BMI: How to Calculate Your Body Mass Index and What Each Range Means

September 29, 2026

Body Mass Index (BMI) relates weight to height and is the most widely used screening to see whether an adult's weight is in a healthy range. It is easy to calculate – but it has important limits worth knowing.

The formula

BMI = weight (kg) ÷ height (m)²

Example: 70 kg and 1.70 m. Height squared is 1.70 × 1.70 = 2.89; 70 ÷ 2.89 = 24.2.

WHO table for adults

BMI	Category
Below 18.5	Underweight
18.5 to 24.9	Normal weight
25 to 29.9	Overweight
30 to 34.9	Obesity class I
35 to 39.9	Obesity class II
40 or more	Obesity class III

Examples

Height	Weight	BMI	Category	Normal weight range
1.70 m	70 kg	24.2	Normal weight	53.5 to 72.0 kg
1.60 m	80 kg	31.3	Obesity class I	47.4 to 63.7 kg
1.75 m	55 kg	18.0	Underweight	56.7 to 76.3 kg

The normal weight range for a height is the weight that gives a BMI between 18.5 and 24.9: 18.5 × height² to 24.9 × height².

Limits of BMI

It does not tell muscle from fat: muscular athletes can have an overweight BMI with little fat.
It does not show where fat is: waist circumference is a better sign of cardiovascular risk.
Older adults: many professionals use slightly different ranges for people over 60.
Children and teenagers: BMI is read on percentile charts by age and sex, not with this table.
Pregnancy: weight gain follows its own prenatal guidance.

Work out yours

The BMI calculator takes kilograms and metres or pounds and inches, and shows the category and the healthy weight range for your height. Nothing you type leaves your device.

BMI is a screening tool, not a diagnosis. For an assessment of your health, see a professional.

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