About the Intermittent Fasting Calculator
This intermittent fasting calculator turns a fasting protocol into a real schedule. Choose a method such as 16:8, 18:6, 20:4, 14:10, one meal a day (OMAD) or the 5:2 diet, enter the time of your first meal, and it shows exactly when your eating window opens and closes, when your fast starts and ends, and how many hours you fast each day and week.
It is built for beginners setting up their first fasting routine and for regular fasters who want to shift their window to fit work, training or family meals. The weekly table lays the plan out day by day, and for the 5:2 method it shows the low-calorie days and how much the plan cuts from your weekly calorie intake.
Time-restricted eating works for weight loss mainly by making it easier to eat less, so what and how much you eat in the window still matters. Intermittent fasting is not advised for people who are pregnant or breastfeeding, have a history of eating disorders, or take insulin or other glucose-lowering medication without medical guidance.
How to use the intermittent fasting calculator
- 1Choose a fasting method — 16:8 is the most popular starting point.
- 2Enter the time you want to eat your first meal.
- 3For 5:2, enter your maintenance calories and your low-day calorie target.
- 4Read your eating window and when to finish your last meal.
- 5Follow the weekly schedule and adjust the start time to suit your routine.
Formula and method
Time-restricted eating splits each 24-hour day into a fasting period and an eating window. The numbers in a protocol name give the hours of each: 16:8 means a 16-hour fast and an 8-hour eating window, so a first meal at noon means the window closes at 8 PM and you fast until noon the next day.
The 5:2 method works differently: you eat normally five days a week and eat around 500–600 kcal on two non-consecutive days. The weekly calorie reduction is two times the gap between your maintenance calories and your low-day intake; dividing by about 3,500 kcal per pound gives the expected weekly weight change if the other days stay at maintenance.
- eating hours
- Length of the daily eating window (e.g. 8 for 16:8)
- TDEE
- Maintenance calories per day
Worked examples
16:8 with the first meal at noon
An 8-hour window from 12:00 PM closes at 8:00 PM. You then fast for 16 hours until noon the next day — 112 hours of fasting across a week.
18:6 starting breakfast at 9:30 AM
Six eating hours from 9:30 AM end at 3:30 PM, followed by an 18-hour fast until 9:30 AM the next morning — 126 fasting hours a week.
5:2 with a 2,200 kcal TDEE and 500 kcal fast days
Each low-calorie day is 2,200 − 500 = 1,700 kcal below maintenance, so two of them cut 3,400 kcal a week — close to 1 lb (0.45 kg) of weight loss per week if the other five days stay at maintenance.
Frequently asked questions
What is the best intermittent fasting schedule for beginners?+
12:12 or 14:10 are easy starting points because most of the fast happens overnight. Once that feels comfortable, 16:8 — for example eating between noon and 8 PM — is the most widely used schedule.
What can I drink while fasting?+
Water, sparkling water, black coffee and plain tea contain little or no energy and are generally fine during the fasting window. Milk, sugar, juice and most “diet” shakes add calories and break a strict fast.
Does intermittent fasting help you lose weight?+
Studies generally find time-restricted eating produces weight loss similar to ordinary calorie restriction. It helps mainly because a shorter eating window makes it easier to eat less, not because fasting burns fat in some special way.
Does it matter what time my eating window is?+
Consistency matters most, but some research suggests earlier windows (for example 8 AM–4 PM) may be slightly better for blood sugar and sleep than late ones. Pick a window you can keep around your work and social life.
Who should not do intermittent fasting?+
People who are pregnant or breastfeeding, under 18, have a history of eating disorders, or take insulin or other glucose-lowering drugs should not fast without medical supervision, because of the risk of low blood sugar or disordered eating.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.