How to Calculate BMI and What the Number Means
Body mass index (BMI) is a quick screening number that compares weight with height. This guide shows how to calculate it and how to read the result.
The BMI formulas
- US units: BMI = 703 × weight in pounds ÷ (height in inches)²
- Metric units: BMI = weight in kg ÷ (height in meters)²
Example in US units
A person who is 5 ft 6 in tall (66 inches) and weighs 150 lb has a BMI of 703 × 150 ÷ 66² = 105,450 ÷ 4,356, which is about 24.2.
Example in metric units
A person who is 1.75 m tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625, which is about 22.9.
BMI categories for adults
- Below 18.5: underweight
- 18.5 to 24.9: healthy weight
- 25.0 to 29.9: overweight
- 30.0 and above: obesity
These ranges are the common screening categories for adults aged 20 and over.
What BMI can and cannot tell you
BMI is a screening tool. It does not measure body fat directly, and it does not show where fat is stored. A very muscular person can have a high BMI and be healthy. An older adult can have a BMI in the healthy range but low muscle mass. BMI also may not fit all body types and ethnic groups equally well.
Children and teens
Adult categories do not apply to children and teens. For them, BMI is compared with growth charts for their age and sex. A child's doctor can explain the result.
Other measures
Doctors may also look at waist circumference, blood pressure, blood sugar, cholesterol, family history and lifestyle. BMI is one piece of information and should not be used alone to judge health.
How to use your result
If your BMI is outside the healthy range, it does not mean something is wrong, and a healthy-range BMI does not guarantee good health. Use the number as a reason to ask a doctor or other health professional about your own situation.
Calculate yours
Enter your weight and height into our BMI Calculator to see your BMI and category instantly. Nothing you type leaves your device.
Healthy weight range for your height
You can reverse the formula to find the weights that give a BMI between 18.5 and 24.9. Weight in pounds = BMI × (height in inches)² ÷ 703. For someone 5 ft 9 in (69 inches), that is about 125 to 169 lb. For someone 5 ft 4 in (64 inches), it is about 108 to 145 lb. These ranges only describe the BMI categories and are not personal targets.
How much a small change moves BMI
For a person 5 ft 9 in tall, a 5 lb change in weight shifts BMI by about 0.74. That is why a BMI is better viewed as a rough guide, and why daily changes in weight do not matter much.
Frequently asked questions
Is BMI accurate?
It is a useful screening number for groups, but less precise for individuals.
Which units should I use?
Either. The formulas give the same result when units are converted correctly.
Is this medical advice?
No. This guide is general information. Talk to a healthcare professional about your health.