Two people can weigh exactly the same and look completely different: one carries more muscle, the other more fat, and the scale cannot tell them apart. Body fat percentage can. It is the share of your total weight that is fat tissue, and it gives a far clearer picture of body composition than weight or BMI alone. This guide shows how to calculate it with a tape measure, with two full worked examples and what your result means.
Body fat percentage is simple arithmetic:
Body fat percentage = (fat mass / total body weight) x 100
Everything that is not fat is lean mass: muscle, bone, organs, and water. At 80 kg and 18.4% body fat, fat mass is 14.72 kg and lean mass 65.28 kg. If you lose 5 kg but only 2 kg is fat, you also lost 3 kg of lean tissue.
Body fat comes in two kinds. Essential fat is the minimum your body needs for organs, temperature, and hormones: about 2 to 5% for men and 10 to 13% for women, and dropping below it is unhealthy, not an achievement. Storage fat is everything above that: fat under the skin (subcutaneous) and fat around the organs (visceral). Visceral fat is most closely linked to insulin resistance, type 2 diabetes, and heart disease.
The most practical at-home method is the US Navy method, developed by Hodgdon and Beckett at the Naval Health Research Center in 1984 from tape measurements compared against underwater weighing. It landed within about 3.5 percentage points of the lab result: good for tracking change, not a medical verdict on one reading.
Measure in the morning, before eating, standing relaxed. Take each measurement twice and average them:
The formulas use base 10 logarithms (the log button on a phone calculator). All measurements are in centimetres:
Men: density = 1.0324 minus 0.19077 x log10(waist minus neck) plus 0.15456 x log10(height)
Women: density = 1.29579 minus 0.35004 x log10(waist plus hips minus neck) plus 0.22100 x log10(height)
Then convert density to a percentage with the Siri equation:
Body fat percentage = 495 / density minus 450
Two complete calculations, worked by hand:
Example 1, a man. Height 180 cm, neck 38 cm, waist 88 cm. Waist minus neck is 50. Log10(50) is 1.6990 and log10(180) is 2.2553. Density = 1.0324 minus 0.3241 plus 0.3486 = 1.0569. Body fat = 495 / 1.0569 minus 450 = 18.4%, in the ACE Average range for a man. At 80 kg, his fat mass is 14.72 kg and lean mass 65.28 kg.
Example 2, a woman. Height 165 cm, neck 33 cm, waist 78 cm, hips 98 cm. Waist plus hips minus neck is 143. Log10(143) is 2.1553 and log10(165) is 2.2175. Density = 1.29579 minus 0.7545 plus 0.4901 = 1.0314. Body fat = 495 / 1.0314 minus 450 = 29.9%, in the ACE Average range and inside the healthy age band for a woman in her 20s or 30s below.
The tape method is free and good for tracking. Each alternative trades cost, convenience, and precision:
| Method | How it works | Accuracy and cost |
|---|---|---|
| Tape measure (US Navy) | Circumferences plus height in a formula | About 3 to 4 points from lab results, price of a tape |
| Skinfold calipers | Pinch skinfold thickness at set sites, apply an equation | About 3 points if trained, cheap but technique dependent |
| Smart scales (BIA) | A small current estimates body water, then predicts fat | Off by 3 to 8 points, shifts with hydration and meals |
| DEXA scan | Low-dose X-ray maps fat, lean tissue, and bone by region | Most precise, but a paid clinic visit |
There is also a no-tape shortcut, the BMI method (Deurenberg formula). For a man: body fat percentage = 1.20 x BMI plus 0.23 x age minus 16.2 (for a woman, replace 16.2 with 5.4). A 30 year old man with a BMI of 24 gets 28.8 plus 6.9 minus 16.2 = 19.5%. Useful with no tape measure, but treat it as a rough estimate only.
There is no single official cut-off, so here are the two most cited systems. The American Council on Exercise (ACE) categories are the fitness framing trainers use:
| Category | Men | Women |
|---|---|---|
| Essential fat | 2 to 5% | 10 to 13% |
| Athletes | 6 to 13% | 14 to 20% |
| Fitness | 14 to 17% | 21 to 24% |
| Average | 18 to 24% | 25 to 31% |
| Obese | 25% and above | 32% and above |
But healthy ranges rise with age, as muscle mass slowly falls. Gallagher and colleagues (American Journal of Clinical Nutrition, 2000) mapped the body fat levels matching a healthy BMI, by age:
| Age | Men, healthy range | Women, healthy range |
|---|---|---|
| 20 to 39 | 8 to 20% | 21 to 33% |
| 40 to 59 | 11 to 22% | 23 to 34% |
| 60 to 79 | 13 to 25% | 24 to 36% |
A 22% reading means something different at 25 than at 65, so use your own age band, not a single number you saw online.
BMI uses only height and weight, so it cannot tell muscle from fat. A muscular person can score as overweight on BMI with low body fat, while someone with little muscle can score a normal BMI with unhealthy body fat. Use BMI for a quick screen, then body fat percentage for the composition behind it.
Your waist adds a third signal: waist-to-height ratio is waist divided by height, and keeping it under 0.5 is a widely used rule of thumb. The man in our example has 88 / 180 = 0.489, just under the threshold, and a rising waist at a stable weight warns of visceral fat gain even when body fat percentage barely moves.
Measure every 4 to 8 weeks, not every day. What matters is the trend over months, and whether lean mass holds steady while fat mass falls.
The Gallagher healthy ranges are a sensible target: roughly 8 to 20% for men aged 20 to 39 and 21 to 33% for women the same age, rising a few points in later bands. The ACE Fitness band is a common training goal.
All you need is a soft tape measure and the US Navy formula in this guide. Without a tape measure, the BMI method gives a rough estimate from height, weight, age, and sex.
They are fine for trends and unreliable for single numbers, because hydration, exercise, meals, and time of day all shift readings. Judge the average over several weeks.
BMI compares weight to height; body fat percentage compares fat mass to total weight. BMI cannot distinguish muscle from fat.
Every 4 to 8 weeks, under identical conditions (morning, before eating, same tape placement). More frequent checks mostly capture measurement noise and water fluctuation.
Yes. Below essential fat (2 to 5% for men, 10 to 13% for women), hormones, temperature regulation, and immunity suffer. If your reading approaches those levels, talk to a healthcare professional.
Our free calculators do the maths for you:
These calculators give estimates for general information only, not professional medical advice. Tape and formula estimates carry several points of error, and one reading is never a diagnosis. If a result worries you, talk to a qualified healthcare professional first.