Methodology and Sources
How ClearBMI calculates every number on the site, and the guidance behind it.
Formula
BMI = weight (kg) ÷ height (m)². Measurements in other units are converted first, using the exact international definitions: 1 lb = 0.45359237 kg, 1 in = 2.54 cm and 1 stone = 14 lb.
Rounding and categories
BMI is rounded to one decimal place, and the category comes from the rounded value, as on the CDC’s calculator.
- Standard adult categories (WHO, CDC): underweight below 18.5; healthy weight 18.5–24.9; overweight 25–29.9; obesity 30 or more, split into class 1 (30–34.9), class 2 (35–39.9) and class 3 (40 or more).
- Lower cut-offs (WHO 2004, and NICE, the UK’s National Institute for Health and Care Excellence): healthy weight 18.5–22.9, overweight 23–27.4, obesity 27.5 or more.
Healthy weight range
Every whole pound or kilogram whose BMI, rounded to one decimal as on the calculator, falls between 18.5 and 24.9: from 18.45 × height (m)² up to just below 24.95 × height (m)². With the lower cut-offs, the range ends at 22.9.
Waist-to-height ratio
Waist ÷ height, rounded to two decimals. Under 0.5: no increased health risk; 0.5–0.59: increased health risk; 0.6 or more: high health risk (NICE).
Children and teens
BMI-for-age percentiles use the CDC growth charts. Up to the 95th percentile we use the LMS method of the 2000 charts; above it, CDC’s 2022 extended method: percentile = 90 + 10 × Φ((BMI − P95) ÷ σ). Age is counted in completed months. The L, M, S and σ parameters come from CDC’s data file, rounded; checked against every percentile CDC publishes for ages 2 to 19, the largest difference is 0.003 percentile points. Categories: under the 5th percentile, underweight; 5th to under the 85th, healthy weight; 85th to under the 95th, overweight; 95th or above, obesity; 120% of the 95th or above, severe obesity.
Accepted inputs
Height 3–9 ft (91–274 cm), weight 55–1,000 lb (25–454 kg) and waist 16–98 in (40–250 cm). A height typed in meters into the centimeter field, such as 1.75, is read as 175 cm. For children: age 2 to 19 years, height 2–7 ft (60–220 cm) and weight 15–400 lb (7–180 kg).
Testing
The calculator’s math is covered by automated checks, including every category boundary, unit conversion and rounding rule. The charts and tables on the site are generated by the same code as the calculator, so they always agree with it.
Where the calculation happens
Everything runs in your browser; your measurements aren’t sent to a server. Privacy →
Sources
- CDC: Adult BMI Calculator
- CDC: Child and Teen BMI Calculator
- CDC: Extended BMI-for-age growth charts, data file (2022)
- NIH NHLBI: Calculate Your BMI
- WHO Expert Consultation (2004): Appropriate body-mass index for Asian populations
- NICE NG246 (2025): Overweight and obesity management
- Rubino et al. (2025): Definition and diagnostic criteria of clinical obesity
- ASMBS and IFSO (2022): Indications for metabolic and bariatric surgery
- NIH NIDDK: Prescription Medications to Treat Overweight and Obesity
Updates
We review these sources when guidance changes and record changes in the changelog. Spotted an error? Tell us.