Estimating body fat from skinfold measurements
Body fat percentage is the share of your body mass that is fat tissue. One classic field method estimates it from skinfold thickness: a caliper pinches a fold of skin and the fat just beneath it at several body sites, and the readings are entered in an equation that predicts body density. Fatter bodies are less dense, so density can be converted to a fat percentage. This calculator does that using seven sites, plus your age.
It suits people who have access to a skinfold caliper and a trained measurer, such as students in an exercise science lab, coaches tracking athletes over a season, or anyone comparing their own repeated measurements. It is an estimate with real uncertainty, and it works best for tracking change measured the same way by the same person, rather than for exact values.
The Jackson-Pollock and Siri equations
The calculator adds the seven skinfolds, estimates body density with the Jackson and Pollock (1978) seven-site equation, then converts density to body fat with the Siri (1961) equation. The coefficients used here are those of the male equation:
- Sum7 = triceps + subscapular + chest + midaxillary + abdominal + suprailiac + thigh, in millimeters.
- Body density = 1.112 − 0.00043499 × Sum7 + 0.00000055 × Sum7² − 0.00028826 × age, in g/cm³.
- Body fat % = 495 ÷ density − 450 (Siri). The result is limited to the range 0 to 75%.
- There is no sex input. The female equation uses different coefficients, so female results from this tool should be treated as unreliable.
Worked example
A 30-year-old man has skinfolds of triceps 12 mm, subscapular 15, chest 10, midaxillary 12, abdominal 20, suprailiac 14 and thigh 15. Sum7 = 12 + 15 + 10 + 12 + 20 + 14 + 15 = 98 mm.
Body density = 1.112 − 0.00043499 × 98 + 0.00000055 × 9,604 − 0.00028826 × 30 = 1.112 − 0.042629 + 0.005282 − 0.008648 ≈ 1.06601 g/cm³, shown as 1.0660. Body fat = 495 ÷ 1.06601 − 450 ≈ 14.35, shown as 14.4%. Both are the outputs the calculator gives for these inputs.
Common mistakes and how to interpret the result
- Inconsistent technique. Site location, the direction of the fold, caliper pressure and timing after exercise all change readings. Take each site two or three times and use the same person and caliper for follow-ups.
- Using it for women without adjustment. This tool applies the male coefficients, so a woman's estimate will not be correct; use a method that includes sex-specific equations.
- Over-trusting small differences. A change of one or two percentage points is within typical measurement error for skinfolds. Look for trends over several sessions.
- Entering circumference or weight. Every site needs a skinfold thickness in millimeters, not a tape measurement.
Body Fat Percentage Chart: Ranges for Men and Women
The American Council on Exercise (ACE) classifies body fat percentage into five categories for men and women. These ranges provide a general guide for interpreting your result.
| Category | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletes | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Average | 18–24% | 25–31% |
| Obese | 25%+ | 32%+ |
Source: American Council on Exercise (ACE).
Important: Body fat estimates from formulas (Navy, BMI-based) have a margin of error of ±3–8%. For medical assessment, consult a healthcare provider. Source: American Council on Exercise.
How Body Fat Percentage Is Measured
Several methods exist for estimating body fat, each with different levels of accuracy and practicality.
- DEXA scan (Dual-Energy X-ray Absorptiometry) — considered the most accurate non-invasive method, with a margin of error of approximately ±1–2%. Requires a clinical or research setting and involves mild radiation exposure.
- Hydrostatic weighing (underwater weighing) — a gold-standard laboratory method based on Archimedes' principle. Accurate to within ±1.5–2%, but requires full submersion and specialized equipment.
- Skinfold calipers — an affordable field method using pinch measurements at multiple body sites. Accuracy is approximately ±3–5% when performed correctly by a trained measurer. This calculator uses the seven-site Jackson-Pollock equation.
- Bioelectrical impedance analysis (BIA) — consumer and gym-grade scales and handheld devices that send a low electrical current through the body. Convenient but accuracy varies widely (±3–8%), and results are significantly affected by hydration status.
- US Navy Method — uses tape measurements of waist, neck, and height (and hips for women) to estimate body fat. Accuracy is approximately ±3–4% compared to DEXA. This method is used in the Navy Body Fat Calculator.