Eating window (hours)

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General estimate for planning only — not medical advice. Consult a doctor about your specific situation.

Calculation details

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Fasting hours gained as eating window shrinks from 10 to 6 hours — Each hour cut from the window adds an hour of fasting: 8:16 down to 6:18.

Method & verification

How This Tool Works

fact-checked against our calculation methodology

Calculation Method

Fasting period = 24 - Eating window

Site methodology

BoringToolsKit methodology and verification standard

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Assumptions to Verify

These tools provide general estimates for planning and education only.
They are not medical advice and not a substitute for a qualified healthcare professional.
Always consult your doctor or another healthcare provider about your specific situation.

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Worked example: the 16:8 split, step by step

Enter 8 as your eating window. The fast is 24 - 8 = 16 hours, so you land on the classic 16:8 pattern. The example clock schedule is built by halving the window around a 12:00 midpoint: 12 + 8/2 = 16:00 start, and 12 - 8/2 = 8:00 end. Eating from 8:00 to 16:00 leaves 16 hours fasting, and the two numbers always sum to 24.

Now try an odd window, say 7 hours. The fast is 24 - 7 = 17 hours. The schedule formula produces 12 + 3.5 = 15:30 as the start and 8:30 as the end, because a 7-hour window cannot be centered on a whole hour. Fasting from 15:30 to 8:30 is exactly 17 hours, which the calculator reports as '17 hours'. One hour trimmed from the window always adds exactly one hour of fasting; there is no nonlinearity in the formula.

Note the validation boundary: a window of 24 or more is rejected because it leaves zero fasting time, and 0 is rejected because it is not an eating window at all. Only 1 through 23 produce a result.

Limits the math does not capture

The 24-hour day is an idealized cycle. A 16-hour fast measured from a 16:00 last meal assumes you actually stop eating at that minute; in practice a 20:30 finish turns a planned 16:8 into a real 14.5:9.5, since 20:30 to 12:00 the next day is only 15.5 hours. Coffee with cream or a 60-calorie snack technically ends the fast, though it barely moves total intake.

Sleep absorbs most of the fasting window, which is why a 14:10 schedule (10-hour window, for example 9:00 to 19:00) is a common starting point: only about 5 waking hours are food-free. Going from 16:8 to 18:6 removes just 2 hours of eating but does not halve calories on its own. Hunger typically peaks around hours 12 to 16 of a fast, and medications that must be taken with food can force an earlier window start, effectively shortening the fast below what you entered.

Shift workers should anchor the clock to their own wake time rather than midnight; a nurse on a 19:00 to 07:00 shift running the standard 8:00 to 16:00 window would eat only during sleep hours, defeating the schedule.