How to use the Intermittent Fasting
- Choose a protocol such as 16:8, 18:6, 20:4 or one meal a day.
- Enter the time you want your first meal.
- Read your eating window close time and the fasting period.
- Check the schedule against your training and sleep times before committing.
How the calculation works
The window is arithmetic: a 16:8 protocol starting at midday closes at 8pm, with the fast running until noon the next day. Sleep does most of the work, which is why skipping breakfast is the common entry point. Protocols differ only in window length: 16:8 is the most sustainable, 18:6 and 20:4 tighten it, and one-meal-a-day compresses everything into roughly an hour.
Evidence suggests most of the benefit comes from the calorie reduction that a shorter window tends to produce, rather than from fasting itself — trials matching calories show similar outcomes with and without time restriction. Water, black coffee and plain tea do not break a fast in any practical sense. Fasting is not appropriate for people who are pregnant, have a history of disordered eating, or take medication requiring food, and anyone on diabetes medication should consult a clinician before compressing meal timing.
Window close = first meal + eating hours ; fasting hours = 24 − eating hoursSource: Time-restricted eating protocols as described in controlled feeding trial literature.
Worked example
An 18:6 schedule with the first meal at 1:00pm, training at 5:30pm.
- Eating window = 1:00pm to 7:00pm.
- Fast runs 7:00pm to 1:00pm, 18 hours.
- Training at 5:30pm falls inside the window — good for fuelling.
- Two substantial meals fit: 1:00pm and 6:30pm.
A six-hour window from 1pm to 7pm with training fed rather than fasted.
Frequently asked questions
Does coffee break a fast?+
Black coffee and plain tea have negligible calories and do not break a fast in practical terms. Milk and sugar do.
Is fasting better than plain calorie restriction?+
Calorie-matched trials show similar results. Fasting helps mainly by making restriction easier to stick to.
Can I train fasted?+
Light and moderate sessions are usually fine. Heavy strength work generally goes better inside the eating window.
Who should avoid it?+
People who are pregnant, have a history of disordered eating, or take medication requiring food should not fast without medical advice.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.