Body composition calculators
How much of you is fat, muscle and bone, and how that compares with recognised reference ranges. BMI is the quickest screen; the tape-based and index tools here add what BMI cannot see.
13 calculators
BMI Calculator
Free BMI calculator with WHO standard and Asia-Pacific classifications side by side, plus the 2025 multi-measure check. Metric, imperial and stone.
Relative Fat Mass (RFM) Calculator
Estimate body fat percentage from just height and waist with the Relative Fat Mass equation, a 2018 index that beat BMI against DEXA in NHANES data.
Weight loss percentage calculator
Work out what percentage of your starting weight has changed, where that sits against the 5 and 10 per cent clinical marks, and your average weekly rate.
Skinfold Body Fat Calculator (Jackson–Pollock)
Estimate body fat from caliper skinfolds with the Jackson-Pollock 3-site and 7-site equations, showing both the Siri and Brozek density conversions.
Asian BMI Calculator
BMI calculator using the WHO Asia-Pacific classification (23 overweight, 25 obese) shown alongside the standard scale, with the 2004 action points explained.
Body Fat Calculator (US Navy Method)
Estimate body fat from tape measurements with the US Navy circumference method, cross-checked against three independent equations and an honest error range.
Army Body Fat Calculator
Estimate body fat with the military tape method, compare it with the AR 600-9 age-band limits, and see what the Army's 2026 waist-to-height rule changed.
Ideal Body Weight Calculator
The four clinical ideal weight formulas side by side (Hamwi, Devine, Robinson and Miller), each with the BMI it implies at your height, and why they disagree.
Healthy Weight Range Calculator
The WHO healthy weight range for your height in kg, lb or stone, plus where your current weight sits in it, the Asia-Pacific band and the evidence after 65.
Lean Body Mass Calculator
Work out lean body mass from a measured body fat percentage, or compare the Boer, Hume and James predictions with the honest spread between them stated.
FFMI Calculator
Calculate your fat-free mass index, normalised for height, with interpretation bands and the honest error propagation from your body fat estimate.
Body Frame Size Calculator
Estimate a small, medium or large skeletal frame from your height-to-wrist ratio and from elbow breadth, with both published threshold tables shown in full.
Body Recomposition Calculator
Two weigh-ins plus body-fat percent: see fat mass and lean mass change when the scale is almost flat. Same method both times, weeks apart, not mixed.
Guides that explain these numbers
- BMI, explained properly
- Why BMI is wrong for athletes
- Asian BMI cut-offs, explained without the muddle
- What changed in the 2025 obesity definition
- How to measure your waist accurately
- Body fat percentage methods, compared honestly
- How accurate is the Navy body fat formula?
- How to track body composition without a DEXA scan
- BMI vs body fat vs waist ratio: which number matters?
- How much protein do you actually need?
- Metabolic adaptation and weight loss plateaus
- How to estimate your one rep max
- Child BMI percentiles, explained
- Pregnancy weight gain, explained
- Body recomposition versus weight loss
- What metabolic age actually is
Every formula on these pages is written out with its primary source on the methodology page. Results are informational. See the medical disclaimer.