BMI Calculator for Adults
Calculate your body mass index in metric or imperial units. BMI is a screening measure for adults. It is not a diagnosis.
Quick answer. BMI equals weight in kilograms divided by height in meters squared. For adults, 18.5 to 24.9 is the WHO normal weight range.
The formula
Metric: BMI equals weight in kilograms divided by height in meters, squared.
Imperial: BMI equals 703 multiplied by weight in pounds, divided by height in inches, squared.
Adult weight categories
The World Health Organization uses these ranges for adults: below 18.5 is underweight, 18.5 to 24.9 is normal weight, 25.0 to 29.9 is overweight, and 30.0 or above is in the obesity range.
Worked example, metric
A person weighs 70 kg and is 175 cm tall. Height in meters is 1.75. Squared, that is 3.0625.
BMI equals 70 divided by 3.0625, which is 22.9 to one decimal place. This is in the normal weight range.
Worked example, imperial
A person weighs 154 lb and is 5 ft 9 in tall. Total height is 69 inches.
BMI equals 703 multiplied by 154, divided by 69 squared (4,761). The result is 22.7, which is in the normal weight range.
Common mistakes
Entering height in feet with decimals. 5.9 feet is 5 feet 10.8 inches, not 5 feet 9 inches. Use feet and inches separately.
Using BMI for children or teenagers. Their assessment uses age and sex specific percentile charts.
Treating BMI as body fat. Muscle, frame size and age change what the same number means.
When to speak to a clinician
BMI is a starting point. A doctor or qualified health professional can assess weight in the context of your history, waist size, blood pressure, blood tests and other factors.
Why BMI is used
BMI is a simple, low cost measure that is used in large population studies and in clinics as a first step. It needs only weight and height, so it is easy to repeat over time.
Because it is simple, it cannot tell muscle from fat, or fat in the belly from fat under the skin. Treat it as a starting point for a conversation with a clinician.
Groups where BMI needs care
Athletes and people with high muscle mass can have a high BMI with a low body fat level. Older adults can have a normal BMI with a high body fat level and low muscle mass.
BMI is not a reliable guide during pregnancy, because healthy weight gain is expected. Ask your midwife or doctor about gestational weight targets.
How to measure correctly
Weigh yourself in the morning, after using the toilet and before eating, in light clothing. Measure height without shoes, standing straight against a flat wall.
Round your height to the nearest millimetre or sixteenth of an inch, and check that you entered the correct units before reading the result.
Questions
Can I use this for children?
No. This page covers adults. Children and teenagers are assessed with age and sex specific percentile charts.
Is BMI a diagnosis?
No. It is a screening measure. Discuss your result with a doctor or qualified health professional.
What does 0 to 11.9 inches mean?
Inches must be at least 0 and less than 12. For example, 5 ft 11 in is valid, while 5 ft 13 in is not.
Is a BMI of 25 unhealthy?
A BMI of 25 is at the start of the overweight range. It is a screening number only. Waist size, blood pressure and blood tests give a fuller picture, and a clinician can interpret them with you.
Does BMI work for athletes?
BMI can overstate body fat in people with high muscle mass. A clinician can use other measures for athletes.
Last updated: 2026-10-08. Adult BMI categories follow the World Health Organization classification. The calculation uses the standard BMI formula. Formula and WHO categories checked against the stated method. Clinical review is pending.