Intermittent Fasting Weight Loss Calculator — Free

The honest answer from controlled research is that intermittent fasting works about as well as continuous calorie restriction — no better, no worse — when total calories are matched. In a year-long randomised trial, people who combined an 8-hour eating window with a calorie limit lost about 8 kg versus 6.3 kg for calorie restriction alone — a difference that was not statistically significant (Liu et al., NEJM 2022). Reviews of the field reach the same conclusion: fasting’s benefit for weight is driven by the calorie deficit it creates, not by a unique metabolic switch (Varady et al., 2021).

What fasting does do well is make a deficit easier to keep. Compressing eating into a window tends to cut snacking, late-night eating and overall portions, so the average person eats less without tracking. Early in a fast your body also burns through stored glycogen and sheds the water bound to it — which is why the scale drops fast in week one. That early drop is real weight but mostly water, so this calculator separates it from the steadier fat-loss rate it projects.

Bottom line: if a fasting schedule helps you eat less and stick with it, it’s an excellent tool. If you eat back the calories in your window, fasting alone won’t move the scale — which is exactly what the deficit figure above is there to check.

Water fasting — consuming nothing but water for 24 hours or more — is a different and far riskier practice than the time-restricted eating this calculator models, which is why we don’t project a weight-loss number for it. People search for a “water fasting weight loss calculator” expecting a big figure, but most of the early loss on a multi-day water fast is water and glycogen that comes straight back once you eat.

More importantly, prolonged water-only fasting carries real medical risk: electrolyte imbalances, dizziness and fainting, loss of lean muscle, and refeeding syndrome — a potentially dangerous shift in fluids and minerals when you start eating again. For these reasons, water fasts beyond about 24 hours should only be undertaken with medical supervision, never from a calculator alone.

If your goal is sustainable fat loss, you’ll get there more safely — and keep more of it off — with a structured eating window (start with 16:8 above) or a moderate daily deficit. Both create the same calorie gap as a fast without the danger of going to zero.

1. Pick your fasting schedule. 16:8 is the best starting point; choose a longer window or a 5:2 plan if you prefer.
2. Add your stats. Weight, height, age, gender and activity let the calculator estimate your maintenance with the Mifflin-St Jeor equation. Already know your maintenance (TDEE)? Type it in and skip the estimate.
3. Set a goal weight (optional). Add a target and you’ll get an estimated number of weeks to reach it.
4. Fine-tune intake (optional). If you know roughly what you eat in your window — or your fast-day allowance for 5:2 — enter it for a precise projection instead of the typical-reduction estimate.
5. Read your projection. You get your estimated daily intake, the deficit fasting creates, a realistic weekly-loss range, time to your goal, and a safety check on minimum intake and pace.

Not sure of your maintenance number? Confirm it across three formulas with the TDEE Calculator, then come back.

• Don’t over-eat in your window. The single biggest reason fasting fails is compensating with bigger or richer meals. The deficit figure above is your reality check.
• Prioritise protein. Aim for around 1.6 g per kg of bodyweight to blunt hunger and protect muscle while you lose fat (Morton et al., 2018). Split your day’s calories with the Macro Calculator.
• Stay hydrated. Water, black coffee and plain tea don’t break a fast and help manage appetite.
• Keep training. Resistance work biases the loss toward fat rather than muscle.
• Judge the trend, not the day. Weigh a few mornings a week and watch the 1–2 week average — daily water swings are noise.