How is BMI calculated?

BMI is calculated as weight (kg) divided by height squared (m²). For imperial units: BMI = (weight in lbs × 703) ÷ (height in inches)². A BMI of 18.5–24.9 is considered normal weight.

What is a healthy BMI range?

For adults, a BMI between 18.5 and 24.9 is classified as normal weight. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obese.

Is BMI accurate for everyone?

BMI is a useful screening tool but has limitations. It doesn't distinguish between muscle and fat, so athletes may score high despite being healthy. It also varies by age, gender, and ethnicity. Use it as one indicator alongside other health measures.

Is this BMI calculator free?

Yes, it is free, needs no sign-up, and runs entirely in your browser — your numbers are never sent to a server.

BMI Calculator — Body Mass Index

How BMI is calculated

Body Mass Index (BMI) is a simple number calculated from your weight and height. It provides a quick screening to categorise weight status and is widely used by doctors and health organisations worldwide.

The formula

BMI = weight (kg) ÷ height (m)²

For imperial units: BMI = (weight in lbs × 703) ÷ (height in inches)². For example, a person who weighs 170 lbs and is 5′9″ (69 inches): (170 × 703) ÷ (69²) = 119,510 ÷ 4,761 = 25.1.

BMI categories

Underweight: BMI below 18.5
Normal weight: BMI 18.5–24.9
Overweight: BMI 25.0–29.9
Obese: BMI 30.0 and above

Limitations of BMI

BMI is a useful starting point but doesn't tell the whole story. It doesn't distinguish between muscle and fat — muscular athletes may have a high BMI while being perfectly healthy. It also doesn't account for bone density, body composition, fat distribution, age, or sex. Women typically have more body fat than men at the same BMI. Older adults may have more body fat than younger people at the same BMI. Use BMI alongside other measures like waist circumference, body fat percentage, and blood work.

Related calculators

Pair your BMI with the TDEE calculator to find out how many calories you need daily. Track your weight goals with the compound interest calculator — see how small consistent savings grow, just like small consistent habits.

Frequently asked questions

How is BMI calculated?: Weight in kilograms divided by height in metres squared. For imperial: (lbs × 703) ÷ inches².
What is a healthy BMI?: 18.5–24.9 for adults. Below 18.5 is underweight, 25+ is overweight, 30+ is obese.
Is BMI accurate for everyone?: It's a screening tool, not a diagnosis. It can misclassify muscular people and doesn't account for age, sex, or fat distribution.
Is this calculator free?: Yes, free, no sign-up, runs entirely in your browser.