Skinfold body fat
calculator

Estimate body fat from caliper skinfolds using the Jackson-Pollock generalized equations — 3-site or 7-site, converted with Siri (1961). You get the body fat %, body density, and lean/fat mass, plus two things most skinfold tools skip: the sum at which the equation stops working, and how much the conversion choice and a millimetre of caliper drift move the number. Skinfolds are in millimetres; bodyweight toggles imperial or metric.

How the math works

Skinfold body fat is a two-step estimate. First a regression predicts your body density from the sum of the skinfolds (in millimetres), your age, and your sex. Then a two-compartment equation converts that density to a body fat percentage. Jackson & Pollock (1978, men) and Jackson, Pollock & Ward (1980, women) built the density regressions; Siri (1961) is the usual conversion.

S is the sum of the skinfolds in millimetres. The 7-site equations use the same shape with different coefficients and add midaxillary, subscapular, triceps, and suprailiac to the men's three (the women's three plus chest, midaxillary, subscapular, abdomen). Notice the S² term: because its coefficient is positive, density has a minimum, then rises again — more on that below. Lean body mass, when you enter a bodyweight, is weight × (1 − body fat / 100); fat mass is the rest.

Worked example

A 30-year-old man, 180 lb (81.65 kg), with 3-site caliper readings of chest 10 mm, abdomen 20 mm, thigh 12 mm — a sum of 42 mm:

• Body density: 1.10938 − 0.0008267×42 + 0.0000016×42² − 0.0002574×30 = 1.069759 g/cc
• Body fat (Siri): 495 / 1.069759 − 450 = 12.7% (Brozek gives 13.0% — the conversion is a model choice, not a second measurement)
• Lean body mass: 180 × (1 − 0.127) = 157.1 lb (71.3 kg); fat mass 22.9 lb (10.4 kg)

A 30-year-old woman, 130 lb (59.0 kg), 3-site readings triceps 18 mm, suprailiac 14 mm, thigh 28 mm — sum 60 mm: BD = 1.0994921 − 0.0009929×60 + 0.0000023×60² − 0.0001392×30 = 1.044022 g/cc, so Siri body fat is 495 / 1.044022 − 450 = 24.1% (Brozek 23.5%), a lean mass of 98.6 lb (44.7 kg). Both sums sit far below the turnover point, so both estimates are inside the equation's range.

When this calculator is wrong

Every calculator on this site gives an estimate, and here the real-world calibration matters more than the formula: the Jackson-Pollock generalized equations carry a standard error of about 3.5 percentage points against hydrostatic weighing, and near a 42 mm sum a single millimetre of caliper drift moves the result about 0.34 points. So a reading of 12.7% could sit anywhere from roughly 9% to 16%, and a 2–3 mm mis-pinch at each of three sites shifts it more than choosing Siri over Brozek does. Re-measure with the same tester, same sites, same day of the week, and read the trend — not the decimal. The counter: the first measurement is noise on its own; it takes two or three sessions before a real change separates from technique scatter.

• The equation has an upper limit built into its shape. Because body density is quadratic in the sum with a positive squared term, it bottoms out at S = c1 / (2·c2) — about 258 mm for the men's 3-site equation and 216 mm for the women's — and past that sum the formula predicts body fat falling as skinfolds rise, which is nonsense. The generalized equations were fitted on lean-to-moderate cohorts and drift badly near and beyond that vertex; the calculator flags when your sum crosses it.
• It is a model of a model. The skinfold-to-density step is a regression; the density-to-fat step (Siri) assumes a fixed fat-free-mass density of about 1.100 g/cc, which is a population average, not your body. People with denser or lighter bone and different hydration — older adults, some ethnic groups, very lean athletes — break that assumption, so the absolute number is off even when the calipering is perfect.
• Different methods are not different truths. A skinfold number, a Navy-tape number, a bioimpedance scale, and a DEXA scan disagree by several points because they model body fat differently. Pick one, hold conditions constant, and compare against yourself — not against another calculator.

What to do with the result

Use the body fat number to get lean body mass, which is the durable output. Feed that into a Katch-McArdle BMR to set calories, or anchor protein to lean mass in the macro calculator — a defensible target for very lean or very heavy people, where weight-based numbers drift.

Then measure the same way every 3–4 weeks and track the direction. A sum of skinfolds that drops 8 mm over two months is a cleaner signal than any single session's converted percentage. If you need an accurate absolute number — for a medical decision or a research-grade baseline — get a DEXA scan; calipers are for trend, not for diagnosis.

Common questions

Is the 3-site or 7-site skinfold test more accurate?

The 7-site equation samples more of the body and is generally the more reliable of the two, but only if you can pinch all seven sites consistently — subscapular and midaxillary are the hardest to self-measure. The 3-site test is faster and its extra error is small next to the tester-technique error both share. For self-testing, a well-taken 3-site beats a sloppy 7-site.

How accurate is a Jackson-Pollock skinfold body fat estimate?

Its standard error against hydrostatic weighing is about 3.5 percentage points, and that is before tester error — a 1 mm change in the sum of skinfolds moves the result roughly 0.34 points. It is good for tracking change over time with the same tester and calipers, not for a precise one-off absolute number. DEXA and hydrostatic weighing are the reference methods when accuracy matters.

Should I use the Siri or Brozek equation?

Both convert body density to body fat and disagree by roughly 0.3–1 percentage point over the normal range, because they assume slightly different densities for fat and fat-free tissue. Siri is the more common default and is fine for most people; Brozek is sometimes preferred at higher body fat. The choice matters far less than measuring the skinfolds consistently — pick one and keep it.

Where do I measure the skinfold sites?

All pinches are vertical unless noted, on the right side, with the caliper 1 cm from your fingers. Men's 3-site: chest (diagonal, between nipple and armpit), abdomen (2 cm beside the navel), thigh (front midline). Women's 3-site: triceps (back of the upper arm), suprailiac (diagonal, above the hip bone), thigh. Take two or three grabs at each and average them.

Why is my skinfold body fat different from my scale or tape number?

Because each method models body fat a different way. A bioimpedance scale infers it from an electrical signal that shifts with hydration; the Navy tape uses circumferences; skinfolds use subcutaneous fat thickness. They can differ by several points on the same person on the same day. Compare a method against itself over time, not against another method.

Can I use skinfold body fat to set my calories?

Yes — that's a good reason to measure it. Body fat gives you lean body mass, and the Katch-McArdle BMR formula (370 + 21.6 × lean mass in kg) uses lean mass directly, which is more accurate than weight-and-height formulas for lean or muscular people. Our TDEE calculator runs Katch-McArdle when you enter a body fat number.