Body Composition Calculator: A Fuller Picture Than BMI
BMI was never designed to describe individuals. It can't tell muscle from fat, or see where your body carries weight. UK guidance now suggests a simpler check alongside it: keep your waist to less than half your height. Starting with nothing more than a tape measure, this calculator builds seven validated measurements into a fuller picture.
Private & anonymous
No account needed
3–5 minutes
Evidence-based methods
Understanding the metrics
What each measurement can — and can’t — tell you
Each measure looks at your body from a different angle. Together they say far more than any single number could.
Waist-to-height ratio
WHtR
Keep your waist to less than half your height.
The rule of thumb UK guidance now highlights alongside BMI, because it sees centrally stored weight that BMI cannot.
The finer detail
Research comparing the two suggests WHtR performs more consistently across ages, sexes and ethnic groups than BMI. NICE aims this at adults with a BMI below 35 kg/m²; above that, treat it as one input among several rather than your primary measure.
Source: NICE, Obesity: identification, assessment and management (waist-to-height recommendation added 2022); Ashwell & Gibson, systematic reviews of waist-to-height ratio.
Waist-to-hip ratio
WHR
Where you carry weight, not just how much.
Two people can carry the same weight in very different places — and weight carried centrally relates more strongly to long-term health than weight on the hips and thighs.
The finer detail
We use the WHO's single "substantially increased risk" cut-off (over 0.90 for men, over 0.85 for women) rather than a softer multi-tier scale. The cut-offs were derived on predominantly White European populations; other ethnic groups can reach equivalent risk at different values.
Source: World Health Organization, Waist circumference and waist–hip ratio: report of a WHO expert consultation (2008).
Body Roundness Index
BRI
Your body's shape, modelled as an ellipse from waist and height.
The middle range carries the lowest risk — so a very low result is not the same thing as "low risk".
The finer detail
The large US cohort study behind our category bands found a U-shaped relationship with mortality: both very low and very high results were linked to higher mortality than the middle range.
Source: Thomas et al. (2013), Obesity (formula); Zhang et al. (2024), JAMA Network Open (category cut-points: one cohort study, not a systematic review).
A Body Shape Index
ABSI
Is your waist larger or smaller than expected for your height and weight?
Isolates what your waist adds beyond BMI, shown as a percentile for your age and sex rather than a low/average/high verdict.
The finer detail
The percentile compares you with adults of the same age and sex in the NHANES 1999–2004 reference population. A percentile is a screening signal, not a diagnosis.
Source: Krakauer & Krakauer (2012), PLoS ONE.
Waist circumference
WC
The most direct measure of central weight.
We use the two-stage NICE "action levels" rather than a single cut-off, since guidance already advises personal action at the lower level.
The finer detail
Action levels are 94/102 cm for men and 80/88 cm for women, derived on predominantly White European populations. South Asian, Chinese, Black African/Caribbean and other groups can reach equivalent risk at smaller waist measurements.
Source: Lean, Han & Morrison (1995), BMJ; NICE obesity guidance.
Body fat percentage
BF%
How much of your weight is fat rather than lean tissue.
The one distinction BMI can never make — estimated here by tape measure or skinfold calipers.
The finer detail
Two validated routes: the US Navy circumference method (tape) and the Jackson-Pollock three-site skinfold method (calipers, sex-specific sites), which we frame as an in-clinic, practitioner-taken measurement rather than a self-measurement guarantee.
Every calculation runs on the published formulas, not simplified approximations: BMI, the direct waist ratios, the Body Roundness Index eccentricity formula (Thomas et al., 2013, with category bands from Zhang et al., 2024), A Body Shape Index (Krakauer & Krakauer, 2012, shown as an age- and sex-matched percentile rather than a risk verdict), and body fat by the US Navy circumference equations (Hodgdon & Beckett, 1984) or the Jackson-Pollock three-site skinfold equations (1978 men; 1980 women).
The Navy tape method sits typically within around ±3.5% of laboratory reference methods and is built for self-measurement. Jackson-Pollock skinfolds can match it, but only with exact site placement and practised technique — which is why we treat that route as an in-clinic option. Measuring the same way each time matters more than which method you pick.
On BMI above 35
NICE's "keep your waist under half your height" guidance is aimed primarily at adults with a BMI below 35 kg/m². Above that, treat waist-to-height ratio as one input among several rather than your primary measure.
On ethnicity
The waist thresholds here come from studies of predominantly White European populations. South Asian, Chinese, Black African/Caribbean and other groups can reach the same metabolic risk at smaller waist measurements — treat these cut-offs as a general guide rather than an exact line.
These results are estimates for information and self-reflection, not a diagnosis — they can't account for your medical history, medication or individual circumstances. If anything in your results concerns you, a GP or practice nurse can put it in proper context.
References · 11 sources
National Institute for Health and Care Excellence. Obesity: identification, assessment and management. Waist-to-height ratio recommendations added in the 2022 update.
Ashwell M, Gibson S. Waist-to-height ratio as an indicator of early health risk. BMJ Open, 2016.
World Health Organization. Waist circumference and waist–hip ratio: report of a WHO expert consultation. Geneva, 2008.
Thomas DM, Bredlau C, Bosy-Westphal A, et al. Relationships between body roundness with body fat and visceral adipose tissue emerging from a new geometrical model. Obesity, 2013.
Zhang X, Ma Y, Yang W, et al. Body Roundness Index and All-Cause Mortality Among US Adults. JAMA Network Open, 2024;7(6):e2415051.
Krakauer NY, Krakauer JC. A new body shape index predicts mortality hazard independently of body mass index. PLoS ONE, 2012.
Lean MEJ, Han TS, Morrison CE. Waist circumference as a measure for indicating need for weight management. BMJ, 1995;311:158–161.
Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men and women from body circumferences and height. Naval Health Research Center, 1984.
Jackson AS, Pollock ML. Generalized equations for predicting body density of men. British Journal of Nutrition, 1978.
Jackson AS, Pollock ML, Ward A. Generalized equations for predicting body density of women. Medicine & Science in Sports & Exercise, 1980.
Kouri EM, Pope HG Jr, Katz DL, Oliva P. Fat-free mass index in users and non-users of anabolic-androgenic steroids. Clinical Journal of Sport Medicine, 1995;5(4):223–228.
