Water Fasting Calculator: How the Estimate Is Calculated

Step 1 — Basal metabolic rate (Mifflin–St Jeor, 1990)

We estimate resting energy use with the Mifflin–St Jeor equations published in The American Journal of Clinical Nutrition (Mifflin MD, St Jeor ST, et al., 1990). Weight is in kilograms, height in centimeters, age in years:

Men: BMR = 10×weight + 6.25×height − 5×age + 5. Women: BMR = 10×weight + 6.25×height − 5×age − 161. Units are kcal/day. Sex, age, height, and weight all enter the formula—so two people of different sizes or ages do not share one “pounds per day” blog figure.

Later reviews (for example Frankenfield et al., 2005) have often ranked Mifflin–St Jeor among the more accurate common predictive equations in healthy adults; it remains an estimate, not a calorimetry measurement.

Step 2 — TDEE with a ×1.35 activity factor

Total daily energy expenditure (TDEE) ≈ BMR × 1.35. That multiplier sits between typical “sedentary” (~1.2) and “lightly active” (~1.375) conventions used in energy-requirement tables—a fixed sedentary-to-lightly-active baseline for educational fasting estimates.

We do not add workout calories. If you train hard during a fast, your real deficit may differ from the projection. The calculator footnote and exercise notes on the site state the same limit clearly.

Step 3 — Fat loss at 3,500 kcal per pound

Estimated fat loss (lb) = (TDEE × fasting days) ÷ 3,500. The 3,500 kcal ≈ 1 lb adipose figure is a long-standing clinical and educational convention associated with Wishnofsky (1958) and later popularizations; modern research notes individual variation, so we treat it as a transparent rule of thumb, not a guarantee.

During a water-only fast the model assumes near-zero dietary energy intake, so the daily deficit is approximately TDEE. Results convert to kilograms for metric display using standard lb↔kg factors.

Step 4 — Glycogen-linked water weight

Stored glycogen binds water in muscle and liver. Classic human work by Olsson and Saltin (1970) related large glycogen shifts to about 3–4 grams of water per gram of glycogen. That physiology helps explain steep early scale drops when stores empty in the first ~72 hours of fasting—and why some water often returns when you refeed carbohydrates.

Our implementation is a practical model, not a biopsy: up to about 3.5 lb (~1.6 kg) of glycogen-related water across the first three days, then roughly 0.35 lb (~0.16 kg) per additional day. Total estimated loss = fat + water. The water line is labeled separately so temporary fluid is not mistaken for adipose loss.

Worked check and limits

Example: 35-year-old woman, 70 kg, 165 cm → BMR ≈ 1,395 kcal/day, TDEE ≈ 1,884 kcal/day. A 5-day water fast projects about 3.1 kg (6.9 lb) total—roughly 1.2 kg fat and 1.9 kg water—matching the engine behind our duration and topic calculators.

Limits: no hormone-cycle or disease modeling; no refeeding-syndrome risk score; no substitute for clinical judgment. Extended fasting carries real medical risks. Read the disclaimer, and use primary sources below to verify the science yourself.