Body fat calculator

How this body fat percentage calculator works

Nothing outside a lab measures fat directly. Every field method measures something easier, a circumference, a fold of skin, a weight and a height, and converts it to a body fat percentage with an equation that was fitted against a lab reference in a sample of real people. The three tabs above use three of the best documented equations, one per type of equipment.

Tape: the US Navy circumference formula

Waist, neck and height for men, plus hip for women, go into the Hodgdon and Beckett equations from 1984. The formula is the same one used on the Navy body fat calculator, which covers the tape method, its measurement sites and its military history in depth.

Skinfold: Jackson-Pollock and the Siri equation

Caliper readings at 3 or 7 sites are summed and turned into body density with the Jackson and Pollock equation for men (1978) or the Jackson, Pollock and Ward equation for women (1980), then density becomes body fat with the Siri equation (1961). The section below gives every equation and site.

BMI: the Deurenberg estimate

With no tape and no calipers, the Deurenberg equation (1991) estimates body fat from BMI, age and sex. It is the quickest of the three and the least accurate, because BMI cannot tell muscle from fat.

One person, three methods

Take a 30 year old man, 178 cm and 82 kg, with an 87 cm waist, a 38 cm neck and skinfolds of 12 mm at the chest, 22 mm at the abdomen and 15 mm at the thigh. The tape gives 18.0%, the 3-site skinfold gives 14.8% and the BMI estimate gives 21.8%. All three are plausible for the same body, and the 7 point spread is the real lesson: compare a result only with a later result from the same method.

Fat mass is weight times the percentage; lean mass is the rest. At 18.0% the same man carries about 14.7 kg of fat and 67.3 kg of lean mass: muscle, bone, organs and water.

Skinfold calipers for body fat percentage: Jackson-Pollock 3-site and 7-site

Jackson and Pollock built their equations by measuring skinfolds and underwater weighing on 308 men aged 18 to 61, then checking them on a second group of 95 men, where predicted and measured body density correlated above 0.90. The women's equations, from Jackson, Pollock and Ward, used 249 women and were checked on 82 more; the authors note they are most reliable for women under 40.

The 3-site equations

Men measure chest, abdomen and thigh. Body density equals 1.10938 minus 0.0008267 times the sum, plus 0.0000016 times the sum squared, minus 0.0002574 times age. Women measure triceps, suprailiac and thigh. Body density equals 1.0994921 minus 0.0009929 times the sum, plus 0.0000023 times the sum squared, minus 0.0001392 times age.

The 7-site equations

Both sexes measure chest, midaxillary, triceps, subscapular, abdomen, suprailiac and thigh. For men, body density equals 1.112 minus 0.00043499 times the sum, plus 0.00000055 times the sum squared, minus 0.00028826 times age. For women it is 1.097 minus 0.00046971 times the sum, plus 0.00000056 times the sum squared, minus 0.00012828 times age. Seven sites average out a bad pinch at any one site, so they suit a client whose fat sits unevenly; three sites are faster and almost as accurate in practice.

The Siri equation: from density to body fat

Fat tissue is less dense than lean tissue, so a lower body density means more fat. Siri assumed fixed densities for fat and fat free mass and arrived at body fat % equals 495 divided by body density, minus 450. The same constants appear in the Navy formula, which was fitted against density from underwater weighing too.

How to measure skinfolds

Most caliper error comes from the pinch, not the equation. A consistent routine removes most of it.

The technique

Measure on the right side of the body, standing relaxed. Grasp the skin and the fat under it between thumb and index finger about 1 cm above the site, and lift it away from the muscle. Place the caliper jaws on the fold at the site, keep holding the fold, and read the dial about 2 seconds after the jaws close. Take each site twice and use the average; if two readings differ by more than 1 to 2 mm, take a third.

Consistency beats precision

Measure at the same time of day, before training, with dry skin. Mark the sites with a skin pen the first few times. The same tester with the same caliper gives a far more useful trend than a better tester once and a different one next time.

When calipers struggle

Very large folds may not fit in the jaws, and with heavy fat tissue the fold is hard to separate from muscle. Peterson and colleagues (2003) found that Jackson-Pollock underestimated body fat in women against a 4-compartment reference, 26.2% versus 32.8% on average, and that all skinfold equations underestimated more as body fat rose. For a client with obesity, the tape tab is usually the better choice.

The BMI-based estimate, and why it is the least accurate

Deurenberg, Weststrate and Seidell fitted body fat measured by densitometry in 1,229 people aged 7 to 83. For adults, body fat % equals 1.20 times BMI, plus 0.23 times age, minus 10.8 times sex (1 for men, 0 for women), minus 5.4. The standard error of estimate was 4.1 percentage points, and the authors noted that it slightly overestimated body fat in people with obesity.

The deeper problem is what BMI cannot see. A muscular lifter and an untrained person of the same height, weight, age and sex get the identical answer. Use this tab as a starting point when you have nothing else, and switch to the tape as soon as you own one. The BMI calculator explains the BMI number itself.

Body fat percentage estimator accuracy: tape, calipers and BMI vs DEXA and BodPod

Every method has an error, including the scans. The table puts the three methods on this page next to the two lab methods most people can book.

DEXA body fat percentage: what the scan measures

DEXA (dual energy X-ray absorptiometry) passes two low dose X-ray beams through the body and splits it into fat, lean tissue and bone mineral, region by region. It is the reference most field studies use today, but it is not perfect: research labs check it against 4-compartment models, which combine density, body water and bone mineral.

BodPod (air displacement)

The BodPod measures body volume from the air you displace in a sealed chamber, turns it into body density and usually converts that with the same Siri equation as the skinfold tab. Against DEXA it agrees reasonably well in normal weight and overweight adults, but Lowry and Tomiyama (2015) found it overestimated body fat in thin people and underestimated it in heavier people, with gaps up to 13 percentage points in some underweight participants.

Reading the table

Typical error means the spread around the reference, not a fixed offset. For the skinfold method, the 0.0077 g/cm³ standard error from Jackson and Pollock's cross-validation works out to about 3.4 percentage points of body fat through the Siri equation. For a single person, any field method can be off by more, in either direction. Pick one method, use it the same way every time, and the trend stays meaningful even when the absolute number is not.

Body fat chart for men and women by age

Body fat rises with age even at a stable weight, as muscle is slowly replaced by fat. Gallagher and colleagues (2000) measured 1,626 adults with 4-compartment models and DEXA and linked body fat to the WHO BMI cut offs of 18.5, 25 and 30, which gives healthy body fat ranges per age group. The chart below is the paper's combined table for African American and white adults; Asian adults have separate, higher values.

The calculator itself reports the ACE category (next section), which is the same for every age. Use this chart to judge whether a result is typical for someone your age.

Body fat categories (ACE)

The American Council on Exercise chart is the classification the calculator reports. It has no age adjustment, so it reads a little strict for older adults compared with the chart above. Treat the bands as coaching reference points, not clinical cut offs.

Women body fat percentage: why the ranges are higher

On the ACE chart every band for women sits about 7 percentage points above the same band for men, and the gap is biological, not a matter of effort. Essential fat for women, 10 to 13% on the ACE chart, includes sex specific fat in the breasts, hips and thighs that supports hormone production and reproduction. That is why every equation on this page has a separate female version and why the Navy formula adds a hip measurement for women.

What changes with age

In the Gallagher chart, the healthy band for women moves up by about 3 percentage points from the 20s to the 60s, and around menopause fat tends to shift from the hips toward the waist. That shift raises a tape result even when weight is stable, which is a real change in where fat is stored rather than a measurement error.

Which method suits women

The female Jackson-Pollock equations were built on women up to about 55 and were most reliable under 40, and they tended to read low against 4-compartment data. For women over 40, or with higher body fat, the tape method is often the more stable tracking tool.

Which method should you use?

You train alone and own a tape measure: use the tape tab. It is repeatable, insensitive to hydration and needs no help. For the measurement sites and the military background, see the Navy body fat calculator.

A coach measures you in person: use 3-site or 7-site skinfolds. In trained hands they track lean clients better than a tape, where small waist changes barely move the Navy formula.

You have no equipment yet: use the BMI tab for a first number, then move to the tape. Do not track progress with it: it cannot show fat loss while you gain muscle.

You want to know your lean mass and muscularity: take the lean mass from here into the lean body mass calculator and the FFMI calculator.

How coaches use a body fat calculator

Having three methods on one screen solves a common coaching problem: in person clients get calipers, remote clients send tape measurements, and a new lead with no equipment still gets a starting number. Each client should stay on one method so their retests compare like with like.

Scraler stores those measurements on each client's profile with weight and progress photos, so the body fat trend builds itself. Pair it with check-ins to make remeasuring every 4 to 6 weeks routine, and feed the result into a nutrition plan once you know whether the client needs a deficit, a surplus or a recomposition.