Fitness calculators

Body fat with a tape measure, water needs and heart rate zones. Free, no sign-up: everything is computed in your browser.

For information only. These estimates do not replace medical advice. Before starting intense exercise, or if you have heart disease or take medication (beta-blockers…), ask your doctor.

Units

You

Sex

Female Male

Age (years)

Height ()

Weight ()

For BMI, healthy weight and waist size, see the BMI calculator.

Body fat (US Navy method)

Body fat percentage estimated from tape-measure circumferences (Hodgdon and Beckett, 1984).

Neck ()

Waist ()

Hips ()

Neck: just below the Adam's apple. Waist: at the navel for men, at the narrowest point for women. Hips (women): around the widest part of the buttocks. Measure without squeezing, after breathing out, and average 2 or 3 readings.

Water needs

Daily water intake recommended by the European Food Safety Authority (EFSA, 2010), including water from food, and the part to drink.

Physical activity (minutes per day)

Situation

Hot weather

Heart rate zones

Estimated maximum heart rate and training zones, as % of max HR, or of heart rate reserve (Karvonen method) if you enter your resting heart rate.

Resting HR (optional, bpm)

Measured max HR (optional, bpm)

Max HR formula

Methods and formulas

Body fat: US Navy method

Formulas by Hodgdon and Beckett (Naval Health Research Center, 1984), measurements in centimetres:

Men: % fat = 495 / (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450
Women: % fat = 495 / (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450

The average error compared with a reference method (DEXA, underwater weighing) is 3 to 4 percentage points. The method is designed for adults. American Council on Exercise ranges: women 10–13% (essential fat), 14–20% (athletes), 21–24% (fitness), 25–31% (average), 32% and above (obese); men 2–5%, 6–13%, 14–17%, 18–24%, 25% and above.

Water needs

EFSA adequate intakes of total water (2010), drinks and food included: 1.1–1.2 L at ages 1–2, 1.3 L at 2–3, 1.6 L at 4–8, 1.9 L (girls) or 2.1 L (boys) at 9–13, then 2.0 L for women and 2.5 L for men; +0.3 L during pregnancy, +0.7 L while breastfeeding. About 80% comes from drinks and 20% from food. During exercise you typically lose 0.4–0.8 L of sweat per hour (more in the heat): drink regularly without waiting until you are thirsty. Older adults feel less thirsty and should remember to drink.

Heart rate

Estimated max HR: Tanaka 208 − 0.7 × age (most accurate on average), Fox 220 − age, Gulati 206 − 0.88 × age (women). Individual error is about ±10 beats: a max HR measured during an exercise test is more reliable.
Karvonen method: target HR = resting HR + % × (max HR − resting HR).
Zones: 1 recovery (50–60%), 2 endurance (60–70%), 3 aerobic / tempo (70–80%), 4 threshold (80–90%), 5 maximum (90–100%). The WHO recommends 150–300 minutes of moderate-intensity activity per week for adults.

Sources

Hodgdon J. A., Beckett M. B. Prediction of percent body fat for U.S. Navy men and women from body circumferences and height, Naval Health Research Center, 1984.
EFSA: Scientific Opinion on Dietary Reference Values for water, 2010.
Tanaka H. et al., J. Am. Coll. Cardiol. 2001; Gulati M. et al., Circulation 2010; Karvonen M. J. et al., 1957.
WHO guidelines on physical activity and sedentary behaviour, 2020.

Worked examples

Body fat: a man 180 cm tall, neck 38 cm, waist 85 cm gets about 16% with the US Navy formula, an average value for an adult.
Maximum heart rate at 40: 208 − 0.7 × 40 = 180 bpm (Tanaka), the same as Fox (220 − 40). The two diverge at the extremes of age.
70% zone: 126 bpm as a percentage of max HR; with a resting HR of 60 bpm and the Karvonen method, 60 + 0.7 × (180 − 60) = 144 bpm. Karvonen accounts for your fitness, hence a higher and often more accurate figure.

Training zones

Zone 1 (50–60%): recovery, brisk walking.
Zone 2 (60–70%): base endurance; you can speak in full sentences. Most endurance training happens here.
Zone 3 (70–80%): tempo; talking gets hard.
Zone 4 (80–90%): threshold, sustained effort for 20–60 minutes.
Zone 5 (90–100%): short intervals, a few minutes at most.

A maximum heart rate measured in a supervised stress test is far more reliable than formulas, which can be off by 10 bpm or more.

Measuring well

Circumferences: a soft tape, not pulled tight, at the end of a breath out, always at the same place and time of day; average 2 or 3 readings.
Resting heart rate: in the morning on waking, lying down, before getting up, over several days.
Tracking: the trend matters more than the absolute value. Measure every 2–4 weeks.

Water needs

EFSA recommends about 2 L of water a day for adult women and 2.5 L for men, food included; roughly 70–80% comes from drinks. Add more for exercise, heat, pregnancy or breastfeeding: the tool accounts for these. Urine colour (pale yellow) remains a good everyday guide.

These estimates do not replace medical advice. For BMI and waist size, see the BMI calculator.

Guide written and checked by Erwan Hautot · updated 29 October 2026.