What this tool does
Pick a protocol, when you wake, and how long you wait to eat. The calculator prints fast length, eating length, and clock times for the first and last meal window. It does not compute calories. People who skip breakfast already “fast”; naming it 16:8 does not burn extra fat by itself.
How it works
Eat start = (wake hour + offset) mod 24. Eat end = (eat start + eating hours) mod 24. Protocols are 14/10, 16/8, 18/6, and 20/4. Overnight sleep counts as fasting time. The tool will happily build a window that fights your job or training — you still have to like living it.
Formula and methodology
For protocol F-E, fastHours = F and eatHours = E. eatStart = (wakeHour + firstMealOffsetHours) mod 24; eatEnd = (eatStart + eatHours) mod 24. No metabolic equation. NEJM reviews of intermittent fasting discuss possible benefits mediated largely through intake; this page is scheduling arithmetic.
How to interpret the result
If protein cannot fit in a 4-hour window, 20:4 is a hobby that undercuts training. Early workouts may need a protocol that allows a pre-session meal — see the early-workout guide rather than forcing coffee-only lifts. Pregnancy, diabetes medications, and eating-disorder history are stop signs, not “advanced fasts.”
Example
Wake at 7:00, 16:8, first meal 4 hours later → eat from 11:00 AM to 7:00 PM. A 14:10 with a 1-hour offset eats 8:00 AM to 6:00 PM. Same TDEE either way if calories match. The 16:8 version only wins if it helps you not graze until 11.
Common mistakes
- Thinking the window erases a calorie surplus.
- Stacking 20:4 with a hard deficit and a lifting block.
- Sleeping badly because dinner is at 3 PM for no social reason.
- Fasting through a prescribed medication that must be taken with food.
When this estimate may be off
- Shift work where “wake hour” changes every day.
- You need clinical medically supervised fasting.
- Time zones and travel; mod 24 will still print a time that is locally wrong for your plan.
- You eat in the “fast” and still count it as 16:8.