Skinfold Body Fat Calculator

Three pinches with a caliper, honestly taken, estimate body fat within a few points of laboratory methods. Enter your sex, age, weight and the three skinfold measurements in millimetres exactly as the caliper read them, and this calculator returns your body fat percentage by the Jackson-Pollock 3-site equations and the Siri conversion — plus your fat mass and lean mass in kilograms.

Two Equations and a Pinch

The method chains two classic results. First, Jackson and Pollock's regression (1978, refined with Ward for women) converts the sum of three skinfolds plus age into body density — the sites chosen because their fat depots track whole-body adiposity best for each sex: chest, abdomen and thigh for men; triceps, suprailiac (above the hip bone) and thigh for women. The squared term in the equation captures a real nonlinearity — each added millimetre means slightly less at high sums than at low ones. Second, the Siri equation (1961) converts density into fat percentage on a two-compartment model: fat at 0.900 g/mL, everything else at 1.100 g/mL, solved for the mixture that matches the measured density: %BF = (4.95 / density − 4.50) x 100. Every number this page prints passes through both gates, and its accuracy is bounded by both: the regression's population fit, and the two-compartment assumption that lean tissue's density is constant — mostly true, and the reason all field methods cluster in the same few-points error class.

Worked Example: Sum of 40 Millimetres at Age 30

The defaults — a 30-year-old man, folds of 12, 16 and 12 mm summing to S = 40: density = 1.10938 − 0.0008267(40) + 0.0000016(1600) − 0.0002574(30) = 1.10938 − 0.03307 + 0.00256 − 0.00772 = 1.07115 g/mL, and Siri gives (4.95 / 1.07115 − 4.50) x 100 = 12.12% body fat — at 80 kg, 9.7 kg of fat and 70.3 kg of lean mass: an athletic reading, consistent with the lean folds entered. Run the women's equation for contrast at age 30 with a sum of 50 mm: density 1.05142, fat 20.79% — the equations are sex-specific in both sites and coefficients because the depots genuinely differ. Note what age does inside the formula: at the same folds, a 50-year-old man's density falls by 0.0051 and his computed fat rises about 2.2 points — the regression pricing the internal fat that accumulates with age at unchanged surface folds, a quiet sophistication the tape methods of this batch handle differently.

Taking the Folds: Where Accuracy Actually Lives

Every site is on the body's right side, taken on dry skin, the fold pinched firmly between thumb and finger a centimetre from the caliper's jaws, read two seconds after the caliper closes (the fold compresses under sustained pressure, and dawdling shaves millimetres). Diagonal fold at the chest midway between nipple and armpit crease for men; vertical fold at the abdomen two centimetres right of the navel; vertical fold on the front of the thigh midway between hip and knee; vertical at the triceps' midpoint for women; diagonal above the iliac crest for the suprailiac. Take each site three times in rotation and record the middle value of each — the median discipline that professional testers use and the single habit that most improves home results. A plastic caliper costing very little performs within a whisker of the laboratory instruments for sums in the normal range; technique, never equipment, is the binding constraint, and a second person measuring you beats the world's best caliper in your own contorted hand.

Skinfolds Among the Methods

Against this hub's other instruments the caliper occupies a considered middle: more composition-aware than the circumference equations (Army and Navy pages, this batch's neighbours), which infer fat from shape alone; cheaper and more repeatable than bioimpedance scales, whose readings swing with hydration by more than a training month's honest change; and less precise than DEXA or hydrostatic weighing, which it trails by roughly two to three points in skilled hands while costing nothing and living in a drawer. Its deepest strength is trend fidelity: the raw sum of folds, before any equation touches it, is itself a superb progress metric — a falling sum at stable weight is recomposition, documented in millimetres, immune to every modelling assumption downstream of the pinch.

A Short History of the Caliper

The method descends from the mid-century project of weighing the body in parts: underwater weighing gave the density criterion in the 1940s (Behnke's work, then Siri's equation of 1961), and researchers hunted surface measurements that could predict density without the dunk tank. Skinfold calipers — the spring-loaded Harpenden instrument of the 1950s set the standard, its jaw pressure specified to the gram per square millimetre — were validated site by site against the tank, and Jackson and Pollock's 1978 regressions, built on hundreds of measured adults, fixed the three- and seven-site forms the field still uses. The plastic calipers in gym bags today are that lineage democratised; the equations, unchanged, are among the longest-serving instruments in body-composition science, surviving every newer technology's audit within their stated few points.

Keeping a Measurement Log

The log that makes this method sing has five columns: date, the three raw folds, their sum, and — only then — the computed percentage. Raw millimetres first, because they are the direct evidence: equations can be re-run, categories can be re-drawn, but a sum that falls from 46 to 40 across a season is recomposition documented beyond argument. Photograph the caliper placement in the first session; the sites you standardise today are the trend you can trust next year.

Related Tools

More from the Health Calculators hub: the Army Body Fat Calculator and Navy Body Fat Calculator — the tape-measure methods needing no caliper — and the Maintenance Calorie Calculator for the energy side of the same project.

Frequently Asked Questions

Why three sites and not seven?

Jackson-Pollock's 7-site protocol (adding subscapular, midaxillary, triceps or suprailiac and supraspinale depending on sex) trims the error modestly — a few tenths of a point in validation studies — at more than double the measuring burden and twice the opportunities for site error. The 3-site equations were published precisely as the pragmatic form, and for self-testing, where the same hand repeats the same three sites monthly, their trend reliability matches the longer protocol for practical purposes. Clinics with trained staff may prefer seven; a person alone with a mirror should master three.

My sum is very high or very low — is the result valid?

The regressions were fitted on general adult samples, and extrapolation frays at the edges: sums below about 15 mm (stage-lean athletes) and above about 120 mm (where folds exceed caliper jaw capacity and compressibility changes) both stretch the model. At the edges, treat the percentage as a neighbourhood, track the raw sum as the primary metric, and consider a reference method for a one-time anchor reading. The calculator computes the equation faithfully at any input; this FAQ marks where the equation's evidence runs out.

Does the Siri equation suit everyone?

Siri assumes lean tissue density of 1.100 g/mL, an average that varies modestly with age, ancestry, athletic development and bone density — the known source of the method's systematic few-point differences between populations at identical true fatness. The Brozek equation (4.57/density − 4.142) is the historical alternative and reads slightly differently; this page uses Siri as the field standard. For personal tracking the choice barely matters (consistency beats selection); for a single absolute reading, hold the error band in mind whichever equation printed it.

Can I test children or teenagers?

Not with these equations: growing bodies violate the adult density assumptions outright (children's lean tissue is less dense, hydrating differently), and youth-specific equations (Slaughter's) with different sites and coefficients exist for that population. This page's formulas are adult instruments, and the honest answer for minors is a paediatric reference, not a calculator's extrapolation.

What do the category labels mean for my reading?

Reference bands differ by source and sex — for men, roughly: essential fat 2-5 percent, athletes 6-13, fitness 14-17, average 18-24, obese 25 and above; for women, each band sits some 8-10 points higher (essential 10-13, athletes 14-20, fitness 21-24, average 25-31, obese 32 and above). The worked example's 12.12 percent therefore reads as athletic for a man, while the women's example at 20.79 percent sits at the athletic-fitness border of her scale — the same fat fraction meaning different things on different frames, which is why the equations, the sites and the bands are all sex-specific. Treat the bands as orientation, not verdicts: health tracks with the trend and the company the number keeps (strength, blood markers, waist), never with one decimal of one morning.

Can skinfolds estimate visceral fat?

Only weakly: calipers sample subcutaneous fat — the layer under the skin — while visceral fat, the depot around the organs that carries the larger metabolic risk, hides from the pinch entirely. Two people with identical sums can carry quite different visceral loads (a pattern that differs by sex, age and ancestry), which is why waist circumference and its ratios are kept alongside skinfolds in serious assessments: the tape sees the waist's total, the caliper sees its surface, and the two together triangulate what neither proves alone. Imaging (MRI, DEXA's android-region estimates) settles the question where it clinically matters.

How often should I re-measure?

Monthly is the professional rhythm: fat change slower than measurement noise is invisible weekly, and a four-week cadence lets a real two-to-three-millimetre sum change — the smallest trustworthy signal — emerge from the wobble. Same tester, same caliper, same time of day, recorded in millimetres before percentages: the log of raw sums is the dataset; the equation can be re-run over it forever.