Body Water Calculator

Water is about 60% of body weight, with a wide range

Water is the largest single component of the human body, and the share depends mostly on how much fat you carry. The figure quoted in physiology texts is about 60% for a lean adult man, about 55% for an adult woman, and lower for anyone with a higher body fat percentage.

Fat is the reason for the spread. Adipose tissue is roughly 10% water by weight, while skeletal muscle is about 79%. A pound of fat brings far less water with it than a pound of muscle, so two people at the same scale weight can hold sharply different amounts of water.

The table gives the ranges that show up in physiology references for each group.

Apply the 60% rule to a 180 lb man and you get 49.0 L. The equation on this page puts the same man at 45.2 L. Both numbers are in the right neighborhood, and the gap between them is the honest uncertainty in any estimate that does not measure body composition directly.

The Watson equation this calculator uses

The estimate comes from a set of regression equations published by Watson, Watson and Batt in 1980. They pooled isotope dilution measurements from 458 adult men and 265 adult women, then fitted total body water against height, weight and age. The result is two formulas, one per sex, that predict body water in liters.

For men: 2.447 - 0.09516 x age + 0.1074 x height in cm + 0.3362 x weight in kg.

For women: -2.097 + 0.1069 x height in cm + 0.2466 x weight in kg.

Height and weight carry most of the signal, which makes sense because body water scales with body size. Age subtracts a little in the male equation because muscle mass falls and fat rises across adulthood. The female equation handles the same effect through its different coefficients rather than an explicit age term.

The equations were fitted on adults, so they do not apply to children, and they were not built for people at the extremes of body composition. A bodybuilder at 8% body fat and a person at 45% body fat will both get a number, and both numbers will be less reliable than the same equation applied to someone in the middle of the range.

Worked example: a 180 pound man at 40

Take a man who weighs 180 lb, stands 5 ft 10 in, and is 40 years old. Convert first, because the equation works in metric units.

• Weight: 180 x 0.4536 = 81.65 kg
• Height: 70 x 2.54 = 177.8 cm
• Age term: 0.09516 x 40 = 3.81
• Height term: 0.1074 x 177.8 = 19.10
• Weight term: 0.3362 x 81.65 = 27.45

Add them up with the constant: 2.447 - 3.81 + 19.10 + 27.45 = 45.19 L. As a share of body weight that is 45.19 ÷ 81.65 = 55.3%.

The same man at 70 years, with the same height and weight, comes to 42.33 L. The only thing that changed is the age term, which moved from 3.81 to 6.66, and 0.09516 x 30 = 2.85 L of water disappeared from the estimate. That is the model's way of saying that aging shifts composition toward fat even when the scale does not move.

A woman at 150 lb, 5 ft 5 in and 45 years gives a different picture: -2.097 + 0.1069 x 165.1 + 0.2466 x 68.04 = 32.33 L, which is 47.5% of her body weight. The lower share is the female equation doing its job, not a mistake in the arithmetic.

Age and sex move the percentage

Sex changes the answer twice over. Women carry more essential fat around organs and in breast tissue, which lowers the water share at any given weight, and the female equation reflects that with a smaller weight coefficient. Two people who weigh the same and are the same height will differ by several liters if they differ in sex.

Age works in one direction after about 30. Muscle mass falls by roughly 3% to 8% per decade in adults who do not train, and fat mass tends to rise, so the water share drifts down even at a stable weight. The Watson age term prices that in at about 0.095 L per year for men, which is 2.85 L over 30 years.

The columns are the same person at three assumed body compositions, and the 11 liter spread on a 200 lb frame is why a single percentage is a poor answer. The percentage matters more than the weight once you are outside the middle of the distribution.

A second estimate from body fat

If you know your body fat percentage, there is a second way to get at the same number, and it does not depend on age or sex at all. Lean mass is about 73% water, and fat mass is about 10% water, so the total is the sum of the two parts.

Total body water = lean mass x 0.73 + fat mass x 0.10.

Take the same 180 lb man at 20% body fat. Lean mass is 180 x 0.80 = 144 lb, or 65.32 kg, and fat mass is 36 lb, or 16.33 kg. The water is 65.32 x 0.73 = 47.68 L from lean tissue plus 16.33 x 0.10 = 1.63 L from fat, for 49.31 L.

That is 4.1 L more than the Watson equation gave, and the gap is informative rather than wrong. Watson's sample averaged over a population with a higher fat share than a 20% man, so its equation pulls toward the middle. When you have a real body fat measurement, the composition method uses more information and should be trusted a little further.

When you do not have one, do not guess. A wrong body fat percentage moves the answer by more than the age term does, and the Watson equation already accounts for the average composition at your age and sex.

Where the water sits: cells, blood and the spaces between

Total body water splits into two compartments. About two thirds sits inside cells as intracellular fluid, and the remaining third sits outside them as extracellular fluid, which includes blood plasma and the fluid between cells. For the 180 lb man that is roughly 30.1 L inside cells and 15.1 L outside.

The split is not decorative. Cell volume is what most of the body's chemistry happens in, and the extracellular space is the delivery route. A change in total water usually shows up first in the extracellular compartment, which is why dehydration and fluid retention both show at the surface before they show anywhere else.

Plasma is the smallest of the three extracellular parts and the one most often measured. A standard blood draw gives a plasma volume that is roughly 3 L in an adult, and that volume is what a clinician watches when fluid balance is in question. The rest of the extracellular water sits in the interstitial space, between cells, where it has no direct route back into the bloodstream except through the lymphatic system.

The tissue figures are rounded reference values from physiology tables, and they explain the whole calculation in one line: the water share of the body is a weighted average of the water share of its parts. Muscle and blood pull it up, fat and bone pull it down, and the mix is what changes with age, sex and training.

What this number is not

Total body water is a description of your body right now. It is not a drinking target, and this page does not tell you how much to drink.

The two numbers get confused because they are both in liters. A 45 L body water estimate says nothing about the 2 to 3 L you cycle through in a day, and drinking more water does not raise your total body water for long. The kidneys adjust within hours, which is why a gallon of water in the morning does not change the figure by evening.

For intake, the reference point is the Institute of Medicine's 2005 adequate intake of 3.7 L of total water a day for men and 2.7 L for women, and that total includes water from food and from every beverage, not just plain water. The CDC's own summary of those figures makes the same point. Individual needs move with activity, heat, pregnancy, illness and medication, and a clinician is the right person to set a target for any of those cases.

This is a physiology estimate, not medical advice. Total body water is measured properly by isotope dilution or bioelectrical impedance, and every equation on this page is a prediction with an error band of a few liters. Do not use it to diagnose dehydration, heart failure, kidney disease or anything else.