Total Body Water Calculator (Watson formula)

Understanding the total body water calculator — concept and principle

This calculator estimates total body water (TBW) with the Watson formula from sex, age, height and weight. TBW is used in clinical calculations such as the free water deficit, sodium correction and drug dosing, and in dialysis to estimate urea distribution volume.

Watson formula: with the default inputs the result is 40.8 L (57% of body weight).

How it is calculated

Men: TBW (L) = 2.447 − 0.09516 × age + 0.1074 × height (cm) + 0.3362 × weight (kg)

Women: TBW (L) = −2.097 + 0.1069 × height (cm) + 0.2466 × weight (kg)

Share of body weight (%) = TBW ÷ weight × 100 (1 L of water ≈ 1 kg)

Water per kg (mL/kg) = TBW × 1,000 ÷ weight

The principle

Watson and colleagues (1980) measured body water by isotope dilution in 458 men and 265 women and fitted these simple equations. Water is mostly held in lean tissue: muscle is about three-quarters water, fat very little. Taller people of the same weight are leaner, so height raises TBW; in men, age lowers it as muscle declines. For women, age did not improve the fit.

Worked example

Default inputs: male, 45 years, 172 cm, 72 kg.

TBW = 2.447 − 0.09516 × 45 + 0.1074 × 172 + 0.3362 × 72
= 2.447 − 4.282 + 18.473 + 24.206 = 40.8 L.
Share = 40.84 ÷ 72 = 57%; water per kg = 567 mL/kg.

How to read the result

Bedside shortcuts often use 60% of weight for adult men and 50% for women, lower in older adults. The Watson formula usually gives a more individual estimate. People with more body fat have a lower share of water at the same weight.

Common mistakes

Using it in children, pregnancy, oedema or severe dehydration, where it is not valid.
Treating the result as a measured value.
Using pounds or inches without converting.

Keep in mind: This is an estimate for clinical calculations and education, not a diagnosis.

Frequently asked questions

What share of the body is water?

Typically about 50–60% in adults, lower with more body fat and with age.

Why is the women's formula different?

It was fitted separately, and age did not improve the prediction for women.

Where is TBW used?

In free water deficit, Kt/V volume estimates and some drug dosing.

Sources: Watson PE, Watson ID, Batt RD, "Total body water volumes for adult males and females estimated from simple anthropometric measurements", Am J Clin Nutr 1980;33(1):27–39

Checked 9 Nov 2026. Watson PE, Watson ID, Batt RD, “Total body water volumes for adult males and females estimated from simple anthropometric measurements”, Am J Clin Nutr 1980;33(1):27–39; coefficients as given by QxMD Calculate “Total Body Water (Watson Formula)” and the MSD Manual medical calculator “Total Body Water in Men (Watson formula)”.

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