Fasting Timer Calculator
Intermittent fasting windows and stages
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Fasting Timer Calculator is built for intermittent fasting windows and stages — fast, free, and private. Drop Last Meal Hour into Fasting Timer — the figure refreshes as you type. Fasting Timer ties each output to its formula. Fasting Timer works for one check or many. Fasting Timer is privacy-first: Health inputs stay offline. Fasting Timer sits in the Health section next to free online fasting timer calculator and intermittent fasting calculator timer autophagy. Fasting Timer is great for comparing scenarios. Change one value and watch. Fasting Timer stays bookmarked for next time.
What the defaults give
A 16:8 fast after a 20:00 meal breaks at 12:00 the next day.
The eating window is 8 hours, running from noon to 8 pm, and the fast repeats from 8 pm.
The formula
Fasting hours = the protocol's first number
Eating window = 24 − fasting hours
Break-fast time = (last meal hour + fasting hours) mod 24
The modulo is what wraps the clock past midnight. A 20:00 meal plus 16 hours is 36, and 36 mod 24 is 12:00.
Worked examples
16:8, last meal 20:00 → break fast at 12:00, eat until 20:00
18:6, last meal 20:00 → break fast at 14:00, eat until 20:00
20:4, last meal 18:00 → break fast at 14:00, eat until 18:00
16:8, last meal 21:00 → break fast at 13:00, eat until 21:00
24-hour fast, last meal 20:00 → next meal at 20:00 the following day
Choosing a window
The protocol decides how long you fast; the last meal decides when that fast ends. Shifting the last meal an hour later moves the whole window an hour later, which is why the same 16:8 plan can mean breakfast at 10:00 for one person and lunch at 14:00 for another.
Earlier windows tend to suit sleep and blood-sugar control, because the eating period ends several hours before bed. Later windows suit people who train in the evening or eat with family. Adherence beats theory: the window you can keep for months is the better one.
18:6 and 20:4 are not automatically better than 16:8. Longer fasts are harder to sustain and make it more difficult to hit protein and calorie needs, especially for anyone active or trying to gain muscle.
What the timer does not model
It assumes a 24-hour clock and a single last-meal time. The 36-hour option runs past a full day, so the eating-window figure no longer describes a same-day schedule. Water, black coffee and unsweetened tea are generally considered compatible with a fast; anything with calories is not.
This is a timing tool, not medical advice. Fasting is not appropriate during pregnancy or breastfeeding, for children and teenagers, or for anyone with a history of disordered eating. People on insulin, sulfonylureas or other glucose-lowering medication should not change meal timing without a clinician, because the risk of hypoglycaemia is real.
Frequently Asked Questions
If I finish eating at 8 pm, when can I eat on 16:8?
At noon the next day. Sixteen hours after 20:00 is 12:00, and the eight-hour eating window then runs to 20:00.
Does a later last meal shift the whole window?
Yes. On 16:8, finishing at 21:00 pushes the first meal to 13:00 and the window to 13:00–21:00.
Is 18:6 better than 16:8?
Not inherently. A longer fast is harder to sustain and can make protein and calorie targets difficult. The best protocol is the one you can keep.
What can I drink while fasting?
Water, black coffee and plain tea are the usual choices. Anything containing sugar, milk or calories breaks the fast.
Who should not fast?
Pregnant or breastfeeding women, children, teenagers, anyone with a history of disordered eating, and people on glucose-lowering medication without medical supervision.
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