About BMI Calculator

Calculate body mass index with WHO categories — metric or imperial. Part of the DevTools Surf developer suite. Browse more tools in the Calculators collection.

Use Cases

• Track personal BMI against WHO category thresholds
• Convert between metric and imperial for health apps
• Add a BMI calculation to a fitness project prototype
• Build a health-check widget for an internal wellness tool

Tips

• BMI = weight (kg) / height² (m²) — or use unit=imperial for inches/pounds
• The WHO cutoffs aren't perfect — athletes often classify as 'overweight'
• Consider waist-to-height ratio as a complementary measure

Fun Facts

• BMI was invented by Belgian mathematician Adolphe Quetelet in the 1830s — it was never meant to be a medical metric, just a population statistic.
• The BMI categories (underweight, normal, overweight, obese) were standardized by the WHO in 1995. Before that, 'overweight' started at BMI 27.8 for men and 27.3 for women.
• Quetelet's original formula used height^2 for mathematical simplicity, but research shows height^2.5 would be more accurate — taller people are systematically penalized by the standard formula.

FAQ

Metric or imperial?

Both — toggle units. Results are the same BMI value; only the input form changes.

What are the categories?

WHO: <18.5 underweight, 18.5-24.9 normal, 25-29.9 overweight, 30+ obese. Subcategories within obese (30-34.9 class I, 35-39.9 class II, 40+ class III).

Is BMI reliable?

For populations yes. For individuals, particularly athletes or people with high muscle mass, BMI overestimates fat. Complement with body-fat percentage for accuracy.

Why isn't BMI accurate for kids?

Children's healthy BMI ranges change with age and sex. Use a child-specific BMI percentile chart rather than adult BMI categories.