What is body fat percentage?
Body fat percentage is the share of your total weight that is fat tissue. The rest, called lean mass, includes muscle, bone, organs and water. Two people with the same weight can have very different body fat percentages, which is why many people find it more informative than weight alone.
Some fat is essential for hormones, organ protection and energy storage. Too little or too much can both be a concern, but the right level for any one person depends on age, health, sport and genetics.
How the US Navy method works
The US Navy method estimates body fat from tape measurements instead of scales or calipers. It uses the idea that the difference between waist and neck size, together with height, tracks how much fat a person carries around the midsection. For women, the hip measurement is added because fat is stored differently.
Because it needs only a tape measure, it is easy to repeat at home. It is an estimate based on population averages, so it can miss for people with unusual builds, such as very muscular or very tall people.
How to calculate it manually
- Convert all measurements to inches (divide centimeters by 2.54).
- Take the base-10 logarithm of each term in the formula.
- Apply the men's or women's formula shown above.
- Multiply the result by your weight to get fat mass.
How to measure correctly
- Neck: measure just below the Adam's apple, with the tape sloping slightly down at the front. Do not flare your neck muscles.
- Waist: measure at the navel, breathing out normally, and do not suck in your stomach.
- Hips (women): measure around the widest part of the buttocks.
- Measure two or three times and use the average. Small errors in the waist and neck change the answer noticeably.
Body fat categories
This calculator labels results with the commonly cited American Council on Exercise (ACE) ranges. They are general reference points, not medical cut-offs.
| Category | Men | Women |
|---|---|---|
| Essential fat | 2-5% | 10-13% |
| Athletes | 6-13% | 14-20% |
| Fitness | 14-17% | 21-24% |
| Average | 18-24% | 25-31% |
| Obese range | 25%+ | 32%+ |
Healthy ranges often shift higher with age, so a single label should not be treated as a verdict.
BMI vs body fat
BMI uses only height and weight, so it cannot tell muscle from fat. The Deurenberg equation turns BMI, age and sex into a rough body fat estimate: 1.20 ร BMI + 0.23 ร age โ 10.8 ร sex โ 5.4, where sex is 1 for men and 0 for women. We show it beside the Navy result so you can compare the two.
If the two numbers are far apart, that is a hint that one method is a poor fit for your body, for example if you carry a lot of muscle. Neither is a replacement for a professional assessment.
Things that change your result
- Time of day, hydration and recent meals can shift waist measurement.
- Tape position and tightness. Always measure the same spots.
- Very muscular builds, pregnancy, and some medical conditions can make tape formulas unreliable.
- Other methods, such as DEXA scans or skinfold calipers, have their own margins of error, and none is perfect.