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Total Body Water Calculator

Work out total body water instantly with clear inputs, formula shown and shareable results.

About Total Body Water Calculator

The Watson equations predict total body water from weight, height, age and sex, and remain the clinical standard for dosing and dialysis calculations. A 40 year old man of 75 kg and 175 cm holds about 42.5 L, roughly 57% of body weight. Water is lost with age as muscle mass declines, which is why age carries a negative coefficient.

Formulas

Watson, men

TBW = 2.447 - 0.09516 x age + 0.1074 x height(cm) + 0.3362 x weight(kg)

Watson, women

TBW = -2.097 + 0.1069 x height(cm) + 0.2466 x weight(kg)

Worked example

With the default values below, Total Body Water Calculator returns Total Body Water of 42.65. Change any input above and every figure here is recalculated in your browser.

Inputs used

Body weightminimum 1 kg
75 kg
Heightminimum 100 cm
175 cm
Ageaccepts 16–100 years
40 years
Sexone of Male, Female
Male

Results

Total Body Water
42.65 L
Water As A Share Of Body Weight
56.9%
Intracellular Water
26.44 L
Approximate Plasma Volume
3.20 L

Step by step

  1. 1Values used
  2. 2Watson, men
  3. 3Watson, women
  4. 4Total Body Water= 42.65 L
  5. 5Water As A Share Of Body Weight= 56.9
  6. 6Intracellular Water= 26.44 L
  7. 7Approximate Plasma Volume= 3.20 L

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • These figures are population-average estimates, not a medical assessment or diagnosis. Body composition equations carry errors of several percentage points, and healthy ranges vary with age, ethnicity, muscularity and medical history. Discuss weight or body composition goals with a qualified clinician or dietitian.
  • This is an estimate for general information and not professional medical advice. See the full disclaimer for the limits that apply to this kind of calculator.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

These figures are population-average estimates, not a medical assessment or diagnosis. Body composition equations carry errors of several percentage points, and healthy ranges vary with age, ethnicity, muscularity and medical history. Discuss weight or body composition goals with a qualified clinician or dietitian.

Frequently Asked Questions

Why does the female equation omit age?

Because in Watson's original regression age did not add predictive value for women once height and weight were included.

What is total body water used for clinically?

Ethanol and lithium dosing, dialysis adequacy (Kt/V) and fluid replacement planning all depend on it.

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