Intermittent Fasting Window Calculator

How it is calculated

The tool is pure clock arithmetic. eating window = 24 − fasting hours, and last meal = (first meal hour + eating window) mod 24, written as hh:mm. First meal hour is on a 24-hour clock and may be decimal, so 9.5 means 09:30, and must be at least 0 and below 24: 23.5 is 23:30 and works (with a 16-hour fast the last meal is 07:30), while 24 gives no result. Fasting hours must be above 0 and below 24 and may be decimal (16.5 is fine). If the eating window runs past midnight the time wraps around, so a first meal at 20:00 with 16 fasting hours ends at 04:00 the next day. The eating and fasting windows are shown to one decimal, in hours. The tool does not judge whether a fasting length suits you.

Common mistakes

• Typing the time as 12:30 or 9.30 for half past nine. The field is a decimal hour, so use 12.5 and 9.5; 9.30 means 09:18.
• Entering the eating window instead of the fasting length. For a 16:8 plan the answer needed is 16 fasting hours, not 8, otherwise the windows are swapped.
• Entering 24 as the first meal hour for midnight. Values from 0 up to just below 24 are accepted, so use 0 for midnight.
• Expecting a full 24-hour fast to work. Fasting hours must be below 24, so one-meal-a-day plans are best entered as 23 or 22.
• Missing that a late window wraps past midnight. A last meal shown as 02:00 is early the next morning, not before your first meal that day.