What is an intermittent fasting calculator?

An intermittent fasting calculator works out your exact eating and fasting windows once you pick a protocol and your first meal time. For example, 16:8 means 16 hours of fasting and an 8-hour eating window each day.

How do the 16:8, 18:6, 20:4, and OMAD protocols differ?

The numbers are fasting hours to eating hours: 16:8 gives an 8-hour window, 18:6 a 6-hour window, and 20:4 a 4-hour window. OMAD means one meal a day, the most restrictive option with a roughly 23-hour fast.

Which intermittent fasting schedule is best for beginners?

16:8 is the most popular starting point because the 16-hour fast often overlaps with sleep, making it easier to sustain. More aggressive protocols like 20:4 or OMAD shorten the eating window further and may suit experienced fasters better.

Intermittent Fasting Calculator

Your eating and fasting windows for any protocol

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Fasting window

16:00:00

Fasting protocol

First meal time (HH:MM)

Intermittent Fasting Calculator

Intermittent fasting cycles between eating and fasting windows. 16:8 means 16 hours fasting and an 8-hour eating window — the most popular protocol. Set your first meal time and the tool maps the exact hours; combine it with the calorie calculator for fat loss.

How to use this fasting calculator

Pick a fasting protocol — 14:10, 16:8, 18:6, 20:4 or OMAD (one meal a day) — and enter the time you want your first meal. The calculator instantly lays out your eating window and fasting window, no sign-up needed. You can also start the built-in live timer: it counts your fast in real time, survives page reloads, and chimes when the window changes.

How to read your result

Your eating window opens at your first meal and stays open for the hours your protocol allows — 8 hours on 16:8, 6 on 18:6, just 1 on OMAD. Everything after that is your fasting window, which runs until the next day's first meal. On 16:8 with a 12:00 first meal, you eat from 12:00 to 20:00 and fast from 20:00 to noon. The live timer shows time remaining in the current phase and counts completed cycles.

The science behind fasting windows

After roughly 10–12 hours without food, the liver's glycogen stores run low and the body shifts toward burning fat and producing ketones — the so-called metabolic switch. Insulin stays low during the fast, which makes stored fat easier to access. Trials show intermittent fasting delivers weight loss comparable to ordinary calorie restriction, largely because a shorter eating window means fewer calories; long-term evidence on other benefits is still emerging.

Limits and practical tips

Intermittent fasting is not for everyone: if you are pregnant or breastfeeding, take diabetes medication, or have a history of eating disorders, talk to a healthcare professional first. Start gently with 14:10 or 16:8 and drink plenty of water during the fast. Pair your schedule with our TDEE and calorie-deficit calculators to keep intake on target — and use the CaloNote app's meal logging to stay consistent inside your window.

Frequently Asked Questions

What is an intermittent fasting calculator?: An intermittent fasting calculator works out your exact eating and fasting windows once you pick a protocol and your first meal time. For example, 16:8 means 16 hours of fasting and an 8-hour eating window each day.
How do the 16:8, 18:6, 20:4, and OMAD protocols differ?: The numbers are fasting hours to eating hours: 16:8 gives an 8-hour window, 18:6 a 6-hour window, and 20:4 a 4-hour window. OMAD means one meal a day, the most restrictive option with a roughly 23-hour fast.
Which intermittent fasting schedule is best for beginners?: 16:8 is the most popular starting point because the 16-hour fast often overlaps with sleep, making it easier to sustain. More aggressive protocols like 20:4 or OMAD shorten the eating window further and may suit experienced fasters better.