How to use this calculator
On the Skinfold tab, choose your sex (which sets the three measurement sites), enter your age, and add your three caliper readings in millimeters. Optionally enter your body weight to also see fat mass and lean mass. The calculator estimates your body density with the Jackson-Pollock 3-site equation, then converts it to body fat percentage with the Siri equation. On the Body Density tab, you can enter a density value directly — from a Bod Pod or underwater weighing — and see both the Siri and Brozek body fat results.
What is the Siri equation?
The Siri equation, published by physicist William Siri at Berkeley in 1961, is the standard way to turn body density into body fat percentage. It is built on a two-compartment model that treats the body as fat (low density) plus everything else (higher density), and solves for the fat fraction.
Body Fat % = (495 ÷ Body Density) − 450
// Brozek equation (1963) — an alternative
Body Fat % = (457 ÷ Body Density) − 414
The catch is that you need a body density value first. Most people do not have access to underwater weighing, so the density is usually estimated from skinfold thickness using a regression equation — most commonly the Jackson-Pollock formula.
How skinfolds become body fat
The skinfold method works in two steps. First, the Jackson-Pollock equation estimates body density from the sum of your skinfolds and your age. Second, the Siri equation converts that density to body fat percentage. This calculator runs both steps for you.
Density = 1.10938 − 0.0008267×S + 0.0000016×S² − 0.0002574×age
// Jackson-Pollock 3-site — Women (triceps, suprailiac, thigh)
Density = 1.0994921 − 0.0009929×S + 0.0000023×S² − 0.0001392×age
// S = sum of the three skinfolds in mm; then apply Siri
Sources: Siri, W. E. (1961). Body composition from fluid spaces and density. Jackson, A. S. & Pollock, M. L. (1978, 1980), Generalized equations for predicting body density of men and women. British Journal of Nutrition / Medicine & Science in Sports.
Which sites do I measure?
The 3-site method uses different sites for men and women because fat distribution differs by sex. The calculator updates the labels automatically when you pick your sex.
| Sex | Site 1 | Site 2 | Site 3 |
|---|---|---|---|
| Men | Chest (diagonal) | Abdomen (vertical, 2 cm right of navel) | Thigh (vertical, mid-front) |
| Women | Triceps (vertical, back of arm) | Suprailiac (diagonal, above hip bone) | Thigh (vertical, mid-front) |
How to take a skinfold measurement
- Measure on the right side of the body, with the muscle relaxed.
- Pinch a fold of skin and fat firmly between thumb and forefinger, about 1 cm away from where the caliper jaws will sit.
- Place the caliper jaws on the fold and read after 1 to 2 seconds, while still holding the pinch.
- Take each site at least twice and average the readings; if they differ by more than 1 to 2 mm, take a third.
- Use the same caliper and the same sites every time so your trend is comparable.
What is a healthy body fat percentage?
The American Council on Exercise publishes widely used body fat categories. They differ by sex because women carry more essential fat than men.
| 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) body fat percentage categories.
How accurate is the skinfold method?
Done well by a consistent tester, skinfold testing estimates body fat to within about 3 to 5 percentage points of a person's true value — clearly better than BMI, which cannot measure fat at all, and less precise than a DEXA scan. The single biggest source of error is technique: inconsistent pinch location, reading the caliper too late, or switching testers between sessions. Because of that, the method is most useful for tracking change over time under the same conditions, rather than chasing one perfect number.
Important
Not medical advice. Skinfold and density estimates are screening tools, not diagnoses. The Jackson-Pollock and Siri equations were derived mainly from younger and middle-aged adults and lose accuracy at the extremes of age and body fat. If precise body composition matters for a medical reason, ask your clinician about a DEXA scan.
- These equations are for adults, not children or teenagers.
- Very lean athletes and people with obesity sit at the edges of the equations' validity — interpret with caution.
- If you have or suspect an eating disorder, please seek professional support before tracking body fat.
Siri skinfold vs the US Navy method vs BMI
There are several ways to estimate body fat, each suited to different tools.
- Siri skinfold (this page) — needs calipers. Most accurate of the at-home methods, especially for lean and athletic people.
- US Navy method — needs only a tape measure for neck, waist and hip circumferences. Easiest to do at home with no special equipment.
- Body Roundness Index — waist and height; focuses on central fat and mortality risk rather than total fat.
- BMI — height and weight only; a fast screen, but blind to body composition.
If you own a caliper, the Siri skinfold method on this page is your most accurate option. If you only have a tape measure, the Navy method is the better fit.
Frequently asked questions
The Siri equation converts body density into body fat percentage. It is written as body fat percent equals 495 divided by body density, minus 450. Published by William Siri in 1961, it is the most widely used formula for turning a density measurement from skinfolds or underwater weighing into a body fat figure.
First estimate body density from your skinfold measurements using the Jackson-Pollock equation, which takes the sum of your skinfolds and your age. Then apply the Siri equation to convert that density to body fat percentage. This calculator does both steps automatically: enter your sex, age and three skinfold sites and it returns your body fat percentage.
For men, the Jackson-Pollock 3-site method uses the chest, abdomen and thigh. For women, it uses the triceps, suprailiac (just above the hip bone) and thigh. The sites differ because men and women store fat in different patterns, and the equations are calibrated separately for each sex.
Both convert body density to body fat percentage. Siri uses 495 divided by density minus 450, while Brozek uses 457 divided by density minus 414. They give very similar results for most people, usually within one percentage point. Siri is more common; Brozek is sometimes preferred at the extremes of body fat. This calculator shows both when you enter a density directly.
When done correctly by a trained tester, skinfold testing is accurate to within roughly 3 to 5 percent of a person's true body fat. The main source of error is measurement technique, so consistency matters: use the same tester, the same caliper and the same sites each time. It is more accurate than BMI but less accurate than a DEXA scan.
They suit different tools. The Siri skinfold method needs caliper measurements and tends to be more accurate for lean and athletic people. The US Navy method needs only a tape measure for circumferences and is easier to do at home. If you own calipers, use this Siri calculator; if you only have a tape measure, use the Navy body fat calculator.
For men, the American Council on Exercise considers 6 to 13 percent athletic, 14 to 17 percent fitness, 18 to 24 percent average, and 25 percent or more in the obese range. For women, 14 to 20 percent is athletic, 21 to 24 percent fitness, 25 to 31 percent average, and 32 percent or more obese. Essential fat is about 2 to 5 percent for men and 10 to 13 percent for women.