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 length | Water weight | Fat loss | Total estimate | BMR / TDEE |
|---|---|---|---|---|
| 3 days | 1.6 kg | 0.9 kg | 2.5 kg | 1,724 / 2,327 kcal |
| 4 days | 1.7 kg | 1.2 kg | 3.0 kg | 1,724 / 2,327 kcal |
| 5 days | 1.9 kg | 1.5 kg | 3.4 kg | 1,724 / 2,327 kcal |
| 7 days | 2.2 kg | 2.1 kg | 4.3 kg | 1,724 / 2,327 kcal |
What happens day by day
General patterns during a water fast—useful alongside reference breakdowns.
| Phase | Hours | Water trend | Fat trend | Notes |
|---|---|---|---|---|
| Day 1 | 0–24h | Low–moderate | Minimal | Glycogen begins depleting; mild scale drop |
| Day 2 | 24–48h | Moderate–high | Low | Water loss accelerates as glycogen falls |
| Day 3 | 48–72h | Peak | Moderate | Most glycogen gone; largest water-weight window |
| Days 4–5 | 72–120h | Tapering | Increasing share | Fat oxidation becomes a larger part of total loss |
| Days 6–7 | 120–168h | Low | Dominant | Extended 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?
Is there a water fasting calculator for women?
How much weight will I lose while water fasting?
Is this water fasting calculator free to use?
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.