Calories & Nutrition tool
Intermittent Fasting Calculator
Plan your eating window and fasting hours for 12:12, 14:10, 16:8, 18:6 or 20:4 intermittent fasting from the time of your first meal. Free, nothing stored.
Nothing you enter leaves your device.
Inputs
Results
Enter values to see your results.
Simple steps
How it works
- 01
Choose a fasting plan, such as 16:8.
- 02
Enter the time of your first meal of the day.
- 03
Select Calculate to see when your eating window ends and the fast begins.
Good to know
Frequently asked questions
What does 16:8 mean?
Sixteen hours of fasting and an eight-hour eating window each day, for example eating from noon to 8 pm.
Which plan should I start with?
Many people start with 12:12 or 14:10 and move to 16:8 if it suits them. Longer fasts are harder to keep up.
Can I drink during the fast?
Water, plain tea and black coffee are usually allowed. Drinks with calories break the fast.
Who should not fast?
It is not advised in pregnancy, while breastfeeding, under 18, with diabetes or a history of eating disorders unless a doctor agrees.
Does fasting help with weight loss?
Research suggests it works about as well as other ways of eating fewer calories. The best plan is one you can keep.
Is anything I enter stored or sent?
No. The calculation runs in your browser. Nothing you type is saved or sent to a server.
How it is calculated
Eating window = from your first meal for the plan's eating hours (12, 10, 8, 6 or 4); the fast lasts the remaining hours of the day until your next first meal. Research overview: de Cabo and Mattson (2019).
Sources
Last reviewed: October 14, 2026