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Extracellular Water Estimate Calculator

Enter your age, height, weight and sex to estimate extracellular water.

How it is calculated

Total body water (TBW, litres) comes from the Watson et al. (1980) adult equation. For men: TBW = 2.447 − 0.09516 × age + 0.1074 × height + 0.3362 × weight. For women: TBW = −2.097 + 0.1069 × height + 0.2466 × weight, which does not use age. Age is in years, height in cm, weight in kg. The tool then assigns a flat 40 % of TBW to extracellular water and the remaining 60 % to intracellular water: ECW = 0.40 × TBW, ICW = TBW − ECW. It also shows ECW as a percentage of body weight. Inputs outside human limits give no result: age above 130, height outside 50–272 cm, weight outside 2–650 kg. Any sex value other than female is treated as male. The 40/60 split is a fixed average and does not reflect swelling, illness or dehydration.

Worked example

A 40-year-old man, 178 cm, 82 kg. TBW = 2.447 − 0.09516 × 40 + 0.1074 × 178 + 0.3362 × 82 = 2.447 − 3.806 + 19.117 + 27.568 = 45.33 L. ECW = 0.40 × 45.33 = 18.13 L. ICW = 45.33 − 18.13 = 27.20 L. ECW as a share of weight: 18.13 ÷ 82 × 100 = 22.11%.

Try this example →

Common mistakes

  • Expecting the split to reveal fluid retention or oedema; the 40/60 ratio is fixed, so a swollen person and a lean one with equal TBW get the same split.
  • Entering height in metres (1.78) or weight in pounds; the equation needs centimetres and kilograms, and a height under 50 gives no result.
  • Changing age for a woman and expecting the result to move; the female equation ignores age.
  • Using the result for children or for clinical fluid management: the equation is an adult formula, and for a child it still prints numbers that are only a rough guide.

Sources

About this calculator

Total body water splits into two compartments: intracellular water (ICW), the fluid inside cells, and extracellular water (ECW), the fluid in blood plasma, lymph and the space between cells. This calculator estimates total body water using a Watson formula based on height, weight and sex (age enters the formula for men only), then splits that total into its ICW and ECW components using a fixed typical ratio of 40% extracellular and 60% intracellular, so it does not reflect individual fluid shifts.

Clinicians and dietitians track the ECW/ICW split, or the related ECW/TBW ratio, as an indicator of fluid balance and cellular health — a rising ECW ratio relative to ICW can signal fluid retention, inflammation, or reduced cell mass, which is why bioimpedance devices report it in dialysis and clinical nutrition settings, and why athletes and sports scientists watch it as a hydration and recovery marker.

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