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Intermittent fasting calculator

Pick a schedule and the time of your first meal, and this gives you the actual clock times: when the window opens, when it shuts, and when the fast runs. That is the question people are really asking. Nothing leaves your browser.

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The schedules, and who each one suits

Longer fasts are harder, not better. The trials do not show a clear winner.
Schedule	Window	In practice
14:10	10 hours	A late breakfast and an early dinner. The gentlest version and a sensible first month.
16:8	8 hours	Skip breakfast, eat noon to eight. The default, and the one most of the research used.
18:6	6 hours	Two solid meals. Protein starts needing planning rather than luck.
20:4	4 hours	The Warrior shape. Hard to hit a protein target and easy to underfeed by accident.
OMAD	1 hour	One meal. Simple to describe, difficult to do well, and awkward around other people.

The window is not the diet. A shorter window helps because it removes a meal, not because the clock does anything to fat. Eat the same calories inside eight hours as you did across fourteen and nothing changes. This is the single most common reason fasting stops working for people.

Making it stick

Start at 14:10 and move in once it is boring. Most people who fail at 16:8 failed at it in week one, from a standing start, on a day they had already planned badly.

Protect protein above everything else. A short window makes it easy to end the day 40 g short without noticing, and on a deficit that is the difference between losing fat and losing muscle. Two meals of 40 g each is a floor worth defending.

Black coffee, tea and water are fine throughout. The hungriest stretch is usually the hour before the window opens, and it passes.

The longer version. Our guide to 16:8 intermittent fasting covers the evidence, the first two weeks and what to do when it stops working.

Fasting is one of the main levers when you are not training; losing weight without exercise covers the others.

Sources

How this was made

Written by Abdullah Javaid, founder of Calfit, drafted with AI assistance and edited by hand. The window and fasting times are clock arithmetic from your first meal and the schedule you pick. The notes on each schedule draw on the time restricted eating trials listed above.

Quick questions

What is the best intermittent fasting schedule?

The one you keep. 16:8 is the usual starting point because it mostly amounts to skipping breakfast, and the research does not show a clear winner between the popular windows. Longer fasts are harder rather than better, and past about 20 hours most people start losing protein intake, which costs muscle.

Is intermittent fasting good for weight loss?

It works when it produces a calorie deficit, and for a lot of people it does, because a shorter window removes a meal without any decision being made. Head to head against ordinary calorie restriction at matched calories, the trials mostly find no advantage. It is a tool for eating less, not a metabolic trick.

Can I drink coffee while fasting?

Black coffee, plain tea and water, yes. Milk, sugar, juice and anything with calories ends the fast in the sense that matters for insulin. A splash of milk in a coffee is a few calories and, for weight loss purposes, will not undo the day, whatever a purist says.

When should my eating window start?

Whenever it fits the life you actually live. An earlier window has a little evidence behind it for blood sugar, and a window that ends at six is useless if your family eats at eight. Adherence beats optimisation by a wide margin here.

Will fasting make me lose muscle?

Not on its own, if you get enough protein inside the window and do some resistance work. The risk goes up as the window shrinks, because getting 130 g of protein into four hours is genuinely difficult. That is the practical ceiling on how aggressive a schedule is worth going.

Is fasting safe for everyone?

No. Anyone pregnant or breastfeeding, anyone with a history of disordered eating, people with type 1 diabetes and anyone on medication timed to meals should talk to a doctor before changing when they eat. This is a scheduling tool, not medical advice.

Intermittent fasting calculator

The schedules, and who each one suits

Making it stick

Sources

How this was made

Quick questions

Related reading

16:8 intermittent fasting

Protein calculator

Calorie calculator