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Age-adjusted BMR

Water Fasting Calculator by Age — Free

Metabolism slows with age. The Mifflin-St Jeor BMR equation includes an age term, so a 25-year-old and a 60-year-old of the same size receive different fat-loss projections.

Enter your age along with sex, height, and weight. Choose any fast length and click Calculate.

Age affects fat-loss estimates more than the glycogen water model—early water weight patterns are similar across ages, but daily calorie burn is not.

Mifflin-St Jeor

Industry-standard BMR formula with age built in.

Female & male

Toggle gender for the correct equation constant.

Fill in your details and click Calculate Weight Loss to see your estimated breakdown.

What to know before you start

How age changes your fasting estimate

Resting metabolic rate typically declines with age—roughly 1–2% per decade in many adults, though individual variation is large. The Mifflin-St Jeor equation captures this with a negative age coefficient: each year reduces calculated BMR slightly, which flows through to TDEE and therefore to projected fat loss over any fast length.

What age does not change much is the glycogen water model. The first 72 hours of carbohydrate depletion follow similar physiology regardless of whether you are 30 or 65. That is why older adults may see a similar early scale drop but slower fat-loss accumulation on days 4–7 compared with a younger person of the same weight.

Older adults and extended fasting safety

Older adults face higher risks during prolonged fasting: muscle loss (sarcopenia), orthostatic hypotension, medication interactions, and refeeding complications. Many geriatric and primary-care guidelines emphasize protein intake and caution with voluntary long fasts without supervision.

Use this age-adjusted calculator to understand how metabolism affects estimates—not to push longer fasts. If you are over 60, on multiple medications, or have any chronic condition, treat extended water fasting as a medical decision, not a DIY experiment.

Water Fasting Weight Loss Reference Table

Sample estimates for a 35-year-old male, 175 cm, 80 kg. Your results will differ—use the calculator for personalized numbers.

Fast lengthWater weightFat lossTotal estimate
3 days1.6 kg0.9 kg2.5 kg
4 days1.7 kg1.2 kg3.0 kg
5 days1.9 kg1.5 kg3.4 kg
7 days2.2 kg2.1 kg4.3 kg

What happens day by day

General patterns during a water fast—useful alongside reference breakdowns.

PhaseHoursWater trendFat trendNotes
Day 10–24hLow–moderateMinimalGlycogen begins depleting; mild scale drop
Day 224–48hModerate–highLowWater loss accelerates as glycogen falls
Day 348–72hPeakModerateMost glycogen gone; largest water-weight window
Days 4–572–120hTaperingIncreasing shareFat oxidation becomes a larger part of total loss
Days 6–7120–168hLowDominantExtended fast; medical oversight often advised

Estimates use Mifflin-St Jeor BMR, TDEE × 1.35, 3,500 cal/lb fat conversion, and glycogen water modeling. Not medical advice.

Common questions

Answers for this fast length—see the main calculator FAQ for the full FAQ.

Does age affect water fasting weight loss?
Yes, meaningfully. Resting metabolism typically declines with age, which lowers BMR and therefore TDEE in standard formulas. This calculator embeds age directly in the Mifflin-St Jeor equation (along with sex, height, and weight), so a 55-year-old and a 25-year-old of the same size do not receive identical fat-loss projections. Age does not change the glycogen water model much by itself—the early scale drop is still driven by carbohydrate stores—but older adults often see slower fat-loss curves over the same fast length. Treat any estimate as educational, not a clinical prediction.
Is there a water fasting calculator for women?
Yes. Choose Female to apply the women’s Mifflin-St Jeor constant (−161 kcal/day in the sex term versus +5 for men at the same age, height, and weight). That adjustment reflects average differences in lean mass and resting expenditure—not hormone cycles, pregnancy, or menopause, which this tool does not model. For the most useful estimate, enter current weight and height accurately, pick your fast length (72 hours through custom days), and review both fat and water lines in the results. Anyone who is pregnant, breastfeeding, or underweight should not use fasting estimates as planning advice without a doctor.
How much weight will I lose while water fasting?
Total scale change is the sum of two very different processes: fat burned for energy and water released as glycogen stores empty. Fat loss scales with your metabolism (sex, age, height, weight) and how many days you fast; water weight can move quickly in the first 72 hours and then slow. This calculator uses the Mifflin-St Jeor BMR equation, a sedentary–lightly active TDEE multiplier (×1.35), and a glycogen water model so you can see what may be temporary versus adipose tissue—not a single “pounds per day” guess.
Is this water fasting calculator free to use?
Yes. Water Fasting Calc is free, requires no account, and runs calculations locally in your browser—your inputs are not stored on our servers. It supports metric and imperial units, separates estimated fat loss from glycogen-linked water weight, and offers presets for common fast lengths plus custom days. Results are grounded in published BMR equations and standard calorie-to-fat conversions, with clear disclaimers that outputs are estimates, not medical advice. Use it to understand mechanisms and ballpark ranges; confirm any real fasting plan with a qualified professional.

More water fasting calculators

Each tool uses the same science with a preset fast length—or browse our main calculator for all options, guides, and FAQs.

Also see: reference table, trusted resources, FAQ

Medical disclaimer · Estimates only—not medical advice.