ReckonBoxHow this calculator works
Body fat percentage is the share of your weight that is fat rather than muscle, bone, and water. It tells you something the bathroom scale cannot: whether the weight you carry is lean or not. This tool uses the US Navy tape method, which estimates body fat from a few circumference measurements and your height. It needs no calipers and no scan.
Measure your neck below the larynx, your waist at the navel, and, for women, your hips at the widest point. The calculator feeds those into the Navy equation, then reports your category, your fat mass, your lean mass, and your fat-free mass index.
For a worked example, a man 183 cm tall with a 38 cm neck and a 92 cm waist comes out near 20.7 percent, which sits in the average range. Trim the waist to 85 cm and the estimate drops toward 16 percent, in the fitness range. A single centimetre at the waist moves the number, so measure twice.
The US Navy formula
The method uses the base-10 logarithm of your measurements, in centimetres.
- Men:
%BF = 495 / (1.0324 - 0.19077 x log10(waist - neck) + 0.15456 x log10(height)) - 450 - Women:
%BF = 495 / (1.29579 - 0.35004 x log10(waist + hip - neck) + 0.22100 x log10(height)) - 450
Women add the hip measurement because fat distributes differently, and the extra term keeps the estimate honest.
Body fat categories
The American Council on Exercise classification is the common standard, and it splits by sex because women carry more essential fat.
| 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 |
Healthy ranges also drift up a little with age, since older adults naturally hold more fat, so read your number with your age in mind.
Two methods, two numbers
This tool also shows a second estimate from the BMI method, the Deurenberg formula. The two rarely match, and the gap is the interesting part. The tape method reads your actual shape, while the BMI method infers fat from weight and height alone. A muscular person reads high on the BMI method and lower on the tape, because muscle weighs heavy but is not fat. If your two numbers are far apart, trust the tape and take it as a sign you carry more muscle than average.
Your ideal, and how far to go
The result also shows an ideal body fat for your age, from the Jackson and Pollock reference, and the fat you would shed to reach it. The ideal climbs with age for both sexes, because a little more fat is normal and healthy as you get older. A 30 year old man sits near 12.7 percent, a 30 year old woman near 19.3 percent. The "fat to lose" figure assumes your lean mass stays put, which is the goal of a sensible cut: lose fat, keep muscle.
What FFMI tells you
Fat-free mass index is your lean mass divided by your height in metres squared. It does for muscle what BMI does for weight, and it is a better progress marker than body fat alone. Add muscle and your FFMI climbs even if your weight holds. A natural, drug-free lifter usually tops out around 22 to 25. Values well above that range are a clue that something other than training is involved.
How accurate is a tape measure?
The Navy method lands within about 3 to 4 percent of a DEXA scan for most people, which is remarkable for a cloth tape. Its weakness is measurement error: pull the tape too tight, or read the waist at the wrong spot, and the number shifts. Take each measurement relaxed, level, and twice. For a true gold-standard reading you would need DEXA, hydrostatic weighing, or a Bod Pod, but those cost money and a trip. The tape gets you close for free and tracks change well over time.
Frequently asked questions
How accurate is the US Navy body fat method? The Navy tape method lands within about 3 to 4 percent of a DEXA scan for most people. Measure carefully, since a centimetre at the waist moves the result.
What is a healthy body fat percentage? For men the fitness range is roughly 14 to 17 percent and average is 18 to 24. For women fitness is about 21 to 24 percent and average is 25 to 31. Some essential fat is needed for health.
What is FFMI? Fat-free mass index is your lean mass divided by height squared. It measures muscularity the way BMI measures weight, and is a better progress marker for lifters than body fat alone.
This tool is an estimate, not medical advice. For decisions about your health or training, talk to a doctor or a qualified coach.