Intermittent Fasting Time Calculator

Enter Calculation

Time of Last Meal (24-hour clock) e.g. 20 for 8:00 PM, 20.5 for 8:30 PM

Fasting Window (hours)

Eating Window (hours)

Results

Fast Ends At (24-hour clock)

12

your eating window opens here

Fasting window	16 hours
Eating window	8 hours
Eating window closes at	20 (24-hour clock)

What this calculator does

The Intermittent Fasting Time Calculator tells you exactly when your fast finishes and when your daily eating window opens and closes. Enter the time of your last meal and the length of your fasting and eating windows (for example 16 and 8 for the popular 16:8 plan), and the tool maps those hours onto a 24-hour clock so you never have to count on your fingers again.

A 16:8 schedule shown as a 16-hour fasting arc and an 8-hour eating window on a 24-hour clock.

How to use it

Pick the time of your last meal on a 24-hour clock — 20 means 8:00 PM, and you can use decimals such as 20.5 for 8:30 PM. Then enter how many hours you want to fast and how many hours your eating window should last. Common schedules are 16:8, 18:6, and 20:4. The calculator adds the fasting hours to your last meal to find when the fast ends, then adds the eating hours to find when the window closes.

The formula explained

The math is simple clock arithmetic. Fast end equals last meal plus fasting hours. Because a day only has 24 hours, the result is wrapped with a modulo-24 operation so a time past midnight rolls back into the 0–24 range. The eating window close is then fast end plus eating hours, again wrapped to 24 hours.

$$\begin{gathered} \text{Fasting Ends} = \left(\text{Last Meal} + \text{Fasting (h)}\right) \bmod 24 \\[1.5em] \text{Eating Ends} = \left(\text{Last Meal} + \text{Fasting (h)} + \text{Eating (h)}\right) \bmod 24 \end{gathered}$$

Adding the fasting hours to your last meal time gives the moment your fast ends and your eating window opens.

Worked example

Suppose your last meal was at 20:00 (8 PM) and you follow a 16:8 schedule. Fast end = \(20 + 16 = 36\), which \(36 \bmod 24 = 12\):00 noon — that's when you can eat. Eating window close = \(12 + 8 = 20\):00, so your next fast begins at 8 PM, completing a clean daily cycle.

FAQ

Do I enter AM/PM? No — use the 24-hour clock. Add 12 to any PM hour (7 PM = 19).

What if my eating window opens after midnight? The calculator wraps past midnight automatically, so a result like 2 means 2:00 AM.

Is intermittent fasting right for everyone? No. This tool is for scheduling only. Consult a doctor before starting fasting, especially if you are pregnant, diabetic, or have a medical condition.