Body fat percentage is the share of your weight that is fat. It says more about body composition than BMI does, but the most accurate ways to measure it need specialist equipment. A tape measure gives a rougher answer that you can get at home for free.
This guide explains the tape method developed for the US Navy, works one example through by hand, and shows how to read the result. Treat the output as an estimate that is most useful for tracking change over time.
How body fat is measured
No home method is perfect. Each approach trades accuracy against cost and convenience.
| Method | How it works | Main weakness |
|---|---|---|
| DEXA scan | Low-dose X-ray separates fat, lean tissue and bone | Needs a clinic or lab; costs more |
| Skinfold calipers | Pinches of skin at set sites are measured and put into a formula | Depends heavily on the skill of the person measuring |
| Bioelectrical impedance (smart scales) | A small current passes through the body; resistance is converted to fat | Results move with hydration, food and time of day |
| Tape measure formula | Circumferences and height are put into an equation | Assumes a typical body shape, so it can miss by several points |
Taking the measurements
Use a flexible, non-stretch tape and measure in centimeters. Keep the tape level and snug without compressing the skin, and take each reading twice.
- Height: standing straight without shoes.
- Neck: just below the larynx (Adam's apple), with the tape sloping slightly down toward the front.
- Waist, men: around the abdomen at the level of the navel.
- Waist, women: at the narrowest part of the natural waist, usually above the navel.
- Hips, women only: around the widest part of the hips and buttocks.
Measure after breathing out normally, without pulling your stomach in. Even a centimeter of difference at the waist moves the result noticeably, so consistency matters more than anything else.
The formulas and a worked example
The equations use base-10 logarithms, which any scientific calculator provides (the key is labeled "log"). All lengths are in centimeters.
Men: body fat % = 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450
Women: body fat % = 495 ÷ (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450
Worked example, man: height 178 cm, neck 38 cm, waist 85 cm. The waist minus neck is 47, and log10(47) = 1.6721. Multiply by 0.19077 to get 0.3190. Next, log10(178) = 2.2504, and 0.15456 × 2.2504 = 0.3478. The bracket is 1.0324 − 0.3190 + 0.3478 = 1.0612. Then 495 ÷ 1.0612 = 466.4, and 466.4 − 450 = 16.4% body fat.
Worked example, woman: height 165 cm, neck 32 cm, waist 74 cm, hips 100 cm. The waist plus hips minus neck is 142, and log10(142) = 2.1523, so the first term is 0.35004 × 2.1523 = 0.7534. Then log10(165) = 2.2175, and 0.22100 × 2.2175 = 0.4901. The bracket is 1.29579 − 0.7534 + 0.4901 = 1.0325, so 495 ÷ 1.0325 = 479.4 and the estimate is 29.4% body fat.
To skip the math, enter the same figures in the body fat calculator. Military tape tests build on the same idea but have their own measuring rules and tables, so use the official one, such as the army body fat calculator, if you need to match that standard.
What the number means
Once you have a percentage, you can split your weight into fat and lean parts. If the man above weighs 80 kg, his fat mass is 80 × 0.164 = 13.1 kg and the rest, about 66.9 kg, is lean mass (muscle, bone, organs and water). The lean body mass calculator estimates the same split.
For context, one widely cited set of ranges comes from the American Council on Exercise. Other organizations use different bands, and healthy values vary with age.
| Category | Women | Men |
|---|---|---|
| Essential fat | 10 to 13% | 2 to 5% |
| Athletes | 14 to 20% | 6 to 13% |
| Fitness | 21 to 24% | 14 to 17% |
| Average | 25 to 31% | 18 to 24% |
| Obese | 32% and above | 25% and above |
Women carry more essential fat than men, which is why the ranges sit higher. Essential fat is the minimum needed for normal body function and is not a target to aim for.
How far to trust it
A tape-based estimate can be off by several percentage points compared with a DEXA scan, so a result of 16.4% is best read as "roughly 16%, give or take several points". The method is better at showing direction than at pinning down an exact value.
- Measure at the same time of day, ideally in the morning before eating.
- Use the same tape, the same person and the same spots each time.
- Compare tape results with tape results. Never compare them with a smart-scale number.
- Expect the formula to work less well for very muscular or very lean bodies and for people who store fat unusually.
- It is meant for adults, not children or teenagers.
See what BMI can and cannot tell you for why body fat is often a better companion measure than weight alone.
This article is general education and not medical advice. A doctor or qualified health professional can advise on what a body fat figure means for you.