This water fasting calculator gives you a fast, evidence-based estimate of how much weight you might lose during a zero-calorie water fast, and—just as importantly—how much of that is real body fat versus water you will regain. Enter your age, sex, height, weight and fast length, and the tool models your daily energy use to project a realistic range. Use it as an educational reference to set expectations, not as medical advice or a prescription to fast.
Water Fasting Calculator
Estimate how much fat you would truly lose on a water-only fast — and how much of the scale drop is temporary water.
Your details
Safety screen (tick any that apply)
The model, step by step
This calculator combines a validated metabolic equation with the pattern of weight change seen in supervised water-fasting studies. It deliberately separates lasting fat loss from temporary water and glycogen loss, because confusing the two is the single most common mistake people make when they read the scale during a fast.
1. Resting energy (BMR) — using the Mifflin–St Jeor equation:
Men: BMR = 10×kg + 6.25×cm − 5×age + 5
Women: BMR = 10×kg + 6.25×cm − 5×age − 161
2. Daily energy use (TDEE) = BMR × an activity factor (1.2–1.725). On a water-only fast your calorie intake is zero, so your daily energy deficit is approximately your whole TDEE.
3. Fat loss = accumulated deficit ÷ 7,700 kcal per kg of body fat (≈ 3,500 kcal per lb). This yields roughly 0.3–0.6 lb of true fat per day for a typical adult, matching supervised trials.
4. Water & glycogen — the body empties ~400–500 g of glycogen early in a fast, and each gram is stored with about 3 g of water. This calculator models that early loss as a front-loaded curve that reaches roughly 2–3 kg by 48 hours and then plateaus. Almost all of it returns within days of eating normally.
5. What the scale shows = fat loss + water/glycogen (+ some lean tissue). Because water dominates early, only about one-third of multi-day scale loss is fat — consistent with published reviews.
The tool shows the full worked arithmetic and an independent back-calculation with every result so you can check it yourself. Estimates are for healthy, non-pregnant adults and are not valid for children, pregnancy, or people with the conditions listed in the safety screen.
Building blocks on their own: BMR calculator · TDEE calculator · BMI calculator.
What happens over a prolonged fast
- 0–24 h: glycogen and its bound water are used first — the biggest early scale drop, little fat.
- 24–48 h: ketone production rises; the body shifts toward burning fat.
- Days 3–5: ketosis deepens; fat becomes the main fuel; supervision advised beyond 72 h.
- Days 5–8: blood ketones plateau (~4–5 mmol/L in people without diabetes); risks accumulate.
- Refeeding: reintroducing food too fast can trigger refeeding syndrome (dangerous phosphate, potassium and magnesium shifts). Break longer fasts gradually.
Who should not water fast
Water fasting is generally contraindicated, or needs specialist supervision, for people who are pregnant or breastfeeding, underweight (BMI <18.5), under 18, have type 1 or insulin-treated diabetes (ketoacidosis risk), have heart, liver or kidney disease, or have any history of an eating disorder. This is a reference point — verify against current clinical guidance for the individual.
Full disclaimer & how to report a problem
This calculator is an educational reference and decision aid provided by Clinical Tools Library. It does not provide medical advice, a diagnosis, a treatment plan or a recommendation to fast, and it is not a substitute for professional clinical judgement. All outputs are population-level estimates that depend entirely on the values you enter and will not match any individual exactly. Reference figures should be verified against current labelling and guidance. Do not begin or extend a prolonged fast without appropriate medical assessment and, where indicated, supervision.
If you believe you have experienced harm related to a health product, in the United States you can report to FDA MedWatch; in the UK use the MHRA Yellow Card scheme. To report an error in this tool, use the contact page on clinicaltoolslibrary.com.
If you feel driven to restrict food or fast to control your weight, that can be a sign of disordered eating. In the U.S. the National Alliance for Eating Disorders helpline offers confidential support and referrals.
Sources
- Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990.
- Ezpeleta M, et al. Efficacy and safety of prolonged water fasting: a narrative review of human trials. NIH/NLM.
- Persaud-Sharma D, et al. Refeeding Syndrome. StatPearls, NCBI Bookshelf (NIH).
- Stephens A, et al. Effects of prolonged water-only fasting and refeeding on markers of cardiometabolic risk. NIH.
How the water fasting calculator works
The water fasting calculator starts from your basal metabolic rate (BMR) — the energy your body uses at complete rest — using the widely validated Mifflin–St Jeor equation. It then multiplies BMR by an activity factor to approximate your total daily energy expenditure (TDEE). During a water-only fast you take in no calories, so your daily energy deficit is roughly equal to your entire TDEE. Dividing the accumulated deficit by about 7,700 kcal per kilogram of fat gives a theoretical fat-loss figure, while a separate allowance accounts for the water and glycogen you shed early on.
What the water fasting calculator estimates
The tool separates two very different things the bathroom scale blends together: true fat loss, which is slow and lasting, and water plus glycogen loss, which is fast but temporary. Muscles and the liver store roughly 500 g of glycogen, and each gram holds on to about 3 g of water, so simply emptying these fuel stores can move several kilograms in the first two days. Understanding this split is the single most useful thing the calculator does.
Want the building blocks on their own? Try our BMR calculator (Mifflin–St Jeor) and TDEE / daily calorie calculator to see the numbers this estimate is built on.
How much weight can you lose water fasting?
Across supervised human fasting studies, most people lose only about 0.3–0.6 pounds of actual body fat per day; the larger drop you see on the scale is mostly water, glycogen and a little lean tissue. A narrative review of prolonged water-fasting trials found that roughly one-third of the weight lost was fat and about two-thirds was lean mass or water. In one study, normal-weight adults lost around 6% of body weight after five days of water-only fasting — then regained it after three months of normal eating. In short: the scale moves quickly, but durable fat loss is modest.
Water fasting weight-loss estimates by duration
| Fast length | Total scale weight lost | Estimated true fat loss | Likely to return on refeeding |
|---|---|---|---|
| 1 day (24 h) | ~0.9–2 lb (0.4–0.9 kg) | ~0.3–0.6 lb | Most of it (water & glycogen) |
| 3 days (72 h) | ~3–7 lb (1.4–3 kg) | ~0.5–1.5 lb | Around 70–80% |
| 5 days | ~4–6% of body weight | ~1.5–3 lb | A large share, plus rehydration |
| 7+ days | Highly variable | Grows slowly; lean-mass loss rises | Water rebounds; fat stays if maintained |
Because so much early loss is fluid, expect a chunk of the number to bounce back within a week of eating normally. That rebound is rehydration and refilled fuel stores — not fat you have “regained.”
Why most early weight loss is water
When you stop eating, your body first burns through stored glycogen, releasing the water bound to it. This is why the first 48 hours often show the biggest scale drop and why that drop reverses once you refuel. Roughly 20–30% of your usual water intake also comes from food, so fasting can leave you mildly dehydrated, exaggerating the early “loss.” None of this is fat, which is exactly why a good body composition or BMI check weeks later tells you more than the scale during the fast.
Is water fasting safe? Risks and who should avoid it
In medically supervised programmes, most reported side effects are mild — hunger, headaches, nausea, dry mouth and fatigue — but more serious events have been documented, including fluid retention (edema), abnormal liver function tests, reduced bone density and metabolic acidosis. Risk rises with the length of the fast. Extended water-only fasting should be undertaken only after a clinical assessment and, ideally, with supervision.
Who should not water fast
Water fasting is generally contraindicated or requires specialist oversight for people who are pregnant or breastfeeding, are underweight or have a low BMI, have type 1 or insulin-treated diabetes (risk of ketoacidosis), have heart, liver or kidney disease, take medications that must be dosed with food, or have any history of an eating disorder. If any of these apply to you, do not use a fast for weight loss without speaking to your clinician first.
Refeeding syndrome and breaking a fast safely
Ending a longer fast is not just about eating again. After prolonged starvation, reintroducing food too quickly can trigger refeeding syndrome — potentially dangerous shifts in phosphate, potassium and magnesium that can affect the heart and nervous system. Clinical guidance (such as the NICE risk criteria) flags low BMI, significant recent weight loss and little or no intake for several days as high-risk features. Break longer fasts gradually with small, gentle meals, and seek medical advice if you have any risk factors.
Water fasting calculator FAQ
How much weight will I lose water fasting?
Expect the scale to drop faster than your fat does. Research suggests about 0.3–0.6 lb of true fat loss per day, on top of 2–3 kg of water and glycogen loss in the first 48 hours that largely returns when you eat again. A water fasting calculator estimates both parts so your expectations stay realistic.
Is there a free water fasting calculator?
Yes. This water fasting calculator is free to use — enter your details and it estimates your projected weight and fat loss instantly, with no sign-up required.
How accurate is a water fasting weight-loss calculator?
It is an estimate, not a guarantee. The model uses validated equations for BMR and daily energy use, but real results vary with your starting weight, sex, activity, hydration and metabolism. Treat the output as a realistic range to plan around, and confirm anything important with your clinician.
Is the calculator different for females?
The underlying Mifflin–St Jeor equation already adjusts for sex, so entering “female” typically yields a lower BMR and TDEE than for a male of the same size, and therefore a slightly lower projected daily loss. The fat-versus-water split works the same way for everyone.
What is the difference between water fasting and intermittent fasting?
Water fasting means zero calories for an extended stretch — often 24 hours to several days. Intermittent fasting cycles daily eating and fasting windows (for example 16:8) while you still eat every day. Intermittent fasting is gentler and easier to sustain; you can explore it with our intermittent fasting calculator.
Will I keep the weight off after a water fast?
Not all of it. The water and glycogen you lose return within days of normal eating; only the true fat loss stays, and only if you don’t overeat afterward. Studies show weight lost during fasting is often partly or fully regained without lasting dietary change.
This page is an educational reference from Clinical Tools Library. It is not medical advice, a diagnosis or a prescription. Always follow your prescriber or clinician and confirm any figures before acting on them.
Sources & further reading
- Ezpeleta M, et al. Efficacy and safety of prolonged water fasting: a narrative review of human trials. NIH/NLM (PMC).
- Persaud-Sharma D, et al. Refeeding Syndrome. StatPearls, NCBI Bookshelf (NIH).
- Stephens A, et al. Effects of prolonged water-only fasting and refeeding on markers of cardiometabolic risk. NIH (PMC).
- Myers TR, et al. Prolonged water-only fasting followed by a whole-plant-food diet for hypertension and obesity. NIH (PMC).
