Three-Site Skinfold Calculator estimates body density and body-fat percentage from age plus three caliper measurements using classic Jackson–Pollock equations. It is designed for repeatable fitness tracking, not diagnosis, and it keeps the formula, measurement sites, and limitations visible.
Men's equation: chest + abdomen + thigh; source sample age 18–61. Enter each caliper reading in millimetres.
Educational estimate calculated from the entered measurements.
Need a broader method? Compare the Seven-Site Skinfold Calculator or review the body-fat measurement guide.
The Three-Site Skinfold Calculator turns three subcutaneous skinfold measurements into an estimate of body density, then converts that density into an estimated body-fat percentage. The method is compact because it uses three standardized sites rather than measuring every possible fold.
The calculator asks for age and for one of two published site combinations. For men, the classic Jackson–Pollock three-site equation uses chest, abdomen, and thigh skinfolds. For women, the Jackson–Pollock–Ward three-site equation uses triceps, suprailiac, and thigh skinfolds. Each measurement is entered in millimetres, and the three values are added before the sex-specific body-density equation is applied.
The Three-Site Skinfold Calculator is most useful when measurements are taken with the same calipers, the same technique, and ideally the same trained measurer over time. A one-off result can be interesting, but repeated measurements under similar conditions are usually more informative for tracking direction. If you are still deciding which body-fat method to use, the Body Fat Calculator and Body Composition Guide provide broader context.
You need a skinfold caliper, age in years, and three site readings in millimetres. The Three-Site Skinfold Calculator does not need body weight or height because the Jackson–Pollock equations predict body density from the summed skinfolds and age. If your next step is to translate a body-fat estimate into lean mass, use the Lean Body Mass Calculator after you have a body weight recorded under comparable conditions.
Correct site selection matters as much as entering the numbers correctly.
Use the men's equation when you want the Jackson–Pollock chest + abdomen + thigh model. Keep the site landmarks and pinch direction consistent across sessions, and enter the resulting skinfold thicknesses in millimetres.
Use the women's equation for the Jackson–Pollock–Ward triceps + suprailiac + thigh model. Do not substitute abdomen or chest measurements into this formula because a different site combination changes the regression model.
Skinfold technique can vary with landmarking, pinch placement, caliper pressure, and measurer skill. Taking repeated readings and using a consistent protocol helps reduce avoidable noise before the values reach the Three-Site Skinfold Calculator.
A useful skinfold log records more than the final percentage. Note the date, selected equation, each raw site value, the caliper used, and whether the same person performed the measurements. That record makes it easier to spot a sudden result caused by one unusual site reading instead of a broad change across all three folds.
For repeat sessions, keep the basic setup as similar as practical. The goal is not to create laboratory conditions at home; it is to reduce avoidable variation so the trend is easier to interpret. When a reading looks unexpectedly different, repeat the site before accepting the number. If your technique or equipment changes, mark that change in your log rather than comparing the new value as though the method were identical.
The formula is simple; the measurement process is the part most likely to create noise.
Select the male or female research equation before measuring. The Three-Site Skinfold Calculator changes the required site labels automatically so the entries match the formula.
Use a consistent anatomical landmark and the same side of the body each session. Review How to Measure Body Fat before collecting data if you are new to skinfolds.
Repeat each site rather than relying on a single pinch. If readings differ substantially, re-check placement and technique before averaging or selecting a representative value.
Put the final three values into the Three-Site Skinfold Calculator, calculate, and record the date and method so future comparisons use the same setup.
The Three-Site Skinfold Calculator first adds the three site readings to create a skinfold sum, shown below as S. It then uses a sex-specific generalized body-density equation that includes age. The final step converts body density to estimated body-fat percentage with the classic Siri relationship.
Body density = 1.10938 − (0.0008267 × S) + (0.0000016 × S²) − (0.0002574 × age)
For this equation, S is chest + abdomen + thigh in millimetres. The original generalized equation was developed from adult men spanning ages 18–61. The calculator therefore accepts that age range rather than silently extrapolating beyond it.
Body density = 1.0994921 − (0.0009929 × S) + (0.0000023 × S²) − (0.0001392 × age)
For this equation, S is triceps + suprailiac + thigh in millimetres. The corresponding published women's study included adults ages 18–55, which is the range enforced by this Three-Site Skinfold Calculator.
Estimated body fat (%) = (495 ÷ body density) − 450
This conversion is commonly called the Siri equation. The result is an estimate produced from two model layers: a regression equation predicts density from skinfolds and age, then the density conversion estimates the proportion of body fat. That chain is why a calculator result should not be treated as laboratory truth or a medical diagnosis.
Illustrative arithmetic using the same formulas as the calculator.
Body density ≈ 1.06566, then 495 ÷ 1.06566 − 450 ≈ 14.5%.
Body density ≈ 1.04990, then the Siri conversion gives approximately 21.5%.
A lower sum can support a trend interpretation, but hydration, technique, site placement, and normal measurement error still matter.
The examples show why the Three-Site Skinfold Calculator should be paired with a record of the underlying measurements, not just the final percentage. If the displayed body-fat estimate changes, look at whether all three site readings moved in the same direction and whether measurement conditions were comparable. For a simpler record of raw values, the Skinfold Sum Calculator can separate measurement tracking from body-fat conversion.
Treat the Three-Site Skinfold Calculator as a repeatable field estimate, not a verdict about health, athletic ability, or appearance. The strongest use case is comparing measurements collected with the same protocol over time.
A change of a few tenths of a percentage point may be smaller than the noise introduced by caliper placement or pinching technique. Rather than reacting to one decimal place, review the skinfold sum, the individual sites, and the direction across several measurements. If your goal is body-composition tracking, combine the result with body weight and other consistent measures rather than forcing one number to carry the whole interpretation.
| Question | Useful interpretation | What not to assume |
|---|---|---|
| Did my estimate decrease? | Check whether the three-site sum also decreased under similar measurement conditions. | Do not assume every small change is true fat loss. |
| Why did two methods disagree? | Different equations, sites, devices, and assumptions can produce different estimates. | Do not assume one method is exact because it gives more decimal places. |
| Can I use this for FFMI? | You can use a reasonable body-fat estimate as one input to an FFMI calculation. | Do not forget that body-fat error carries into FFMI. |
To connect the estimate to fat-free mass, use the Lean Body Mass Calculator. If your goal is FFMI, continue to the FFMI Calculator and keep the body-fat method consistent between check-ins. The FFMI vs Body Fat % guide explains why those two measures answer different questions.
The Three-Site Skinfold Calculator is practical when you have appropriate calipers, know the required anatomical sites, and want a short field protocol that can be repeated. It can also be useful when seven-site measurement is too time-consuming, provided you accept that fewer sites do not eliminate technique error.
If you cannot reliably locate or pinch the required sites, a different body-fat estimation method may be easier to repeat. If you already collect seven skinfolds, use the Seven-Site Skinfold Calculator rather than discarding data. If you want to compare uncertainty around a body-fat estimate, the Body Fat Measurement Error Calculator is designed for that next question.
Useful equations still depend on who was studied and how well the measurements are taken.
This Three-Site Skinfold Calculator restricts ages to the reported source-study ranges: 18–61 for men and 18–55 for women. That does not make every in-range result exact; it simply avoids pretending the original equation was validated at ages outside those samples.
Small changes in site location, pinch depth, timing, or caliper placement can alter the readings. The Three-Site Skinfold Calculator cannot detect whether a measurement was taken at the correct landmark, so measurement skill remains part of the method.
The equations were derived from specific adult research samples. Individual differences in fat distribution and tissue density mean the estimate can differ from reference methods even when the skinfolds are measured carefully.
If you are tracking changes, the Body Fat Percentage Change Calculator can quantify the difference between two estimates, while the Progress Tracker can help organize repeated values. Keep the measurement method noted beside the result so a later method switch is not mistaken for a physiological change.
The men's generalized body-density work by Jackson and Pollock was published in 1978, and the women's generalized equations by Jackson, Pollock, and Ward were published in 1980. Those studies used hydrostatic body-density methods and regression models to relate skinfold measurements and age to body density. Later research has continued to test how well Jackson–Pollock equations perform in different populations.
The Three-Site Skinfold Calculator keeps those classic equations transparent and does not present them as a direct measurement. The linked research is useful if you want to review the original study populations, cross-validation approach, and later discussions of prediction error.
Short answers for the measurement choices that most often change the result.
The men's equation uses chest, abdomen, and thigh skinfold thicknesses in millimetres plus age in years. The calculator adds the three skinfolds, applies the Jackson–Pollock body-density equation, and then converts density to estimated body-fat percentage.
The women's equation uses triceps, suprailiac, and thigh skinfolds in millimetres plus age. Those sites belong to the Jackson–Pollock–Ward three-site model; do not replace one of them with a chest or abdomen value from another protocol.
Age is a term in both generalized body-density equations. Two people with the same three-site sum but different ages can therefore receive different density estimates. The Three-Site Skinfold Calculator follows the published equation rather than removing that age term.
Repeating readings can help you identify inconsistent measurements. A practical protocol is to repeat each site and resolve readings that differ materially before entering a representative value. Consistency of landmarking and technique matters more than simply averaging every number.
Not automatically. A seven-site protocol uses more measurement locations, but both approaches depend on technique and the fit of their regression equations to the person being measured. Choose the method you can perform consistently and compare like with like.
Yes, as an estimated body-fat input. Remember that any error in the skinfold estimate carries into fat-free mass and FFMI. Use the same body-fat method over time and continue with the FFMI Calculator.
Those limits mirror the reported age ranges of the original generalized equation studies used here. The Three-Site Skinfold Calculator avoids automatic extrapolation outside those ranges rather than implying that the equations were validated for every adult age.
No. The Three-Site Skinfold Calculator is an educational fitness estimate. Body-fat percentage alone does not diagnose a health condition, and a skinfold equation cannot replace individualized clinical assessment when medical evaluation is needed.
Record the three raw skinfolds, age, selected equation, and date. Reassess with the same method so a change in technique is less likely to look like a change in body composition.
Educational fitness information only. The Three-Site Skinfold Calculator estimates body composition from field measurements and cannot diagnose health conditions or replace professional assessment.