Eating Window Calculator — When Yours Opens

The eating window is the block of hours in which all your food happens. Everything outside it is the fast. Enter the time of your last meal and a protocol below and the calculator returns both edges of the window — when it opens and when it shuts again.

Window length comes from the protocol

There is no separate decision to make here. Fast and window always sum to 24 hours, so choosing the protocol chooses the window.

Protocol	Eating window	Meals it comfortably holds
16/8	8 hours	2 to 3
18/6	6 hours	2, sometimes with a snack
20/4	4 hours	1 main meal, maybe a snack

Shorter is not better. It is only harder — and the trials do not reward it. The whole difference between a 6-hour and an 8-hour window is two hours of the clock, not two levels of result.

Fitting a full day of food into fewer hours

This is the most-searched follow-up question, and the answer is unglamorous: the same total you would eat anyway. Compressing the hours does not change how much energy you need, and StatPearls is explicit that the rationale for weight loss under intermittent fasting rests on caloric restriction rather than on the timing itself.

So the sequence is:

1. Work out your daily energy needs — the TDEE calculator does this.
2. Apply your goal to that number, whether that is maintenance or a deficit.
3. Split the result across the 2 or 3 meals your window holds.

The practical trap is the opposite of the one people expect. Very short windows more often produce under-eating followed by a compensatory binge than steady restriction, and they make hitting a day’s protein genuinely difficult. If you want the macro split for those meals, the intermittent fasting macro calculator does that arithmetic.

Where to put the window on the clock

Position is the flexible part. Two common placements:

• 12:00 to 20:00 — skip breakfast, keep lunch and dinner. The default for most people, because it leaves the evening meal intact.
• 08:00 to 16:00 — breakfast and lunch, fasting through the evening. Better for people who wake up hungry, worse for anyone with family dinners.

Weight-loss trials do not give a clear winner between the two. Choose around your work schedule, your training and the meals you eat with other people, because the placement you can repeat is the one that works.

Regularity over rigidity

Some consistency helps the routine settle. Rigidity does not. Moving the window two hours because of a dinner invitation is a better outcome than dropping the day entirely, and adherence is the one advantage StatPearls credits fasting with over ordinary calorie restriction.

To find the exact moment your window opens, see when to break my fast; for the full picture of the protocols and what the trials found, the intermittent fasting calculator.

This content is informational and does not replace evaluation by a physician or dietitian.