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- Body mass index (BMI) compares your weight with your height. It is your weight in kilograms divided by your height in meters squared.
- For adults, the World Health Organization groups BMI into underweight (under 18.5), normal (18.5 to 24.9), overweight (25 to 29.9) and obesity (30 and over).
- The calculator also shows the weight range that matches a BMI of 18.5 to 24.9 for your height.
- BMI is a screening tool, not a diagnosis. It does not measure body fat or muscle. This page is general information, not medical advice.
How to use the BMI calculator
Choose your units
Pick Metric for centimeters and kilograms, or US for feet, inches and pounds.
Enter height and weight
Type your height and your weight. In US units, leave inches blank if you are an exact number of feet tall.
Add your age if you like
Open More details to enter your age. Under 18, the calculator gives the BMI number but does not apply the adult categories.
Read your result
You get your BMI, the category, the healthy weight range for your height and the steps used.
How BMI is calculated
BMI formulas
Metric: BMI = weight (kg) ÷ height (m)²
US units: BMI = 703 × weight (lb) ÷ height (in)²
Healthy weight range = 18.5 × height² to 24.9 × height², with height in meters
The factor 703 converts pounds and inches into the metric formula.
Squaring the height means taller people are expected to weigh more, but not in direct proportion. BMI was designed for studying groups of people. Doctors use it as one quick check alongside other measures such as waist size, blood pressure and family history.
BMI categories for adults
WHO adult BMI categories
World Health Organization groups for people aged 18 and over.
Some health bodies use lower cut-offs for people of Asian descent, so ask your doctor which ranges apply to you.
Worked examples
Example 1: a typical adult
A person is 170 cm tall and weighs 70 kg.
This person sits inside the healthy range for their height.
Example 2: US units
A person is 5 ft 7 in tall and weighs 154 lb.
The same method works in either system of units.
Example 3: below the range
A person is 170 cm tall and weighs 50 kg.
A doctor can say whether this matters for the person.
Example 4: above the range
A person is 170 cm tall and weighs 80 kg.
BMI says nothing about muscle, so context matters.
Healthy weight by height
Healthy weight range by height
Weights that give a BMI of 18.5 to 24.9, in kilograms.
A weight inside the range does not guarantee good health, and a weight outside it does not guarantee poor health.
Limits of BMI
BMI cannot tell fat from muscle, so a muscular athlete can have a high BMI with low body fat. It can also miss people who look slim but carry little muscle. It does not show where fat is stored, and fat around the waist carries more health risk than fat elsewhere. Age, sex, ethnicity and pregnancy all change how a BMI should be read. For children and teenagers, doctors compare BMI with growth charts for their age and sex instead of using adult categories. If you are worried about your weight, a doctor can look at the whole picture.
Where a BMI calculator helps
Health check-ins
Get a quick starting number to talk through with a doctor.
Fitness goals
Track how your BMI changes as one part of a wider plan.
Healthy range for your height
See the weight range that matches a normal BMI.
Forms and insurance
Many forms ask for a BMI, and the calculator gives it in seconds.
Work out your daily energy needs with the BMR calculator, or compare formulas with the ideal weight calculator. The protein intake calculator estimates daily protein, and the percentage calculator helps with changes in weight. BMI is general information and not medical advice. Please talk to a doctor or other health professional about your own health.
BMI calculator FAQ
What is a healthy BMI?
For adults, a BMI from 18.5 to 24.9 is considered the normal weight range by the World Health Organization. It is a guide, not a guarantee of good health.
How is BMI calculated?
Divide your weight in kilograms by your height in meters squared. With US units, multiply your weight in pounds by 703 and divide by your height in inches squared.
Is BMI accurate for athletes?
Not always. BMI cannot tell muscle from fat, so people with a lot of muscle can have a high BMI without carrying extra body fat.
Does the calculator work for children?
The number can be calculated, but adult categories do not apply under 18. Children are compared with growth charts for their age and sex, which a pediatrician can use.
Do the BMI categories change with age or sex?
The WHO adult categories are the same for men and women. Age, ethnicity and pregnancy can change how a BMI should be read, so ask a health professional about your own result.
What can I do if my BMI is outside the normal range?
BMI is only a screening tool, so talk to a doctor. They can check other measures and give advice that fits you before you make any changes.