Total Body Water Calculator
Estimate total body water in litres using the Watson and Hume & Weyers equations, plus the quick percentage-of-weight rule.
Total body water (Watson)
41.5 L
59.3% of body weight
Hume & Weyers estimate
39.9 L
57% of body weight
Quick estimate
42 L
60% rule of thumb
About 50-60% of adult body weight is water. Watson and Hume & Weyers are validated prediction equations used in clinical research; actual TBW depends on muscle mass, hydration and body composition.
Last updated: August 2026
How this calculator is verified
Checked by True Calculator automated test suite on
- Formula verified against a published worked example in the automated test suite
- Edge cases (zero, negative, boundary and unit-mismatch inputs) covered by unit tests
The full verification method is on our how we verify page. Found an error? Tell us and we will re-check it.
When to Use This Calculator
Total body water figures are used by clinicians to size medication doses and judge fluid balance, by researchers estimating body composition without expensive scanning, and by fitness enthusiasts comparing their hydration percentage against the typical adult band of 50-60%. On body-composition scales, the water percentage helps explain sudden weight swings — a 1 kg overnight drop is usually water, not fat. Athletes use the estimate before and after training sessions to gauge sweat losses, and dietitians reference it when advising clients on high-sodium or high-protein intakes that shift fluid balance. Because the Watson and Hume & Weyers equations were validated against isotope-dilution measurements, they remain standard references decades later. Like all equations, they assume an average build — a very muscular person will read lower than their true water volume.
How to Use This Calculator
- Step 1: Select your sex — body water differs between men and women.
- Step 2: Enter your weight in kilograms, height in centimetres and age in years.
- Step 3: Read the Watson and Hume & Weyers estimates in litres, plus the quick percentage-based rule.
- Step 4: Use the percentage-of-weight figure to compare your hydration against the typical 50-60% adult range.
Worked Example
A 30-year-old man weighing 70 kg at 170 cm gets a Watson estimate of about 41.49 litres — 2.447 minus 0.09156 × 30 plus 0.1074 × 170 plus 0.3362 × 70. That is about 59.3% of his body weight. The Hume & Weyers formula returns about 39.88 litres (56.97%), while the quick rule of 60% gives exactly 42 litres. The three estimates bracket his likely true total body water near 40-42 litres, or roughly 41,490 ml — a useful reference for hydration planning and clinical calculations.
Tips and Common Mistakes
- •Tip 1: Measure weight and height fresh each time — the estimates move with both.
- •Tip 2: Compare the Watson and Hume figures; large gaps hint the input measurements deserve a recheck.
- •Tip 3: Use the same time of day (morning, after toilet, before breakfast) for trend tracking.
- ✗Mistake 1: Treating the estimate as an exact hydration 'score' — equations are population averages.
- ✗Mistake 2: Using the result to over-hydrate; drinking beyond thirst and urine colour does not increase health.
Frequently Asked Questions
What is total body water and why does it matter?
Total body water is all the water in your body — inside cells, in blood plasma and between tissues — typically 50-60% of adult body weight. It matters because it determines medication dosing, fluid needs and how dehydration affects you.
Which formulas does this calculator use?
It uses the Watson (1980) equations and the Hume & Weyers (1971) equations, both validated prediction formulas used in clinical studies, plus a simple rule of thumb (60% of weight for men, 50% for women). The two formulas usually agree within a litre or two.
Why do women have a lower body water percentage than men?
Men typically carry more lean mass and less fat, and muscle holds about 73% water while fat holds only about 10%. Because women on average have a higher fat percentage, their total body water as a share of weight is lower.
How does age affect total body water?
Total body water declines gradually with age as lean mass is lost and body fat rises — the Watson formula accounts for this for men through the age term. Older adults also have reduced thirst sensation, which is why dehydration is more common in the elderly.
Can I use this to plan my daily water intake?
Partially. Total body water is a snapshot of your hydration state, not a daily target. Daily intake needs depend on climate, activity and sweat — the water-intake calculator on this site is the better tool for that. Kidney patients should follow their doctor's fluid advice.
What is body water percentage used for in fitness?
Body composition scales estimate body water along with fat and muscle. Tracking water percentage over time helps spot hydration swings — a sudden drop is usually dehydration, while a rise can mean fluid retention. Day-to-day, aim to measure at the same time each morning.
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