FitCalc

Intermittent Fasting Calculator

Eating window and fasting schedule for 16:8, 18:6, 20:4 and 14:10 plans.

Quick answer: The 16:8 plan fasts for 16 hours and eats within an 8-hour window. If your last meal is 20:00, the eating window runs roughly 12:00–20:00 the next day. Fasting itself does not cause fat loss — the calories you eat still decide.

HH:MM

24-hour clock, e.g. 20:00.

16 h fast, 8 h eating window — the most common

Fasting window
16 hours
Eating window
8 hours
Last meal20:00
Fast begins20:00
Eating window12:00 – 20:00
Next fast begins20:00

Common fasting protocols

PlanFastEating window
14:1014 h10 h
16:816 h8 h
18:618 h6 h
20:420 h4 h
5:22 low-calorie days/week5 normal days

Intermittent fasting is an eating schedule, not a diet — total calorie and protein intake still decide results. It suits some people and not others; those who are pregnant, underweight, or managing diabetes or an eating disorder should speak to a healthcare professional first. Not medical advice.

Core facts
Popular plan16:8
Eating window8 hours
Popular start14:10
Key pointCalories still decide results

What is Intermittent Fasting Calculator?

Intermittent fasting is an eating pattern that alternates periods of fasting with an eating window. The most common protocol is 16:8 — sixteen hours fasting, eight hours eating — with 14:10 as a gentler starting point and 18:6 or 20:4 for those who prefer longer fasts. This calculator shows exactly when your window opens and closes given the time of your last meal, so you can plan meals and training around it. What fasting is not is a metabolic shortcut: the evidence indicates it produces the same fat loss as any other approach at equal calories. Its real value for many people is behavioural — fewer eating occasions means fewer chances to overeat, which makes a calorie deficit easier to hold.

Common Uses for Intermittent Fasting Calculator

  • Plan a 16:8 or 18:6 eating schedule
  • Find your daily eating window from a last-meal time
  • Try a gentler 14:10 plan as a starting point
  • Structure meals around training while fasting

Frequently Asked Questions

Does intermittent fasting cause weight loss?
It causes weight loss only if it produces a calorie deficit. Studies comparing fasting with conventional calorie restriction at equal intake show similar results. Fasting helps adherence for some people, not metabolism.
What can I drink while fasting?
Water, plain tea and black coffee have essentially no calories and do not break a fast for most purposes. Anything with calories — milk, sugar, juice, cream — does.
Is 16:8 safe for everyone?
No. People who are pregnant, underweight, or managing diabetes, or who have a history of eating disorders, should not start fasting without medical advice. Even healthy people can feel fatigued or irritable at first.
Does fasting burn more fat?
Extended fasting does increase fat oxidation during the fast, but over 24 hours the total fat balance depends on overall energy balance. Fasting changes when you burn fat, not how much, at equal calories.

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