Body Fat Calculator
Estimate your body fat percentage from a few tape measurements, and see your fat mass, lean mass and the healthy range for your age.
Neck just below the Adam's apple. Waist level with your belly button, relaxed, after breathing out.
Skinfold calipers (optional, in mm)
Pinch a vertical fold on the right side of the body (chest and thigh folds run diagonally or along the thigh) and read the caliper after 2 seconds. Leave blank to skip.
How your body fat is worked out
Your numbers, step by step, with the US Navy tape method.
- 1
Tape the girth
88 − 38 = 50 cm
- 2
Add your height
log 50, log 178 → 1.0561
- 3
Density to fat %
495 ÷ 1.0561 − 450 = 18.7%
- 4
Split your weight
15.0 kg fat + 65.0 kg lean
Read more: the formula, a worked example, all five methods and the ranges
What body fat percentage means
It's the share of your weight that is fat. The rest, your lean mass, is muscle, bone, organs and water. Two people with the same weight and BMI can have very different body fat, which is why it's a useful extra to BMI. Every home method here is an estimate; a DEXA scan or a trained tester with calipers gets closer.
The formula
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
That's the US Navy method (Hodgdon and Beckett, 1984), with all lengths in centimetres. The part in brackets is an estimate of body density, and "495 ÷ density − 450" is the Siri equation that turns density into body fat. Fat mass is your weight × body fat ÷ 100; lean mass is your weight minus that.
Worked example
A 30-year-old man, 178 cm tall, weighing 80 kg, with a 38 cm neck and an 88 cm waist. Waist − neck = 88 − 38 = 50 cm. log10(50) = 1.69897 and log10(178) = 2.25042. Density = 1.0324 − 0.19077 × 1.69897 + 0.15456 × 2.25042 = 1.0324 − 0.32411 + 0.34782 = 1.05611. Body fat = 495 ÷ 1.05611 − 450 = 468.70 − 450 = 18.7%, in the ACE "average" band and inside the healthy range of 8–20% for men aged 20–39. Fat mass = 80 × 18.7 ÷ 100 = 15.0 kg, so lean mass is 80 − 15.0 = 65.0 kg.
A woman with the same height and weight, a 38 cm neck, 88 cm waist and 98 cm hips: 88 + 98 − 38 = 148, log10(148) = 2.17026, density = 1.29579 − 0.35004 × 2.17026 + 0.221 × 2.25042 = 1.03345, body fat = 495 ÷ 1.03345 − 450 = 29.0%.
The other methods
- BMI-based (Deurenberg, 1991): 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4, where sex is 1 for men and 0 for women. Under 16: 1.51 × BMI − 0.70 × age − 3.6 × sex + 1.4. For the example, BMI = 80 ÷ 1.78² = 25.25, so 1.20 × 25.25 + 0.23 × 30 − 10.8 − 5.4 = 21.0%. It can't tell muscle from fat, so it reads high for muscular people.
- Relative Fat Mass (Woolcott and Bergman, 2018): 64 − 20 × (height ÷ waist), plus 12 for women. Example: 64 − 20 × 178 ÷ 88 = 23.5%. Adults only.
- YMCA waist method: (4.15 × waist in inches − 0.082 × weight in pounds − 98.42) ÷ weight in pounds × 100, using 76.76 instead of 98.42 for women. Example: 34.65 in and 176.4 lb give 17.5%. Adults only.
- Jackson–Pollock 3-site skinfolds: men add chest, abdomen and thigh; women add triceps, suprailiac and thigh (all in mm). Density for men = 1.10938 − 0.0008267 × sum + 0.0000016 × sum² − 0.0002574 × age; for women = 1.0994921 − 0.0009929 × sum + 0.0000023 × sum² − 0.0001392 × age. Example: a man of 30 with 20 + 25 + 18 = 63 mm has density 1.05593 and body fat 18.8%.
- Durnin–Womersley 4-site skinfolds: biceps, triceps, subscapular and suprailiac; density = c − m × log10(sum), with c and m from an age table (men 30–39: 1.1422 and 0.0544). Example: 4 + 10 + 15 + 12 = 41 mm gives density 1.05446 and 19.4%.
- Siri or Brozek: the two common ways to turn density into body fat. Siri is 495 ÷ density − 450; Brozek is (4.57 ÷ density − 4.142) × 100. They usually differ by less than one point.
Different methods disagreeing by a few points is normal. Pick one, measure the same way at the same time of day, and watch the trend.
Body fat categories (ACE)
| Category | Women | Men |
|---|---|---|
| Essential fat | 10 – 13% | 2 – 5% |
| Athletes | 14 – 20% | 6 – 13% |
| Fitness | 21 – 24% | 14 – 17% |
| Average | 25 – 31% | 18 – 24% |
| Above average (ACE: "obese") | 32% and above | 25% and above |
From the American Council on Exercise. We judge the band on the figure shown, rounded to one decimal place.
Healthy body fat by age
| Age | Women | Men |
|---|---|---|
| 20 – 39 | 21 – 33% | 8 – 20% |
| 40 – 59 | 23 – 34% | 11 – 22% |
| 60 – 79 | 24 – 36% | 13 – 25% |
Ranges from Gallagher and others, American Journal of Clinical Nutrition (2000), which most smart scales use. Under 20 we show the 20–39 range as a rough guide only. The Jackson & Pollock ideal in Advanced is a single figure that rises with age: for men 8.5% at 20, 12.7% at 30, 15.3% at 40, 18.9% at 50 and 20.9% at 55+; for women 17.7%, 19.3%, 22.2%, 25.2% and 26.3%. Ages in between are interpolated. Checked on 10 October 2026.
FFMI
Fat-free mass index is lean mass in kg ÷ height in metres squared, a BMI for lean mass only. The normalised figure adds 6.1 × (1.8 − height in metres) to compare people of different heights. For the example: 65.0 ÷ 1.78² = 20.5, normalised 20.6.
Its limits
- These formulas were built on adults. For children and teenagers, and during pregnancy, they're a rough guide at best.
- Tape results swing with where you measure. Keep the tape level and snug, not tight.
- Very muscular or very lean people, and older adults, often get less accurate estimates.
General information, not medical advice. Talk to a doctor or dietitian about what your numbers mean for you.
Common questions
How is body fat percentage calculated?
By default this calculator uses the US Navy method: it estimates your body density from your height, neck and waist (plus hips for women), then turns density into body fat with the Siri equation, 495 ÷ density − 450. You can also use the BMI-based formula, Relative Fat Mass, the YMCA waist method or skinfold calipers.
What is a healthy body fat percentage?
It depends on your age and sex. For adults aged 20 to 39, the healthy range is about 21–33% for women and 8–20% for men. It rises a little with age: 23–34% and 11–22% at 40 to 59, and 24–36% and 13–25% at 60 to 79.
How accurate is the US Navy body fat method?
Every home method is an estimate. Tape results swing with where you measure, and very muscular, very lean or older people often get less accurate figures. A DEXA scan or a trained tester with calipers gets closer. Different methods disagreeing by a few points is normal, so pick one and watch the trend.
How do I measure for the most reliable result?
Keep the tape level and snug, not tight, and measure the same way at the same time of day each time you check. Small changes in where the tape sits can move the result, so the trend over several weeks tells you more than any single reading.
What is the difference between body fat and BMI?
BMI compares your weight with your height only. Body fat percentage is the share of your weight that is fat; the rest, your lean mass, is muscle, bone, organs and water. Two people with the same weight and BMI can have very different body fat, which is why it's a useful extra to BMI.
What is lean body mass?
Lean mass is your weight minus your fat mass, and fat mass is your weight × body fat ÷ 100. For example, an 80 kg man at 18.7% body fat has about 15.0 kg of fat mass and 65.0 kg of lean mass.
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