Intermittent fasting calculator and timer
This intermittent fasting calculator shows when your eating window starts and ends and how long you fast – for 16:8, 14:10, 18:6, 20:4 and 12:12. As a timer it also tells you whether you are in your eating window or fasting right now.
Example: Fasting schedule: 16:8 (fast 16 hours, eat 8) · Start of eating window (first meal): 12:00 → Eating window: 12:00 – 20:00. Source: de Cabo R, Mattson MP (2019): Effects of intermittent fasting on health, aging, and disease. N Engl J Med 381:2541–2551. Updated: .
How it is calculated
How the intermittent fasting calculator works
With time-restricted eating all meals fall into a fixed window and the rest of the day is fasting time. The most common schedule is 16:8: 16 hours without food, 8 hours to eat. The calculator adds the length of the eating window to your start time and gives the end. Example: a first meal at 12:00 on 16:8 gives an eating window from 12:00 to 20:00 and fasting from 20:00 to 12:00 the next day.
How to use the calculator
- Choose the schedule, for example 16:8.
- Choose the time of your first meal.
- Read the eating window, fasting phase, daily schedule and the status “right now”. If you leave the page open it updates every 30 seconds.
Common schedules
| Schedule | Fasting | Eating |
|---|---|---|
| 12:12 | 12 hours | 12 hours |
| 14:10 | 14 hours | 10 hours |
| 16:8 | 16 hours | 8 hours |
| 18:6 | 18 hours | 6 hours |
| 20:4 | 20 hours | 4 hours |
What research says
Reviews such as de Cabo and Mattson (2019) describe effects of fasting on metabolism and ageing, much of it from animal studies. In a randomised trial in people with overweight (Lowe et al., 2020) 16:8 did not lead to greater weight loss than the control group. The German Nutrition Society notes that long-term data are missing and that intermittent fasting alone does not produce a more balanced food choice.
Notes
- A schedule is a timetable, not a promise: what and how much you eat in the window remains decisive.
- With pregnancy, breastfeeding, diabetes, medication, underweight or eating disorders talk to a doctor first.
- Stop if you feel dizzy, weak or unwell.
Related: daily energy needs are in the TDEE calculator, how much to drink in the water intake calculator, and the way to a goal weight in the weight loss calculator.
Frequently asked questions
What does intermittent fasting 16:8 mean?
You eat only within 8 hours and fast for the other 16. With a first meal at 12:00 your eating window would run from 12:00 to 20:00.
How long should you do intermittent fasting?
There is no established recommendation. The German Nutrition Society points to missing long-term data. Discuss the duration with your doctor.
What can you drink while fasting?
What matters is drinking enough. Water and unsweetened tea are usual; if you take medication or have a medical condition, clarify drinks and timing with your doctor.
Does intermittent fasting help you lose weight?
The evidence is mixed. One trial of 16:8 found no greater weight loss than the control group; for weight, the energy balance counts most.
Who should not do intermittent fasting?
Children and teenagers, pregnant and breastfeeding women, and people with eating disorders or underweight. With diabetes and medication tied to meals, talk to a doctor first.
Sources and legal basis
- de Cabo R, Mattson MP (2019): Effects of intermittent fasting on health, aging, and disease. N Engl J Med 381:2541–2551
- Lowe DA et al. (2020): Effects of time-restricted eating on weight loss and other metabolic parameters in women and men with overweight and obesity: the TREAT randomized clinical trial. JAMA Intern Med 180:1491–1499
- DGE: DGE: Intervallfasten (intermittent fasting) – types, state of the evidence, assessment (expert information of the German Nutrition Society)
As of: · Editorial policy
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