Your exact eating and fasting window clock times for any IF protocol
Anchor on the time you already know
Last meal: when you STOP eating (fast starts). Answers "when does my window open?"
Time format
This calculator answers the question every intermittent-fasting tool should and most fumble: exactly what time does my eating window open and close? Pick your protocol (16:8, 18:6, 20:4, OMAD, or a custom fast), tell it the one time you already know — when you finished your last meal, when you plan your first meal, or when you wake up — and it derives the rest: your last-bite deadline, your window open, your window close, and the length of each fasting and eating block. It handles windows that cross midnight correctly, so you never see a negative or 25:00-style time. Everything is computed privately in your browser, free.
8:00 PM or 20:00 — both work.Save as link: the link encodes your protocol, anchor, and time so the calculator reopens pre-filled. It stays in your browser, but the values are visible in the URL — don't post it publicly if you'd rather keep them private.
The moment you stop eating is the start of your fast, not the start of your eating window. Those two moments are separated by your entire fasting length. If you mix them up, your whole schedule slides by the eating-window length. That's why this tool asks which fixed time you actually know and derives everything else, instead of making you guess. Also remember: 16:8 means a 16-hour fast, not 16 hours of eating — the first number is always the fast.
You finish dinner at 8:00 PM on a 16:8 protocol, anchored on Last meal:
Switch the anchor to First meal and enter 12:00 PM instead, and you get the same window — the tool also tells you the latest you should have stopped eating the day before (8:00 PM).
F = fasting hours, E = eating hours, F + E = 24
named protocols: 14:10, 16:8, 18:6, 20:4, OMAD/23:1
custom: F = your input (1..23), E = 24 - F
all arithmetic in integer minutes; wrap every result:
t = ((t mod 1440) + 1440) mod 1440 // 1440 min = 24 h
Anchor = Last meal (eating STOPS):
fastStart = anchor
windowOpen = (fastStart + F*60) mod 1440
windowClose= (windowOpen + E*60) mod 1440
Anchor = First meal (window OPENS):
windowOpen = anchor
windowClose= (windowOpen + E*60) mod 1440
fastStart = windowClose
priorStop = (windowOpen - F*60) mod 1440 // stop eating yesterday by
Anchor = Wake time:
windowOpen = (wake + gap*60) mod 1440 // gap independent of F
windowClose= (windowOpen + E*60) mod 1440
fastStart = windowClose
label any time past midnight relative to the anchor "(next day)"No account, no coding — grab the copy-paste embed code from the "Add this calculator to your site" box below the guide. See it live: view a real embed example →
Fast 16 hours, eat within an 8-hour window. The first number is always the FAST length — not the eating window. Stop eating at 8 PM and your window opens at noon the next day, closing at 8 PM.
Add the fasting hours. On 16:8, 8:00 PM + 16 h = 12:00 PM (noon) the next day; it then closes 8 hours later at 8:00 PM. Set the anchor to "Last meal" and the tool labels the cross-midnight time "(next day)".
The moment you STOP eating starts the fast; the window opens a full fasting-length later. Mixing them up shifts your whole schedule by the eating-window length, which is why the tool derives them from one known time.
No — plain water, black coffee, unsweetened tea and zero-calorie electrolytes keep you fasted. Anything with calories (cream, sugar, milk, juice — even "a little") breaks the fast.
16:8 is the most-studied, safest start; 14:10 is gentler. 20:4 and OMAD rest largely on extrapolated data and make hitting protein needs harder — ramp up gradually rather than starting aggressive.
Yes. It works in integer minutes with a modulo-1440 wrap, so times roll past midnight correctly and are labelled "(next day)" — never negative or over 24:00. A 16:8 fast from 8 PM legitimately opens at noon next day.
No — it only computes clock times. It does not say whether fasting is safe or effective for you. Talk to a doctor or dietitian first, especially with medication (insulin), a chronic condition, pregnancy/breastfeeding, or a past eating disorder.
This tool is a scheduling calculator only. It computes clock times from a chosen protocol and does not assess whether intermittent fasting is safe or effective for you, and it gives no medical, nutritional, or weight-loss advice. Times that cross midnight are correct, not errors — a 16:8 window starting at noon legitimately spans into the evening, and a last-meal-at-8 PM fast legitimately opens at noon the next day. All times are in your own local time zone as entered; the tool does not adjust for travel, shift work, or daylight-saving changes — re-anchor manually after a clock change. Window adherence matters far less than overall food quality, adequate protein, total calories, hydration, and consistency; a 2023 study found meal timing mattered less than total calories for long-term weight loss. The calculator picks WHEN; it cannot make an unbalanced diet healthy.
Disclaimer — not medical advice. Talk to a doctor or registered dietitian before starting intermittent fasting, especially if you take medication (insulin, sulfonylureas/glinides can cause dangerous hypoglycemia when fasting), are managing a chronic condition, or are pregnant or breastfeeding. Intermittent fasting is generally not recommended for people who are pregnant or breastfeeding, have type 1 diabetes, or have a current or past eating disorder — fasting windows can trigger or worsen disordered-eating patterns. Evidence is mixed: 16:8 is the most-studied and safest entry point, while 20:4 and OMAD rest largely on extrapolated/extended-fasting data. Start conservative (14:10 or 16:8) and lengthen gradually. During the fast, only zero-calorie fluids (plain water, black coffee, unsweetened tea, zero-calorie electrolytes) preserve a fasted state; anything with calories breaks the fast.
Everything is calculated in your browser. We don't store or sell your data. Scheduling estimate only — not medical advice.