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Intermittent Fasting Calculator

By · Last reviewed · How we write this

Plan your fasting schedule: choose a method and when you would like to eat, and this shows your exact windows. It also answers the question most fasting calculators skip — whether the clock itself does anything, or whether it simply helps you eat less.

Fasting methods compared Longer fasts leave a shorter eating window. Adherence generally falls as the window narrows, which matters more than the schedule itself. Fasting methods compared Hours spent fasting each day 12:12 (beginner) 12 h fast · 12 h eating 16:8 (most common) 16 h fast · 8 h eating 18:6 18 h fast · 6 h eating 20:4 (Warrior) 20 h fast · 4 h eating OMAD 23 h fast · 1 h eating
Longer fasts leave a shorter eating window. Adherence generally falls as the window narrows, which matters more than the schedule itself.

How intermittent fasting works

Intermittent fasting divides the day into an eating window and a fasting window. The common 16:8 pattern means 16 hours without calories and an 8-hour window — for instance eating between noon and 8 pm. During the fast you have water, black coffee, or plain tea.

Note that a substantial part of any fasting window is spent asleep. A 16-hour fast for someone sleeping eight hours means skipping breakfast and nothing more dramatic, which is part of why 16:8 is the most widely adopted version.

Common methods

Does it actually do anything special?

Here is the honest summary. When total calories and protein are matched, trials comparing intermittent fasting to conventional continuous calorie restriction generally find similar results for weight and fat loss. The clock is not doing independent metabolic work.

What fasting does reliably is make eating less easier for some people. A narrower window naturally limits opportunities to eat, removes decisions, and suits anyone who prefers fewer, larger meals to constant small ones. That is a genuine behavioural benefit and a perfectly good reason to do it — it is just not the mechanism usually advertised.

Research reviewed in the New England Journal of Medicine has explored broader metabolic effects of fasting — on insulin sensitivity, cellular repair processes, and markers of ageing — much of it from animal models or short-term human studies. It is a legitimately interesting area. It is also some distance from established human clinical benefit, and considerably further from the claims made for it in marketing.

Who it suits, and who it does not

It tends to work well for people who are not hungry in the morning, prefer larger meals, have predictable schedules, and find rules simpler than moderation.

It tends to work badly for people who train hard early in the day, need consistent fuelling, or find that restriction triggers overeating later. It is also worth noticing whether a narrow window pushes you toward eating past comfortable fullness — for some people the compressed window produces binge-like meals that undo the intended deficit.

Do not use intermittent fasting, or speak to a doctor first, if you are pregnant or breastfeeding, under 18, have a current or past eating disorder, have type 1 diabetes or take insulin or sulfonylureas, have a history of hypoglycaemia, or take medication that must be given with food. These are not hypothetical cautions — fasting while on glucose-lowering medication carries a real risk of hypoglycaemia.

Making it work

Frequently asked questions

What can I drink while fasting?

Water, black coffee, plain tea and other zero-calorie drinks. Anything with meaningful calories — milk, sugar, juice — ends the fast. Small amounts of milk in coffee will not undo the benefits for most people, though purists disagree.

Does intermittent fasting burn more fat than other diets?

Not when calories and protein are matched. Trials comparing it with continuous calorie restriction generally find similar results. Its real advantage is behavioural: for some people a narrower window makes eating less considerably easier.

Is 16:8 good for beginners?

It is the most sustainable common starting point, and for most people amounts to skipping breakfast. If that feels like a jump, 12:12 or 14:10 first is sensible — narrowing gradually has better adherence than starting aggressively.

Will intermittent fasting cause muscle loss?

Not inherently. Muscle loss during any diet comes mainly from too large a deficit, insufficient protein, or no resistance training — not from meal timing. The practical risk is that a narrow window makes hitting your protein target harder.

Who should avoid intermittent fasting?

Anyone pregnant or breastfeeding, under 18, with a current or past eating disorder, with type 1 diabetes or taking insulin or sulfonylureas, with a history of hypoglycaemia, or on medication requiring food. Speak to a doctor before starting if any of these apply.

References

  1. Effects of Intermittent Fasting on Health, Aging, and Disease — de Cabo R, Mattson MP — New England Journal of Medicine (2019)
  2. Quantification of the effect of energy imbalance on bodyweight — Hall KD, et al. — The Lancet (2011)
  3. International Society of Sports Nutrition Position Stand: protein and exercise — Journal of the International Society of Sports Nutrition (2017)
  4. Treatment for Overweight & Obesity — National Institute of Diabetes and Digestive and Kidney Diseases (NIH)

Links open on the publisher's own site. We check every reference still resolves before each deploy.

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