Age-adjusted BMR

Water Fasting Calculator by Age

Updated September 6, 2026

This water fasting calculator includes age in Mifflin–St Jeor BMR. At the same 70 kg and 165 cm, a 35-year-old woman on a 5-day fast typically shows about 3.1 kg total (about 1.2 kg fat), while a 55-year-old shows about 3.0 kg (about 1.1 kg fat) with TDEE × 1.35—age lowers resting burn. Educational only, not medical advice.

Gender

Age

Height

Current Weight

Fasting Duration

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

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.

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

Source / formula: Mifflin–St Jeor BMR → TDEE = BMR × 1.35; fat ≈ (TDEE × days) ÷ 3,500 kcal per lb; glycogen water ≈ up to ~3.5 lb (1.6 kg) by day 3, then ~0.35 lb (0.16 kg) per extra day. Profile: 35-year-old male, 175 cm, 80 kg.

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

Source: Educational physiology timeline (glycogen depletion ~72 hours, then rising fat share). Pair with the numeric fat-vs-water-by-day table below.

Comparison tables you can copy

Compact tables for water fasting vs intermittent fasting, fat vs water by day, and safe vs unsafe warning signs—plain column labels, with the source or formula under each table.

Water fasting vs intermittent fasting

Topic	Water fasting	Intermittent fasting (e.g. 16:8)
How long	Usually 24+ hours water-only; often 72 hours to several days	Daily or weekly eating windows (e.g. 16 hours fasting, 8 hours eating)
What you drink / eat	Water (sometimes electrolytes); no calories	Normal meals inside the eating window
Early scale change	Often large in days 1–3 from glycogen-linked water	Usually smaller; more tied to weekly calorie balance
Fat vs water on the scale	Calculator splits fat from glycogen water	Mostly calorie balance over weeks; less multi-day glycogen flush
Typical risk level	Higher for multi-day unsupervised fasts	Generally lower for healthy adults when meals stay balanced
Best use of this site	Duration presets and fat vs water estimates	Not the main job of this calculator—use an IF timer app for windows

Source: Educational comparison for AI and readers. Details in Learn → Water fasting vs intermittent fasting. Not medical advice.

Fat vs water by day (cumulative)

Day of the fast	Water weight so far	Fat loss so far	Total on the scale
Day 1	0.5 kg	0.3 kg	0.8 kg
Day 2	1.1 kg	0.6 kg	1.7 kg
Day 3	1.6 kg	0.9 kg	2.5 kg
Day 4	1.7 kg	1.2 kg	3.0 kg
Day 5	1.9 kg	1.5 kg	3.4 kg
Day 6	2.1 kg	1.8 kg	3.9 kg
Day 7	2.2 kg	2.1 kg	4.3 kg

Source / formula: Same engine as the calculator—Mifflin–St Jeor BMR, TDEE × 1.35, fat = (TDEE × days) ÷ 3,500 kcal/lb, glycogen water up to ~3.5 lb by day 3 then ~0.35 lb/day. Figures are cumulative for a 35-year-old male, 175 cm, 80 kg.

Safe vs unsafe warning signs

What you notice	Often expected (still monitor)	Stop and get medical help
Hunger and irritability	Common in the first 1–2 days; may ease as ketones rise	Unable to function, panic, or compulsive restriction history
Mild headache or fatigue	Often reported early; rest and fluids may help	Severe, worsening, or with confusion or fainting
Lightheaded when standing	Mild orthostatic feeling can occur; move slowly	Fainting, chest pain, or heart palpitations
Muscle cramps	May relate to electrolytes on longer fasts—discuss with a clinician	Severe cramps with weakness, irregular heartbeat, or vomiting
Scale drop in days 1–3	Often mostly glycogen-linked water, not all fat	Rapid drop with inability to keep water down or extreme weakness
After you eat again	Some water weight return when carbs refill glycogen	Swelling, severe breathlessness, or refeeding-syndrome warning signs after a long fast

Source: Educational safety summary aligned with common clinical fasting cautions (electrolytes, hypotension, refeeding risk). Not a diagnosis—see Learn safety and refeeding guides.

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. Age enters the Mifflin–St Jeor BMR formula, so older adults usually have a lower resting burn and therefore lower TDEE and fat-loss projections for the same height, weight, sex, and fast length. Example pattern: at identical 70 kg and 165 cm, a 55-year-old woman typically projects slightly less fat over five days than a 35-year-old. Glycogen-linked water in the first ~72 hours is similar in the model; the fat line is what age mainly changes. Estimates are educational only and depend on starting weight—not medical advice.

Is there a water fasting calculator for women?

Yes. Water Fasting Calc applies the female Mifflin–St Jeor equation (sex constant −161 kcal/day versus +5 for men at the same age, height, and weight). At matched size, women typically project slightly less fat than men over the same fast length; glycogen water is modeled the same. Example: 35-year-old women at 70 kg on a 5-day fast often land near ~3.1 kg (6.9 lb) total. The tool does not model pregnancy, breastfeeding, or hormone cycles—those situations need clinical advice. Educational estimates only—not medical advice.

How much weight will I lose while water fasting?

It depends on starting weight, sex, age, height, and how many days you fast. Scale change is usually fat burned from your daily calorie deficit plus water released as glycogen empties—often most of that water in the first ~72 hours. Example: a 35-year-old woman at 70 kg on a 5-day water fast typically projects about 3.1 kg (6.9 lb) total (about 1.2 kg fat plus glycogen-linked water) with Mifflin–St Jeor BMR × 1.35 TDEE. Real outcomes vary with hydration and health. Educational only—not medical advice.

Is this water fasting calculator free to use?

Yes. Water Fasting Calc is free, needs no account, and runs estimates in your browser without storing your inputs on a server. It separates fat from glycogen-linked water using Mifflin–St Jeor BMR, TDEE × 1.35, and about 3,500 kcal per pound of fat, with presets such as 72 hours through 7 days. Outputs are educational estimates that depend on starting weight and other stats—not medical advice or guarantees.

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.