Four skinfold measurements. One transparent body-fat estimate.
Find the four measurement sites
Select an input to highlight its site.
Biceps · Vertical fold · Front of the mid-upper arm.
Estimated body fat
—Durnin–Womersley · Siri conversion
Your estimate in context
Adult descriptive bands; not a diagnosis or personal target.
Four sites, one sum
Measured fold thicknesses in mm · shared scale
Estimated body composition
Fat-free mass includes water, bone, organs and muscle.
Text summary · PDF in member tools
Show calculation steps
From skinfolds to body fat
Four subcutaneous folds estimate body density. The Siri equation converts that density into a two-compartment body-fat estimate.
Body density = c − m × log₁₀(S)
Body fat (%) = 495 / body density − 450
Selected coefficients appear with valid measurements.
An estimate, not a direct scan of fat or a diagnosis. Consistent site placement and caliper technique matter more than extra decimal places.
Frequently asked questions
What measurements do I need?
Enter the biceps, triceps, subscapular and suprailiac skinfold thicknesses in millimeters. Select the biological sex and age group used by the original equations. Weight is optional and only adds estimated fat mass and fat-free mass.
Why are 17–19 and 20–29 separate?
The published age-specific regression coefficients differ between these groups. Combining them would change the estimate. The original study included participants aged 16–72; the youngest option here is 17–19. The 50+ equation should not be interpreted as validated at every older age.
Where should each fold be measured?
Biceps: a vertical fold at the front of the mid-upper arm. Triceps: a vertical fold at the back of the mid-upper arm, between shoulder and elbow landmarks. Subscapular: an oblique fold just below the lower tip of the shoulder blade. Suprailiac: an oblique fold just above the iliac crest at the side of the torso. Use a trained assessor and a consistent documented protocol; the schematic is a locator, not a measurement substitute.
What does fat-free mass include?
Fat-free mass is estimated total body mass minus fat mass. It includes water, organs, bone and muscle; it is not a measurement of muscle mass. Both mass estimates inherit the uncertainty of the body-fat equation.
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How accurate is this method?
Caliper technique, site placement, instrument pressure and the match to the study population affect the estimate. Siri conversion assumes a two-compartment body model. These estimates are not interchangeable with DXA or a clinical assessment. Track repeated measurements using the same protocol and assessor.
Are the reference bands a diagnosis?
No. The labeled adult bands are descriptive fitness references, not age-specific health targets or diagnostic thresholds. Adult categories are not applied to the 17–19 group. The calculated percentage should be interpreted with measurement quality, age and personal context.
Why can the calculator reject a positive measurement?
Every fold must be 0.1–100 mm. Even positive inputs can place the regression outside its usable range. If the equation gives a negative body-fat percentage or 100% or more, results clear instead of displaying a misleading capped value.
What does the Durnin–Womersley Body Fat calculate?
Four skinfold measurements. One transparent body-fat estimate.
Which values should I enter?
Enter the biceps, triceps, subscapular and suprailiac skinfold thicknesses in millimeters. Select the biological sex and age group used by the original equations. Weight is optional and only adds estimated fat mass and fat-free mass.
What formula does this calculator use?
Density = c − m × log10(sum of four skinfolds in mm), with age- and sex-specific Durnin–Womersley coefficients; body fat (%) = 495 / density − 450 (Siri).
Why might my result differ from another source?
Results can differ because measurement methods, rounding and assumptions vary. This calculator is an estimate and is not a medical diagnosis.
Does the result update automatically?
Yes. Valid inputs update the estimate, site bars, density and worked steps automatically. Optional weight adds mass estimates. Invalid measurements clear earlier results.
Research & reference context
Durnin & Womersley1974 · Original 4-site study
British Journal of NutritionBody-density research · 481 adults
Fitness reference contextDescriptive adult body-fat bandsEquation coefficients and site protocol: MRC DAPA Measurement Toolkit. Logos identify sources, not endorsement.