Body Water Calculator
Total body water, percent of weight, and fluid split from sex, age, and size.
What it does
Total body water is the body's largest single compartment: every cell bathes in it and blood plasma is mostly it. The Watson equations predict its liters from sex, age, height, and weight, fitted to isotope dilution studies in hundreds of adults.
The tool above takes those four inputs in either system. It returns total liters with percent of weight and the inside versus outside cell split beside them.
The default 35 year old man at 175 cm and 75 kg holds 43.3 L at 57.7%: typical. A 30 year old woman at 165 cm and 60 kg holds 30.3 L at 50.6%, also typical.
The equations follow the published Watson dilution research with the Merck clinical calculator matching each coefficient. Both split water roughly two thirds intracellular and one third extracellular.
How to use
- Pick metric or imperial for height and weight.
- Pick sex for the matching equation.
- Enter age: adult formulas start at 18.
- Enter height and weight honestly.
- Read liters first, then the percent band.
Formula
- age
- Years; lowers the male estimate with time.
- cm / kg
- Height and weight feeding the regression.
- 2/3 - 1/3
- Inside cells versus outside split.
Imperial converts first (1 in = 2.54 cm, 1 lb = 0.45359237 kg), then runs one path, so systems agree. Lean builds read high since muscle holds more water than fat.
Worked example
Example: man 35, 175 cm, 75 kg
Metric defaults: male, 35 years, 175 cm, 75 kg.
- Age term: 2.447 - 0.09156 x 35 = -0.7576.
- Height term: 0.1074 x 175 = 18.795.
- Weight term: 0.3362 x 75 = 25.215.
- Total: -0.7576 + 18.795 + 25.215 = 43.3 L (57.7%).
- Tool shows 43.3 L, 57.7 % of weight, 28.8 L cells / 14.4 L outside.
Reference table
| Person | Water | Share |
|---|---|---|
| Man 35, 175 cm, 75 kg | 43.3 L | 57.7 % |
| Man 35, 69 in, 165 lb | 43.2 L | 57.8 % |
| Woman 30, 165 cm, 60 kg | 30.3 L | 50.6 % |
Limits
- Adults only; children need pediatric equations.
- Estimates, not measurements: dilution testing decides.
- Obesity, edema, and pregnancy shift real volumes.
- Athletes read high through muscle, not error.
Common mistakes
- Using it under 18 where formulas never validated.
- Reading percent as hydration status for the day.
- Mixing cm with the inch constants.
- Treating the split as a diagnosis.
- Dosing medicine off estimates without a clinician.
Try it yourself
Quick checks. Answers sit tucked under each question.
Woman 30, 165 cm, 60 kg?
-2.097 + 17.64 + 14.80 = 30.3 L (50.6%): typical.
Why do lean people read higher?
Muscle holds far more water per kilo than fat does.
Age 12 entered?
Rejected: enter an age between 18 and 100.
Questions
What percent of weight is water?
Typically 45 to 65% in adults: near 60% for men, near 50% for women.
What is the Watson formula?
The 1980 regression predicting liters from sex, age, height, and weight.
Inside versus outside cells?
Roughly two thirds intracellular, one third extracellular including plasma.
Does drinking change it?
Barely day to day. Total body water is a compartment size, not a thirst gauge.
Why lower with age?
Muscle gives way to fat, which carries little water.
Can I use it for kids?
No. Pediatric formulas differ; this page covers adults 18 and up.
Sources
- Europe PMC: Watson total body water volumes (1980)
- Merck Manuals: Total body water calculators (Watson)