (Int J Obesity, 2011) — 5:2 two-day restriction origin study
- MedlinePlus (NIH) — Safe rate of weight loss — Safe ~1-2 lb per week rate
The thresholds and formulas used by this calculator follow the primary sources above. This tool provides general information, not personalised medical advice.