What does “eating window start” mean on this calculator?

It is the clock time you plan to take your first calories of the day (meal, caloric drink, or anything your clinician counts as breaking a fast). Default 16:8 with start at 12:00 PM → eat until 8:00 PM, then fast ~16 h until 12:00 PM the next cycle. Start 12:00 PM + 8 h eating → stop ~8:00 PM.

16:8 fasting schedule — what times if I start lunch at noon?

16:8 = 16 h fasting, 8 h eating. Start 12:00 PM → stop eating ~8:00 PM, fast until 12:00 PM next day. Same protocol at 7:00 AM start: eat 7:00 AM–3:00 PM (good for early dinner). At noon, all presets from one start: 18:6 ends 6:00 PM, 20:4 ends 4:00 PM.

18:6 vs 16:8 — how do stop-eating times differ?

Eating window length changes, not the start you pick. Both starting at 11:00 AM: 16:8 → eat until 7:00 PM (8 h eat); 18:6 → until 5:00 PM (6 h eat, 18 h fast). 18:6 closes the kitchen earlier; total fast is 2 h longer per day.

What if my eating window crosses midnight?

Evening starters often finish the next calendar morning. Example: 16:8 starting 8:00 PM → eat until 4:00 AM (crosses midnight), then fast until 8:00 PM. The calculator flags midnight crossing so “stop eating” is not confused with same-evening times.

How is OMAD scheduled here?

OMAD-style = ~23 h fast, 1 h eating for clock math. Dinner start 6:00 PM → eat ~6:00 PM–7:00 PM, fast ~23 h until 6:00 PM next day. Real life may be one 45–90 min meal inside a looser window—shift start time to match when you actually sit down.

How does custom fasting hours work?

Choose fasting length 12–23 h/day; eating = 24 − fasting. Example: 19 h fast (5 h eat), start 12:00 PM → eat until 5:00 PM, fast 19 h. Custom mode accepts 12–23 h fast per day; for a 12 h fast / 12 h eat split, use the 12:12 preset.

Does black coffee break a fast?

This page does not judge fast purity—only clock times. Many people drink plain black coffee during the 16-hour fast block; strict definitions count any non-water intake. Follow your clinician or coach; the schedule still shows fast until 12:00 PM regardless.

How does this relate to the intermittent fasting calorie calculator?

This tool answers when to eat and fast on the clock. Our intermittent fasting calorie calculator splits your daily kcal across that window. Example: 16:8 with 8 h eating and 2,000 kcal/day → ~1,000 kcal per meal if split evenly. Typical flow: pick protocol here → set stop/start times → enter the same eating hours in the calorie calculator for kcal per meal.

Is intermittent fasting safe for everyone?

No. Avoid unsupervised IF with type 1 diabetes, pregnancy, underweight status, active eating disorders, or medications that require food. Evening 20:4 (stop ~6:00 PM from a 2:00 PM start) is still not appropriate if your care team forbids fasting. This is educational scheduling—not medical advice.

Can I align IF with morning training?

Set eating start after your session if you train fasted. Example: lift at 6:00 AM, eat from 7:00 AM on 16:8 → window 7:00 AM–3:00 PM, protein in first meal. Others train at 5:00 PM inside a noon-start 16:8 window (12:00 PM–8:00 PM). The tool does not plan workouts—only eat/fast clocks.

20:4 Warrior Diet — what does the schedule look like?

20:4 = 20 h fast, 4 h eating. Start 2:00 PM → eat 2:00 PM–6:00 PM, fast ~20 h until 2:00 PM. Fits late lunch through early dinner; most calories land in that 4 h block.

Is this calculator medical advice?

No. It adds eating hours to a start time on a 24 h loop. It does not replace diabetes education, pre-surgical fasting orders, or RDN meal plans. Use Harvard/NIH resources for background; use our calorie and macro tools for energy targets.

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16:8 · 18:6 · OMAD · clock times

Intermittent Fasting Window Calculator — Eat & Fast Times

Free intermittent fasting window calculator: pick 16:8, 18:6, 20:4, 12:12, OMAD-style, or custom fast hours, set when your eating window starts, and get stop eating and next fast break times on a daily loop.

Last updated: May 25, 2026

Presets + 12–23 h custom fast

12-hour clock · 15-minute steps

Flags windows that cross midnight

Know your kcal target? IF calorie calculator

Fasting window planner

Daily eat / fast times from your protocol and start clock

Protocol

When does your eating window start?

Hour

Minute

Eating

~8 h

Fasting

~16 h

Start eating: 12:00 PM

Stop eating: 8:00 PM

Fast until: 12:00 PM next cycle

• Eat (roughly) between 12:00 PM and 8:00 PM.
• Then fast about 16 hours until eating begins again at 12:00 PM the next cycle.
• That is about 8 hours per day for meals and snacks in one window.

Default snapshot (16:8 · eating starts 12:00 PM)

Start eating

First calories

12:00 PM

Stop eating

8 h window

8:00 PM

Break fast again

After ~16 h fast

12:00 PM

Next cycle

Quick examples (computed)

Default — 16:8 from 12:00 PM

Eat 12:00 PM–8:00 PM · fast 16 h

Early bird — 16:8 from 7:00 AM

Eat 7:00 AM–3:00 PM · fast 16 h

18:6 from 11:00 AM

Eat 11:00 AM–5:00 PM · fast 18 h

20:4 from 2:00 PM

Eat 2:00 PM–6:00 PM · fast 20 h

Late eater — 16:8 from 8:00 PM

Eat 8:00 PM–4:00 AM (+1 day) · fast 16 h

OMAD-style from 6:00 PM

Eat 6:00 PM–7:00 PM · fast 23 h

All protocols starting at 12:00 PM

Protocol	Fast	Eat	Stop eating	Midnight?
12:12	12 h	12 h	12:00 AM	Yes
16:8	16 h	8 h	8:00 PM	No
18:6	18 h	6 h	6:00 PM	No
20:4	20 h	4 h	4:00 PM	No
OMAD-style	23 h	1 h	1:00 PM	No

Protocol reference

12:12

12 h fast · 12 h eat

Gentle; equal fast and eat

16:8

16 h fast · 8 h eat

How the schedule is calculated

stop_eating = start_eating + eating_hours (mod 24 h)

fasting_hours = 24 − eating_hours (presets) or your custom fast length

next_meal = same clock as start_eating after the fast block

Default: Start 12:00 PM + 8 h eating → stop ~8:00 PM. Then fast ~16 h until 12:00 PM.

Pair with calories

This tool answers when to eat and fast on the clock. Our intermittent fasting calorie calculator splits your daily kcal across that window.

Midnight crossing

Evening windows that cross midnight are normal—the stop-eating time may read as early morning on the calendar.

Limitations

Clock math only—does not track calories, macros, or whether coffee “breaks” your fast
Assumes the same start time every day; shift work and travel need manual adjustment
OMAD modeled as 23 h fast + 1 h eating for timing; real single meals may be looser
Custom fast length must be 12–23 h per day; eating hours = 24 − fasting
Diabetes on insulin, pregnancy, eating disorders, and some medications need clinician supervision—not DIY IF

Harvard T.H. Chan — intermittent fasting overview

Worked example — 20:4 from 2:00 PM

Warrior-style window

Four-hour eating block

Protocol: 20:4 → 20 h fast, 4 h eat
Start eating: 2:00 PM
Start 2:00 PM + 4 h eating → stop ~6:00 PM
Fast ~20 h until 2:00 PM next cycle
Split 2,000 kcal in that window → ~500 kcal/h or 1,000 kcal × 2 meals in the IF calorie calculator

Frequently Asked Questions

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16:8 · 18:6 · OMAD · clock times

Intermittent Fasting Window Calculator — Eat & Fast Times

Last updated: May 25, 2026

Presets + 12–23 h custom fast

12-hour clock · 15-minute steps

Flags windows that cross midnight

Know your kcal target? IF calorie calculator

Fasting window planner

Daily eat / fast times from your protocol and start clock

Protocol

When does your eating window start?

Hour

Minute

Eating

~8 h

Fasting

~16 h

Start eating: 12:00 PM

Stop eating: 8:00 PM

Fast until: 12:00 PM next cycle

• Eat (roughly) between 12:00 PM and 8:00 PM.
• Then fast about 16 hours until eating begins again at 12:00 PM the next cycle.
• That is about 8 hours per day for meals and snacks in one window.

Default snapshot (16:8 · eating starts 12:00 PM)

Start eating

First calories

12:00 PM

Stop eating

8 h window

8:00 PM

Break fast again

After ~16 h fast

12:00 PM

Next cycle

Quick examples (computed)

Default — 16:8 from 12:00 PM

Eat 12:00 PM–8:00 PM · fast 16 h

Early bird — 16:8 from 7:00 AM

Eat 7:00 AM–3:00 PM · fast 16 h

18:6 from 11:00 AM

Eat 11:00 AM–5:00 PM · fast 18 h

20:4 from 2:00 PM

Eat 2:00 PM–6:00 PM · fast 20 h

Late eater — 16:8 from 8:00 PM

Eat 8:00 PM–4:00 AM (+1 day) · fast 16 h

OMAD-style from 6:00 PM

Eat 6:00 PM–7:00 PM · fast 23 h

All protocols starting at 12:00 PM

Protocol	Fast	Eat	Stop eating	Midnight?
12:12	12 h	12 h	12:00 AM	Yes
16:8	16 h	8 h	8:00 PM	No
18:6	18 h	6 h	6:00 PM	No
20:4	20 h	4 h	4:00 PM	No
OMAD-style	23 h	1 h	1:00 PM	No

Protocol reference

12:12

12 h fast · 12 h eat

Gentle; equal fast and eat

16:8

16 h fast · 8 h eat

How the schedule is calculated

stop_eating = start_eating + eating_hours (mod 24 h)

fasting_hours = 24 − eating_hours (presets) or your custom fast length

next_meal = same clock as start_eating after the fast block

Default: Start 12:00 PM + 8 h eating → stop ~8:00 PM. Then fast ~16 h until 12:00 PM.

Pair with calories

This tool answers when to eat and fast on the clock. Our intermittent fasting calorie calculator splits your daily kcal across that window.

Midnight crossing

Evening windows that cross midnight are normal—the stop-eating time may read as early morning on the calendar.

Limitations

Clock math only—does not track calories, macros, or whether coffee “breaks” your fast
Assumes the same start time every day; shift work and travel need manual adjustment
OMAD modeled as 23 h fast + 1 h eating for timing; real single meals may be looser
Custom fast length must be 12–23 h per day; eating hours = 24 − fasting
Diabetes on insulin, pregnancy, eating disorders, and some medications need clinician supervision—not DIY IF

Harvard T.H. Chan — intermittent fasting overview

Worked example — 20:4 from 2:00 PM

Warrior-style window

Four-hour eating block

Protocol: 20:4 → 20 h fast, 4 h eat
Start eating: 2:00 PM
Start 2:00 PM + 4 h eating → stop ~6:00 PM
Fast ~20 h until 2:00 PM next cycle
Split 2,000 kcal in that window → ~500 kcal/h or 1,000 kcal × 2 meals in the IF calorie calculator