How BMI Logic works
Every result comes from a published formula. Here's exactly what we calculate and where each cutoff comes from. Everything runs in your browser; nothing you enter is sent anywhere.
Standard BMI
Weight (kg) ÷ height (m)². Categories from the World Health Organization, also used by the CDC: under 18.5 underweight, 18.5–24.9 healthy weight, 25–29.9 overweight, 30+ obesity (30–34.9 class 1, 35–39.9 class 2, 40+ class 3).
Height-corrected BMI ("New BMI")
1.3 × weight (kg) ÷ height (m)2.5, proposed by Oxford mathematician Nick Trefethen to reduce standard BMI's bias against tall people and toward short people. Shown for comparison; the same category cutoffs are applied. Source: Trefethen, University of Oxford.
Waist-to-height ratio
Waist ÷ height. Bands from the UK's National Institute for Health and Care Excellence (NICE, 2022) for adults: 0.40–0.49 healthy, 0.50–0.59 increased risk, 0.60+ high risk. Waist is measured halfway between the lowest rib and the top of the hip bone after breathing out. Source: NICE guideline summary.
Relative Fat Mass (estimated body fat %)
Men: 64 − 20 × (height ÷ waist). Women: 76 − 20 × (height ÷ waist). Developed and validated against DEXA body scans of U.S. adults (NHANES). The study used 22.8% (men) and 33.9% (women) as obesity cutoffs. Source: Woolcott & Bergman, Scientific Reports, 2018.
Body Roundness Index
364.2 − 365.5 × √(1 − (WHtR ÷ π)²). Most adults score about 3 to 7; a large U.S. study linked 6.9 or higher with higher mortality. Source: Thomas et al., 2013, and follow-up research.
Fat Mass Index and Fat-Free Mass Index (if you enter body fat %)
FMI = fat mass (kg) ÷ height (m)². Normal is 3–6 for men and 5–9 for women; above 9 (men) or 13 (women) is in the obesity range, based on DEXA reference data from U.S. adults. FFMI = lean mass (kg) ÷ height (m)², normalized with + 6.1 × (1.8 − height in m). A 1995 study of bodybuilders placed the drug-free limit for men near 25. Sources: FMI classification (Kelly et al. 2009, NHANES); FFMI method (Kouri et al. 1995).
Age-aware view (65+)
A 2014 meta-analysis in The American Journal of Clinical Nutrition found the lowest mortality risk for adults 65 and older at a BMI of about 27.5, with higher risk below about 23. Source: Deakin University summary of Winter et al., 2014.
Frame size
Height ÷ wrist circumference. Men: over 10.4 small, 9.6–10.4 medium, under 9.6 large. Women: over 11.0 small, 10.1–11.0 medium, under 10.1 large.
BMI Logic gives screening estimates for adults 20 and older. It isn't medical advice, isn't for use during pregnancy, and doesn't replace your doctor.