Enter your numbers
12 questions. Questions that only apply to some people appear once your earlier answers decide that they do.
From your health data
8 of these questions can be answered from lab results and device data you have already connected: Sex at birth, Age, Height, Weight, Waist circumference, Neck circumference, Hip circumference, Your measured body fat. Connect them once in your member account and they arrive here pre-filled and labelled as coming from your health data — and you can always type over the value.
Set up health data in your member account
This public page never reads your health data. Pre-filling happens inside the signed-in app, which is a separate, access-controlled surface.
How this is calculated
Formula version lll-body-composition-1.0.0
BMI is weight divided by height in metres squared, classified against the WHO international categories or, where selected, the lower NICE NG246 thresholds. Waist-to-height ratio is waist divided by height and classified against the NICE NG246 bands (0.4-0.49 healthy, 0.5-0.59 increased, 0.6 and above high), which NICE states apply below a BMI of 35. Body fat is estimated either from the US Department of Defense circumference equations (inputs converted to inches inside the logarithm, as published) or from the Jackson-Pollock three-site skinfold equations converted to percent fat with Siri or Brozek, or taken as entered. Fat-free mass index is fat-free mass over height squared, with the Kouri 1995 height normalisation of 6.3 x (1.8 - height in m); the widely implemented 6.1 variant is shown alongside because it cannot be sourced to the paper.
What it cannot do
- BMI cannot distinguish muscle from fat and says nothing about fat distribution. It misclassifies muscular people and older people in opposite directions.
- The Kouri 1995 height-normalisation coefficient is published as 6.3 in the paper abstract. The 6.1 in near-universal use online could not be sourced to the paper; both are reported. See GAPS.md.
- FFMI interpretation bands in common circulation have no peer-reviewed source and are deliberately not used.
- The DoD circumference equations carry a standard error of about 3.5 percentage points of fat plus a documented systematic bias against DXA that runs opposite ways in men and women.
- Skinfold equations depend heavily on technique and on caliper model, and the original work used Lange calipers.
- Siri and Brozek conversions differ by about one percentage point of body fat, and two different equations both circulate under the name Brozek 1963. The one implemented here is 457/density - 414.2.
- The WHO did not redefine BMI categories for Asian populations; national bodies have adopted different lower thresholds from the same evidence, and this tool implements the NICE version.
Sources
- World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894. Geneva: WHO; 2000View source (opens in a new tab)
- NICE. Overweight and obesity management. NICE guideline NG246; January 2025, updated January 2026View source (opens in a new tab)
- Ashwell M, Gunn P, Gibson S. Waist-to-height ratio is a better screening tool than waist circumference and BMI for adult cardiometabolic risk factors. Obes Rev 2012;13(3):275-286DOI 10.1111/j.1467-789X.2011.00952.x (opens in a new tab)PMID 22106927 (opens in a new tab)
- Kouri EM et al. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clin J Sport Med 1995;5(4):223-228DOI 10.1097/00042752-199510000-00003 (opens in a new tab)PMID 7496846 (opens in a new tab)
- Schutz Y, Kyle UUG, Pichard C. Fat-free mass index and fat mass index percentiles in Caucasians aged 18-98 y. Int J Obes 2002;26(7):953-960DOI 10.1038/sj.ijo.0802037 (opens in a new tab)PMID 12080449 (opens in a new tab)
- US Department of Defense Instruction 1308.3, DoD Physical Fitness and Body Fat Programs Procedures, 5 November 2002, Enclosure 3
- Jackson AS, Pollock ML. Generalized equations for predicting body density of men. Br J Nutr 1978;40(3):497-504DOI 10.1079/BJN19780152 (opens in a new tab)PMID 718832 (opens in a new tab)
- Jackson AS, Pollock ML, Ward A. Generalized equations for predicting body density of women. Med Sci Sports Exerc 1980;12(3):175-181PMID 7402053 (opens in a new tab)
- Siri WE. Body composition from fluid spaces and density. In: Techniques for Measuring Body Composition. National Academy of Sciences; 1961:223-244
- Brozek J, Grande F, Anderson JT, Keys A. Densitometric analysis of body composition. Ann N Y Acad Sci 1963;110:113-140PMID 14062375 (opens in a new tab)
- Potter AW et al. Comparison of Military Body Composition Methods with Dual-Energy X-Ray Absorptiometry. Front Physiol 2022;13:868627DOI 10.3389/fphys.2022.868627 (opens in a new tab)
The formula version is pinned so that a result you saved or shared can be reproduced. When we change a coefficient, the version changes and old results stay attributed to the version that produced them.
Questions people ask
- Why lead with waist-to-height rather than BMI?
- Because it tracks central adiposity, which is where the cardiometabolic risk lives, and it needs one tape measure. BMI is included because clinical thresholds are still written in it, not because it is the better measure.
- Is a BMI in the overweight range a problem?
- On its own it is close to uninformative for an individual, especially a muscular one. Read it alongside the waist ratio, and if the two disagree, trust the waist.