Body Fat Calculator

What this calculator estimates

Body fat percentage is the share of your weight that is fat tissue, with the rest counted as lean mass: muscle, bone, organs and water. A 180 pound man at 17.4% carries about 31 pounds of fat and 149 pounds of everything else. Two people can weigh the same and have different percentages, which is the whole reason the number exists.

This page estimates it two ways from things you can do at home. The first is the U.S. Navy tape method, which uses height and a few circumferences to predict body density and then converts that density to a percentage. The second is a formula built on body mass index and age, included as a cross-check because it needs nothing but a scale.

Neither one is a lab measurement. A DEXA scan, which reads bone, fat and lean tissue directly, is the usual research standard, and the tape method typically lands within 3 to 4 percentage points of it. That is close enough to see a trend over 12 weeks and not close enough to argue about a single point.

If you are a woman and want the hip measurement walked through step by step, the body fat calculator for women covers the female formula on its own terms. This page runs both sexes, compares the methods, and explains what the result does and does not mean.

How the tape formula works

The Navy equation was built from measured sailors and Marines in the 1980s, and it is still the standard field method because a tape costs nothing and the arithmetic is short. All three inputs go in as centimeters, so inches are multiplied by 2.54 first.

• Men: body fat % = 495 / (1.0324 - 0.19077 x log10(waist - neck) + 0.15456 x log10(height)) - 450
• Women: body fat % = 495 / (1.29579 - 0.35004 x log10(waist + hip - neck) + 0.22100 x log10(height)) - 450

Three details make or break the result. The log is base 10, not the natural log. The waist is measured at the navel for men and at the narrowest point of the torso for women, with the tape level all the way around. The neck is measured just below the larynx, without tensing or tucking the chin.

The 495 and 450 come from the Siri equation, which turns an estimated body density into a fat percentage. The rest of the formula is the density prediction, and it assumes an average build. A lifter with a 32 inch waist and a 17 inch neck will read differently from a person with the same waist and a 14 inch neck, and that difference is real enough that the method is best used on yourself over time rather than as a verdict on a stranger.

Worked example: a 5-foot-10 man at 180 pounds

Inputs: height 70 inches, weight 180 pounds, age 40, neck 15 inches, waist 34 inches.

• Convert to centimeters: height 70 x 2.54 = 177.8 cm, neck 15 x 2.54 = 38.1 cm, waist 34 x 2.54 = 86.4 cm
• Waist minus neck: 86.4 - 38.1 = 48.3 cm
• log10(48.3) = 1.684 and log10(177.8) = 2.250
• Bracket: 1.0324 - 0.19077 x 1.684 + 0.15456 x 2.250 = 1.0590
• Body fat: 495 / 1.0590 - 450 = 17.4%

At 180 pounds that is 180 x 0.174 = 31.3 lb of fat and 180 - 31.3 = 148.7 lb of lean mass. On the categories most gyms use, the fitness band for men ends at 17% and the average band starts at 18%, so 17.4% sits in the narrow gap between them: leaner than the average man and just short of the fitness band.

The cross-check is worth doing. BMI is 703 x 180 / 70 / 70 = 25.8, which is just inside the overweight range, and the age-based formula gives 1.20 x 25.8 + 0.23 x 40 - 10.8 - 5.4 = 24.0%. The two estimates differ by 6.6 points. That gap is normal: the BMI formula assumes an average amount of muscle for a given height, and a man with a 34 inch waist usually carries more muscle than the assumption allows.

Worked example: a 5-foot-4 woman at 135 pounds

Inputs: height 64 inches, weight 135 pounds, age 32, neck 12.5 inches, waist 28 inches, hip 38 inches.

• Height 162.6 cm, neck 31.8 cm, waist 71.1 cm, hip 96.5 cm
• Waist plus hip minus neck: 71.1 + 96.5 - 31.8 = 135.9 cm
• log10(135.9) = 2.133 and log10(162.6) = 2.211
• Bracket: 1.29579 - 0.35004 x 2.133 + 0.22100 x 2.211 = 1.0377
• Body fat: 495 / 1.0377 - 450 = 27.0%

Her fat mass is 135 x 0.27 = 36.5 lb and her lean mass is 135 - 36.5 = 98.5 lb. On the same categories, 27.0% is in the average band for women, which runs from 25% to 31%.

Her BMI is 703 x 135 / 64 / 64 = 23.2, a normal weight, and the BMI based formula returns 1.20 x 23.2 + 0.23 x 32 - 5.4 = 29.8%. Here the two estimates are 2.8 points apart, closer than the man's gap because her build is closer to the average the BMI formula assumes. When the two numbers disagree by more than about 5 points, trust the tape more and check that the waist was measured in the right place.

How much the tape moves the number

Measurement error is not spread evenly. The waist carries most of the signal, which means a half inch mistake at the navel moves the result more than anything else you can do wrong. This table holds the 5-foot-10 man at 180 pounds and his 15 inch neck fixed, and changes only the waist.

One inch of waist is worth about 1.9 points at this height and weight, and two inches is worth 3.7 points. Three inches moves a man from the fitness band into the middle of the average band without a single pound changing. That is why the categories should be read as a range and why the same person should measure the same way every time: same spot, same tension on the tape, same time of day.

Weight and hip change the picture less than most people expect. Adding 10 pounds of fat to the same waist is unlikely in real life, but the formula would barely register it if the waist stayed put, which is a known weakness of circumference methods on people who carry fat in the hips and thighs.

The reference chart for both sexes

These are the categories published by the American Council on Exercise, the chart most gyms and trainers quote. They describe fitness levels, not medical thresholds.

The gap between the columns is not arbitrary. Women carry roughly 8 to 10 points more essential fat for reproductive and hormonal tissue, so a woman at 24% and a man at 15% can be at comparable levels of leanness. Comparing a percentage across sexes is a mistake, and comparing it across ages is a smaller one: the same person tends to gain fat and lose muscle with age, so 20% at 30 and 20% at 60 are not the same body.

Where the chart is silent is the middle. A man at 17.4% is a rounding error from both bands, and half an inch of waist is inside the measurement error of the tape. Someone living at that boundary should look at a 12 week trend, a waist measurement, and how clothes fit, rather than at a single reading.

Body fat, BMI and the bathroom scale

Four numbers describe the same body, and each one is blind to something the others see.

BMI is the cheapest screen and the least personal. Its cutoffs come from population data, and the CDC adult BMI calculator is the standard reference for them. A 5-foot-10 man at 180 pounds scores 25.8, which is labeled overweight, while his tape estimate of 17.4% is in the fitness band. Both are correct about different things: he is heavy for his height and lean for his weight.

The practical order is to use BMI once to see whether the range is worth worrying about, then use the tape for the actual number, then use a DEXA scan only if you are training for something where the difference matters. The bathroom scale stays useful for daily noise control, not for judging body composition.

Reading your number without overreading it

A body fat percentage is a description, not a diagnosis. The number is most useful in three situations: as a starting point before a training block, as a check every 8 to 12 weeks, and as a way to tell whether a weight change came from fat or from muscle.

The floor matters more than the target. Essential fat is 2% to 5% for men and 10% to 13% for women, and staying near those numbers for long stretches is not a healthy place to live. Men who drop under about 6% and women under about 14% commonly report disrupted sleep, poor training recovery and hormonal changes. If a plan is pushing you toward the bottom of the chart, that is a conversation for a doctor or a registered dietitian, not a spreadsheet.

At the other end, the obese category starts at 25% for men and 32% for women, and it is associated with higher risk of type 2 diabetes and heart disease. That association is about populations. One person at 26% with a 32 inch waist and normal blood pressure is not in the same position as another at 26% with a 44 inch waist, and only a clinician can tell the difference.

The calculator runs in your browser, keeps nothing, and is not medical advice. Use it to get a direction and a trend, then take the trend to someone qualified if it worries you.