How is the 7-site skinfold sum measured?

Take three readings at each of the seven sites (chest, midaxillary, tricep, subscapular, abdominal, suprailiac, thigh) with a calibrated caliper, average each site's readings, then add the seven averaged values. Read each pinch at approximately 2 seconds after the caliper is released to allow the skinfold to compress. Use the same caliper and the same measurer across sessions — operator-to-operator variation can shift the result by a few percentage points.

Is Jackson-Pollock 7-site more accurate than 3-site?

Yes, marginally — averaging seven sites instead of three reduces the variance from any single mis-measurement and better samples regional fat distribution. The two equations both report ±3-4% versus hydrostatic weighing on the validation populations, but 7-site is generally preferred in research and athletic monitoring when time permits. 3-site is faster for routine tracking.

Why are the male and female equations different?

Adult males and females show systematically different patterns of subcutaneous fat distribution at the same total body fat percentage — more central and android in males, more gluteofemoral and gynoid in females. The Jackson-Pollock equations were derived separately for each sex from hydrostatic weighing validation samples to absorb this difference. Applying the male equation to a female (or vice versa) produces a result that is internally consistent but systematically biased.

How precise should the skinfold sum input be?

Each individual skinfold reading is conventionally recorded to the nearest 0.5 mm, but the sum is entered as a whole number here (the JP equation is calibrated against measurements summed to 1 mm). Small differences in the sum produce small differences in the final BF% — a 5 mm change in the sum typically shifts BF% by about 0.5%.

Can I track body fat change over time with this tool?

Yes — and tracking change is more reliable than absolute single measurements, because operator and technique noise mostly cancels when the same person measures the same sites the same way each session. Track the skinfold sum and BF% from this tool at fixed intervals (e.g. every 4-6 weeks) and look for trends across 3+ data points rather than reacting to single readings. Variation under ±0.5% between sessions is typically within measurement noise.

Body Composition

Body Fat Percentage (7-Site Caliper)

Jackson-Pollock 7-site skinfold body fat percentage from caliper measurements.

Last updated: 22 April 2026

What this tool does

Enter the sum of seven skinfold measurements in millimetres, your age, and sex. The calculator applies the sex-specific Jackson-Pollock body density equation, then converts to body fat percentage via the Siri equation. Result updates live.

Inputs

Result

—

Formula Used

D = c_{0} - c_{1} Σ + c_{2} Σ^{2} - c_{3} a (sex-specific JP coefficients), BF % = \frac{495}{D} - 450

Σ

Sum of seven skinfolds (chest, midaxillary, tricep, subscapular, abdominal, suprailiac, thigh) in millimetres

a

Age in years

D

Body density in g/cm³; computed from Σ and age via the sex-specific Jackson-Pollock equation

c_{0} .. c_{3}

Sex-specific coefficients. Male: c₀=1.112, c₁=0.00043499, c₂=0.00000055, c₃=0.00028826. Female: c₀=1.097, c₁=0.00046971, c₂=0.00000056, c₃=0.00012828.

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The Jackson-Pollock 7-site skinfold protocol is one of the most cited field methods for estimating body fat percentage in the absence of laboratory equipment. Published in Medicine and Science in Sports and Exercise (Jackson & Pollock, 1978; Jackson, Pollock & Ward, 1980), it remains a standard reference in exercise-science textbooks and ACSM practitioner guidelines. This tool implements the canonical equation directly: it asks for the sum of all seven sites (saving you typing the individual values), your age, and sex, then reports body fat percentage rounded to one decimal place.

The seven sites

The standard Jackson-Pollock seven sites are: chest, midaxillary, tricep, subscapular, abdominal, suprailiac, and thigh. Each site has a specific anatomical landmark and a defined pinch direction (vertical or diagonal, depending on the site). Sum the seven millimetre readings and enter that total here. A standard sequence and landmark reference is published by NSCA and ACSM; both Lange and Harpenden calipers calibrated to a standard 10 g/mm² jaw pressure produce equivalent readings.

How the math works

The calculator first computes body density from the skinfold sum using the sex-specific Jackson-Pollock equation. For males: D = 1.112 − 0.00043499 × Σ + 0.00000055 × Σ² − 0.00028826 × age. For females: D = 1.097 − 0.00046971 × Σ + 0.00000056 × Σ² − 0.00012828 × age. Body density is then converted to body fat percentage via the Siri equation: BF% = (495 / D) − 450. The Siri conversion assumes a fat density of 0.9007 g/cm³ and a fat-free mass density of 1.100 g/cm³ — assumptions that hold reasonably well for adults of typical body composition but begin to diverge at the extremes (very lean, very heavy, very young, very old).

Where this method is most accurate

Jackson-Pollock 7-site was validated against hydrostatic weighing in adults aged 18-61 and produces estimates within ±3-4% of underwater densitometry for that population. Accuracy depends critically on caliper technique: consistent pinch site, consistent pressure, consistent timing (read at ~2 seconds after the caliper is released), and the same operator across sessions. A different operator can introduce ±2% noise just from technique variation, which is why tracking change is more reliable than absolute single-measurement values. The equation is least accurate at very low body fat (sub-6% in men, sub-14% in women) where the assumed fat-free mass density diverges, and at high body fat (above ~30% men, ~40% women) where subcutaneous-to-visceral ratios shift.

What this tool does not do

It does not interpret the result, tell you whether the number is good or bad, recommend a target, or convert directly into health risk. Body fat percentage is one number among many; population-level associations between BF% and outcomes do not predict individual outcomes. For clinical body composition assessment, DEXA scanning produces higher accuracy and additional regional breakdowns; for athletic monitoring, consistent skinfold technique with the same operator gives reliable trend data even when the absolute number has measurement uncertainty.

Disclaimer

This calculator is for educational and informational purposes only. It does not provide medical, nutritional, or training advice. Results are mathematical estimates based on general formulas and may not reflect individual circumstances. Consult a qualified coach, registered dietitian, medical professional, or physiotherapist for personal guidance.

Questions

How is the 7-site skinfold sum measured?: Take three readings at each of the seven sites (chest, midaxillary, tricep, subscapular, abdominal, suprailiac, thigh) with a calibrated caliper, average each site's readings, then add the seven averaged values. Read each pinch at approximately 2 seconds after the caliper is released to allow the skinfold to compress. Use the same caliper and the same measurer across sessions — operator-to-operator variation can shift the result by a few percentage points.
Is Jackson-Pollock 7-site more accurate than 3-site?: Yes, marginally — averaging seven sites instead of three reduces the variance from any single mis-measurement and better samples regional fat distribution. The two equations both report ±3-4% versus hydrostatic weighing on the validation populations, but 7-site is generally preferred in research and athletic monitoring when time permits. 3-site is faster for routine tracking.
Why are the male and female equations different?: Adult males and females show systematically different patterns of subcutaneous fat distribution at the same total body fat percentage — more central and android in males, more gluteofemoral and gynoid in females. The Jackson-Pollock equations were derived separately for each sex from hydrostatic weighing validation samples to absorb this difference. Applying the male equation to a female (or vice versa) produces a result that is internally consistent but systematically biased.
How precise should the skinfold sum input be?: Each individual skinfold reading is conventionally recorded to the nearest 0.5 mm, but the sum is entered as a whole number here (the JP equation is calibrated against measurements summed to 1 mm). Small differences in the sum produce small differences in the final BF% — a 5 mm change in the sum typically shifts BF% by about 0.5%.
Can I track body fat change over time with this tool?: Yes — and tracking change is more reliable than absolute single measurements, because operator and technique noise mostly cancels when the same person measures the same sites the same way each session. Track the skinfold sum and BF% from this tool at fixed intervals (e.g. every 4-6 weeks) and look for trends across 3+ data points rather than reacting to single readings. Variation under ±0.5% between sessions is typically within measurement noise.

Sources & Methodology

Jackson-Pollock 7-site skinfold protocol (Jackson & Pollock 1978; Jackson, Pollock & Ward 1980). The seven sites are chest, midaxillary, tricep, subscapular, abdominal, suprailiac, and thigh. The calculator accepts the sum of all seven readings (in mm), computes body density via the sex-specific Jackson-Pollock generalised equation, then converts density to body fat percentage via the Siri equation BF% = (495 / D) − 450. The sex coefficients differ because subcutaneous fat distribution patterns differ systematically between male and female adults. Accuracy versus hydrostatic weighing is reported as ±3-4% for adults aged 18-61; the equation is less accurate at the extremes of body composition and assumes consistent caliper technique (a single operator across measurements is the strongest predictor of trend reliability).

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