Body Water Percentage Calculator
Estimate body water percentage with the Watson equations for total body water (age, sex, height, and weight). Use it as a rough wellness indicator alongside BMI and body fat percentage — accurate measurement requires bioelectrical impedance analysis (BIA) or DEXA scans.
Last updated: September 2026
Formula below · 3 sources (CDC, acsm.org, Wikipedia) · Updated Sep 2026
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About this calculator
The calculator uses the Watson et al. (1980) anthropometric equations for total body water (TBW), which were fitted against deuterium/tritium dilution measurements in adults. Men: TBW (litres) = 2.447 − 0.09516 × age + 0.1074 × height_cm + 0.3362 × weight_kg. Women: TBW (litres) = −2.097 + 0.1069 × height_cm + 0.2466 × weight_kg (age did not improve the female equation). One litre of body water weighs about 1 kg, so body water percentage = TBW ÷ weight × 100. Lean tissue is about 73% water and fat tissue only about 10–15%, so for the same height a heavier person usually has a lower percentage: the extra weight is mostly fat. That is why the result falls as weight rises and is lower for women (higher average body fat). Normal ranges: men roughly 50–65% of body weight; women 45–60%; lean athletes can reach 65%; people with obesity often fall below 50%. Edge cases: the equations were derived from healthy adults and are less accurate for children, the elderly, athletes with very high muscle mass, and people with oedema, ascites, kidney or heart failure. The estimate reflects body build, not today's hydration: drinking a litre of water or a hard sweaty workout will not change it. For a measurement, bioelectrical impedance analysis (BIA) — used in smart scales — sends a small electrical current through the body and measures resistance; premium devices (InBody, Tanita professional) reach ±2-3%; consumer scales ±5-8%. Isotope dilution is the reference method.
How to use
Example 1 — Adult male. 30-year-old man, 175 cm, 70 kg. Enter Age = 30, Height = 175, Gender = Male, Weight = 70. TBW = 2.447 − 0.09516 × 30 + 0.1074 × 175 + 0.3362 × 70 = 2.447 − 2.855 + 18.795 + 23.534 = 41.92 litres. Percentage = 41.92 ÷ 70 × 100 ≈ 59.9%. ✓ Inside the typical 50–65% range for men. Example 2 — Adult female. 45-year-old woman, 165 cm, 65 kg. Enter 45, 165, Female, 65. TBW = −2.097 + 0.1069 × 165 + 0.2466 × 65 = −2.097 + 17.639 + 16.029 = 31.57 litres. Percentage = 31.57 ÷ 65 × 100 ≈ 48.6%. ✓ Inside the typical 45–60% range for women. For meaningful body composition tracking, use a smart scale with BIA (Withings, Garmin, Tanita) and measure consistently (same time of day, hydration state, post-bathroom).
Frequently asked questions
What is a normal body water percentage?
Population averages: adult men 50-65% of body weight; adult women 45-60% (lower due to higher average body fat percentage); infants 75-80%; elderly 45-55% (water decreases with age). Athletes typically have higher percentages (65-70%) due to high lean mass. Body composition: lean tissue is 70-75% water; adipose (fat) tissue is only 10-15% water. So leaner people have higher body water percentage even at the same weight. For tracking over time, focus on direction rather than absolute number — increasing body water percentage during a fitness program typically reflects muscle gain and fat loss; decreasing percentage in an aging adult may reflect sarcopenia (muscle loss). Day-to-day variation is normal: high-salt meals, low fluid intake, intense exercise, illness, and menstrual cycle in women all cause 1-3% fluctuations.
How accurate are smart scale BIA measurements?
Consumer smart scales (Withings, Garmin, Fitbit, RENPHO) use single-frequency bioelectrical impedance analysis (BIA), with accuracy ±5-8% vs lab gold standard. Professional multi-frequency BIA devices (InBody, Tanita professional) achieve ±2-3%. Both are influenced by: hydration status (drink water → higher reading; dehydration → lower); time since last meal (food in gut affects measurement); recent exercise (post-exercise dehydration); body position (standing vs sitting); skin moisture and temperature; menstrual cycle. For meaningful tracking, measure at the same time of day (typically first morning after bathroom), same hydration state (avoid measuring after intense exercise or large meals), and same body position. Compare trends over 4-6 weeks rather than day-to-day. DEXA (dual-energy X-ray absorptiometry) is the lab gold standard with ±1-2% accuracy but requires specialized equipment (typically $50-150 per scan at gyms or clinics offering it).
How does dehydration affect body water?
Mild dehydration (1-2% body weight loss): noticeable thirst, decreased exercise performance, cognitive impairment. 3-5% loss: significant performance decline, headache, dizziness, muscle cramping. 5-10% loss: serious risk of heat illness, organ stress, death possible at extreme levels. For a 70 kg person, 2% body weight loss = 1.4 kg = roughly 1.4 liters of fluid. Athletes can lose 1-2 liters of fluid per hour of intense exercise in hot conditions. For exercise hydration: drink 5-10 mL per kg body weight 2-4 hours before exercise; 150-250 mL every 15-20 minutes during exercise; 1.5 liters per kg lost during exercise afterwards. For everyday hydration: aim for pale yellow urine throughout the day; thirst is a late indicator of dehydration. The "drink 8 glasses of water per day" rule is oversimplified — actual needs vary 1.5-4 liters daily based on body size, activity, climate, and diet (high-water-content foods reduce drinking needs).
What are the most common mistakes people make tracking body water?
The biggest is treating any single measurement as meaningful — day-to-day variation of 1-3% is normal; track 4-week rolling averages instead. The second is measuring inconsistently across body position, time of day, and hydration state — all affect BIA readings. The third is interpreting absolute body water percentage without context; a 65% body water percentage is normal for an athletic male, low for a child, high for an elderly woman. The fourth is using cheap smart scales for clinical-grade tracking; consumer BIA has ±5-8% error, fine for tracking trends but not for diagnostic precision. The fifth is forgetting that body water percentage is affected by fat mass (lower fat = higher water percentage); body composition changes show up as opposite movements in fat percentage and water percentage. The sixth is over-hydrating to "improve" the reading; consistent moderate hydration produces better long-term health than chasing reading targets through forced water intake.
When should I not use this calculator?
Skip it for clinical purposes where precise hydration status matters (post-surgery monitoring, kidney disease management, sports performance optimization for elites) — use medical-grade hydration assessment tools. It is the wrong tool for children (the Watson equations are for adults) or very old individuals, where the estimate is less reliable. Do not use it for tracking exercise performance hydration; instead, weigh yourself before and after exercise to measure fluid loss, then drink 1.5L per kg lost. For body composition tracking, BIA on a consumer smart scale or DEXA scan gives more meaningful data than this calculator's estimate. For medical edema or fluid retention concerns (heart failure, kidney disease, pregnancy complications), see a doctor; calculator-based water-content estimates don't diagnose abnormal fluid distribution. And for athletic performance optimization, work with a sports medicine professional who can assess hydration through blood markers (urine specific gravity, plasma osmolality) and individual sweat rate testing.