BMI Calculator

What Is BMI?

Body Mass Index (BMI) is a number derived from your height and weight. It was developed in the 1830s by Adolphe Quetelet as a population-level statistical tool, not as an individual diagnostic measure. Despite this, it became the most widely used weight classification system globally because it is cheap, fast, and requires no special equipment.

BMI Formula

Metric:

Imperial:

This calculator uses metric inputs (cm and kg). Convert your measurements before entering if needed.

WHO BMI Categories

Practical Worked Example

Person: Height 175 cm (1.75 m), Weight 75 kg

$BMI = \frac{75}{1.75^2} = \frac{75}{3.0625} \approx 24.5$

Result: Normal weight (18.5–24.9 range).

Now raise weight to 85 kg:

$BMI = \frac{85}{3.0625} \approx 27.8$

Result: Overweight — moved into the 25–29.9 zone.

Asian Population Thresholds

The standard WHO thresholds were derived from predominantly European populations. For East and South Asian adults, cardiometabolic risk rises at lower BMI values. The WHO Expert Consultation recommends:

Several Asian countries have adopted these lower cutoffs in national clinical guidelines. If you are of East or South Asian descent, the standard thresholds in this calculator may understate your risk.

What BMI Cannot Tell You

BMI has well-documented limitations:

• Ignores body composition. A 200 lb competitive weightlifter and a sedentary 200 lb person at the same height share the same BMI. The weightlifter's excess weight is muscle.
• Ignores fat distribution. Visceral fat (around the abdomen) is far more metabolically harmful than subcutaneous fat. Two people with identical BMI can have very different disease risk depending on where their fat is stored.
• Not accurate for children. For ages 2–19, BMI is plotted against age- and sex-specific CDC growth charts and expressed as a percentile, not a fixed category.
• Not designed for the elderly. Older adults often have less muscle mass, so normal BMI can still reflect unhealthy body fat percentage.

Complementary metrics: Waist circumference (risk elevated above 88 cm for women, 102 cm for men), waist-to-hip ratio, and body fat percentage via DEXA scan provide a more complete picture.

Estimated Body Fat Percentage (Deurenberg Formula)

BMI alone says nothing about how much of your weight is fat. This calculator adds an estimated body fat percentage using the Deurenberg formula, which factors in sex and age:

$\text{BF\%} = (1.20 \times \text{BMI}) + (0.23 \times \text{age}) - (10.8 \times \text{sex}) - 5.4$

Where:

• $\text{BMI}$ — body mass index from height and weight
• $\text{age}$ — age in years
• $\text{sex}$ — 1 for male, 0 for female

Example — 175 cm, 75 kg, Male, 30 years: $\text{BF\%} = (1.20 \times 24.5) + (0.23 \times 30) - (10.8 \times 1) - 5.4 = 29.4 + 6.9 - 10.8 - 5.4 \approx 20.1\%$

Same profile, Female: $\text{BF\%} = 29.4 + 6.9 - 0 - 5.4 \approx 30.9\%$

This illustrates the sex difference: women carry approximately 6–12% more body fat than men at the same BMI and age — a normal physiological difference. Typical healthy ranges:

The Deurenberg formula carries a standard error of ±3–4 percentage points — treat it as an estimate, not a clinical measurement.

Common Mistakes to Avoid

Confusing weight with BMI. BMI is not your weight — it is weight divided by the square of height. A tall person can weigh more and still have a lower BMI than a shorter, lighter person.

Applying adult BMI cutoffs to children. Adult thresholds (18.5/25/30) do not apply to anyone under 20 years old. Use age-and-sex-specific growth charts for minors.

Treating BMI as a health verdict. BMI is a screening flag, not a diagnosis. It should prompt further evaluation by a healthcare provider — not standalone action.

Ignoring population context. If you are of Asian descent, the standard 25 overweight threshold likely underestimates your risk. Check the 23.0 threshold and discuss with your doctor.