Total Body Water (TBW) Calculator
About 60% of the human body is water — including plasma, intra- and extra-cellular fluid, and digestive secretions. Enter age, sex, height and weight to estimate total body water (TBW) in litres, the percentage of body weight, and how it compares to the typical adult reference range.
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Total body water (TBW)
42.1 L
Within typical range
% of body weight
60.2 %
Within the typical range for healthy adults. Maintain steady fluid intake (~30–35 mL per kg of body weight per day).
Estimated using the Watson formula. For general reference only — not a substitute for clinical measurement (e.g. bioimpedance).
Formula
Male: TBW = 2.447 − 0.09156·age + 0.1074·height(cm) + 0.3362·weight(kg) Female: TBW = −2.097 + 0.1069·height(cm) + 0.2466·weight(kg) % of body weight ≈ TBW(L) / weight(kg) × 100
- · Uses the Watson formula (Watson PE, Watson ID, Batt RD. Am J Clin Nutr 1980;33:27-39) — a standard TBW estimator used in dialysis dosing and pharmacokinetics.
- · Typical adult reference: men ~50–65 % of body weight (avg ~60 %), women ~45–60 % (avg ~55 %), because women on average carry more body fat and less muscle.
- · TBW % declines with age as muscle mass falls; infants are 70–80 % water by weight.
- · The percentage assumes water density ≈ 1 kg/L, so % = TBW(L) ÷ weight(kg) × 100.
- · For general / educational use only — not a substitute for clinical measurement such as bioelectrical impedance (BIA) or isotope dilution.
- · Sources: Mayo Clinic "Water: How much should you drink every day?"; USGS Water Science School; Watson et al., Am J Clin Nutr, 1980.
Frequently asked
Why does the female Watson formula have no age term?
In the original 1980 Watson study, age was a statistically significant predictor of TBW in men (older = less water) but the age coefficient was not significant for women, so it was dropped from the female regression. The female formula effectively says TBW depends mostly on height and weight for adult women across the usual age range.
What body water percentage is considered "normal"?
Healthy adult men: roughly 50–65 % (avg ~60 %). Healthy adult women: roughly 45–60 % (avg ~55 %). Body fat heavily influences this — muscle is ~70–75 % water, fat is only 10–20 % water — so higher body fat means a lower body-water percentage. Lean / athletic builds tend toward the upper end.
How does TBW relate to how much water I need to drink?
TBW is the total water reservoir in your body, not how much you need to drink each day. Daily requirement depends on losses through sweat, urine, breath and digestion — a common rule of thumb is 30–35 mL per kg of body weight (around 2.1–2.4 L for a 70 kg adult), more in heat, exercise, fever or lactation. About 20–30 % of intake typically comes from food (soup, fruit, vegetables), so you do not need to drink all of it.
How accurate is the Watson formula compared to bioelectrical impedance (BIA)?
In healthy adults the Watson formula typically estimates TBW to within about ±3–4 L (5–8 % of TBW) and has been used in clinical dialysis dosing and pharmacokinetics for over 40 years. BIA and deuterium-dilution are more accurate but require equipment. Accuracy degrades at extreme body fat, in oedema, in critical illness, or in dialysis patients — those cases call for BIA, DEXA or D₂O dilution.
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