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Water Fasting Calc is a free, independent educational tool that estimates how your weight may change during an extended water fast — separating temporary water weight from real fat loss.

Why we built this

Most fasting content online repeats vague "lose X pounds a day" claims. We built this calculator to give a transparent, personalised estimate instead — one that clearly separates the water weight you lose early from the body fat you actually burn over time.

How our estimates work

Every result is calculated from the details you enter using the Mifflin-St Jeor equation for basal metabolic rate (BMR), a sedentary-to-lightly-active TDEE multiplier of 1.35, a fat-loss conversion of 3,500 calories per pound, and a glycogen-based model for early water weight. The calculation runs entirely in your browser, and your inputs are never stored on our servers.

Our sources

The formulas behind the calculator are published, widely used standards in nutrition and metabolism science. On the calculator page we link to independent government, medical, and peer-reviewed research resources so you can read further and verify the science yourself.

Accuracy & editorial standards

We label every output as an estimate, avoid hype, and never present fasting as a replacement for medical care. Pages are reviewed regularly and corrected whenever something is unclear, inaccurate, or out of date. If you find a mistake, please tell us and we will fix it.

Independence & funding

Water Fasting Calc is not affiliated with any clinic, supplement brand, or diet program. The calculator is free to use. We fund the site through optional digital planners and advertising, which never influences the numbers the calculator produces.

Contact us

Questions, feedback, or a correction? Visit our contact page — we read every message.