BMI Calculator
Calculate your body mass index from height and weight — in imperial (ft+in/lb) or metric (cm/kg), with the WHO category band.
Healthy weight
24.4 BMI
BMI 18.5 – 24.9
A BMI in this range is associated with the lowest population-level health risk for most adults — but it is a single rough indicator, not a diagnosis.
Results update as you type. BMI is a population-level screening tool, not a diagnosis. Athletes, pregnant people, the very tall or very short, and people with very high muscle mass should treat the result with caution.
Formula
BMI is one division:
BMI = weight_kg / (height_m × height_m).
The imperial form folds in the lb→kg and inches→metres conversions:
BMI = (weight_lb × 703) / (height_in × height_in).
Both give the same number for the same person — only the input units change.
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Frequently asked questions
What's the BMI formula in imperial units?▾
BMI = (weight in pounds × 703) / (height in inches)². For a 5'9" person at 165 lb: height 69 in, BMI = (165 × 703) / (69 × 69) = 24.4. The 703 conversion factor folds in the lb→kg and inches→metres conversions so you get the same BMI number as the metric formula on the same person.
What are the standard BMI categories in the US?▾
The CDC uses the WHO bands: under 18.5 = underweight; 18.5–24.9 = healthy weight; 25–29.9 = overweight; 30+ = obesity (Class I 30–34.9, Class II 35–39.9, Class III ≥40). The American Diabetes Association recommends earlier diabetes screening at BMI ≥23 for Asian Americans because cardiometabolic risk in that population rises at a lower threshold.
Is BMI a good measure of health?▾
It's a rough screening tool — useful at population scale, blunt at the individual level. BMI doesn't distinguish muscle from fat (athletes commonly score 'overweight'), doesn't measure where fat is stored (visceral fat is far more harmful than subcutaneous), and doesn't account for age, sex, or ethnicity. Combine it with waist circumference, blood pressure, lipid panel, and HbA1c for a meaningful picture of cardiometabolic health.
What's a healthy waist size?▾
CDC and ADA guidance: men with a waist over 40 inches (102 cm) and women over 35 inches (88 cm) face increased cardiometabolic risk independent of BMI. Asian American thresholds are lower — about 35.4 in (90 cm) men and 31.5 in (80 cm) women — because risk rises at smaller circumferences.
Are children's BMI bands different?▾
Yes — children and teens (2–19) use BMI-for-age percentiles plotted on CDC growth charts, not the adult bands. Under the 5th percentile is underweight; 85th–95th is overweight; 95th+ is obese. The CDC's online BMI calculator handles paediatric cases properly. Use this calculator for adults 18+.
Why is BMI used despite its limits?▾
Because it's cheap, fast, and reasonably correlated with body-fat percentage at a population level. Doctors use it as a screening trigger — a high BMI prompts further tests, not a diagnosis on its own. More accurate measures (DEXA scan, hydrostatic weighing, bioelectrical impedance) are slower, costlier, or both.