BMI Calculator

The BMI Calculator computes body mass index from your height and weight using the standard WHO formula, classifies the result into the conventional categories (underweight, healthy, overweight, obese I-III), and shows the weight range that would place you in the healthy category for your height. Enter values in either metric (kg and cm) or imperial (lb and ft/in) — the calculator converts internally and shows the result in both systems. This is the calculation doctors, public health agencies, and health apps use to get a quick population-level view of weight relative to height.

BMI is useful but imperfect. It is cheap to compute (you only need a scale and a measuring tape), it correlates with population health outcomes, and it is the standard metric on medical forms. It is also notoriously inaccurate for muscular athletes (who often classify as overweight despite low body fat), pregnant women, growing children (who need age-specific pediatric BMI charts, not adult ranges), the elderly (where age-adjusted ranges may differ), and people with unusual body frames. The calculator shows the BMI number, the WHO category, a visual position on the BMI scale, and an honest note about limitations so you can interpret the number in context.

BMI is defined as body mass in kilograms divided by the square of height in meters: BMI = kg / m^2. The formula is dimensionally weight per area (kg per m^2), which is what the units should be written as formally, though almost nobody does that in casual use.

Converting imperial to the same formula: BMI = (lb × 703) / (inches^2). The factor 703 comes from unit conversion: 1 lb / inch^2 × (0.453592 kg / lb) × (39.37^2 inches^2 / m^2) ≈ 703.07. Using 703 exactly gives accuracy to four significant figures, which is plenty for BMI reporting.

WHO classification uses these thresholds (adults, 20+ years):
- Underweight: BMI < 18.5
- Healthy: 18.5 ≤ BMI < 25
- Overweight: 25 ≤ BMI < 30
- Obese class I: 30 ≤ BMI < 35
- Obese class II: 35 ≤ BMI < 40
- Obese class III (severe): BMI ≥ 40

Some sources use different thresholds for Asian populations (overweight at 23, obese at 27.5) based on population-specific research showing higher metabolic disease risk at lower BMI. The calculator offers both WHO and Asian-Pacific variants.

Healthy weight range for a given height: weight_min = 18.5 × m^2 and weight_max = 25 × m^2 (exclusive). For a 175 cm tall adult: weight_min = 18.5 × 1.75^2 ≈ 56.7 kg; weight_max = 25 × 1.75^2 ≈ 76.6 kg. The calculator shows both in kg and lb.

Known limitations are well documented. BMI cannot distinguish muscle from fat, so a body-builder with 8% body fat often scores overweight or obese. BMI also fails for pregnancy, children (who need age-and-sex-specific percentile charts from the CDC or WHO growth standards), and for some populations where the thresholds do not reflect disease risk accurately. For a more accurate body composition assessment, use body-fat percentage (DXA, skinfold, BIA) or waist-to-hip ratio alongside BMI.

Compared to BMI charts in doctors\u2019 offices, this tool gives exact values with metric and imperial input at once, avoids the rounding of printed charts, and includes category boundaries precise to one decimal place.

Compared to fitness app BMI calculators, this tool is faster (no account required) and shows the context and limitations clearly, which many apps skip or hide. For serious body composition tracking, use an app plus a DXA scan or skinfold measurement — not BMI alone.

Compared to body-fat percentage measurement, BMI is much faster and cheaper (needs only a scale and measuring tape) but less informative. They are complementary: BMI for population-level screening, body-fat for individual body-composition tracking.