BMI Calculator for Adults
BMI is your weight in kilograms divided by your height in metres squared. A 70 kg adult who is 1.70 m tall has a BMI of 24.2, which is in the healthy range of 18.5 to 24.9.
Your BMI
24.2
Healthy weight
- Under
- Healthy
- Over
- Obese
For adults aged 18 and over. BMI is a screening number, not a diagnosis: it does not measure body fat or muscle. Talk to a doctor about your health.
Show the math
- 1
Turn height into metres
170 cm ÷ 100 = 1.7 m
= 1.7 m
Read the steps as text
- Turn height into metres. 170 cm ÷ 100 = 1.7 m
- Square the height. 1.7 × 1.7 = 2.89 BMI compares weight with height squared, so taller people are not unfairly penalised for being heavier.
- Divide weight by the squared height. 70 kg ÷ 2.89 = 24.22 Rounded to one decimal place: 24.2.
- Read the category. 24.2 → Healthy weight WHO adult ranges: under 18.5 underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight, 30 and above obesity.
- Find the healthy weight range for your height. 18.5 × 2.89 = 53.5 kg to 24.9 × 2.89 = 72 kg Multiplying the squared height by the lower and upper ends of the healthy BMI range gives the weights that fall inside it.
How BMI is calculated
Body mass index compares your weight with your height. The formula is BMI = weight (kg) ÷ height (m)². In pounds and inches it is BMI = weight (lb) ÷ height (in)² × 703, which gives the same answer. The calculator converts imperial measurements to metric first and shows that step, then squares your height and divides your weight by it.
Squaring the height matters. It stops the number from simply rising with height, so a tall person and a short person of the same build get similar BMIs.
BMI categories for adults
The World Health Organization groups adult BMI into ranges: under 18.5 is underweight, 18.5 to 24.9 is a healthy weight, 25 to 29.9 is overweight, and 30 or more is obesity, which is further split into class 1 (30 to 34.9), class 2 (35 to 39.9) and class 3 (40 and above).
The coloured bar shows where you sit on that scale, and the calculator also works backwards to give the range of weights that would put you between 18.5 and 24.9 at your height.
What BMI can and cannot tell you
BMI is a quick screening tool, not a diagnosis. It cannot tell muscle from fat, so very muscular people can score as overweight, and it does not show where body fat is carried. Age, sex and ethnicity also affect what a healthy BMI looks like, and some health bodies use lower thresholds for people of South Asian and some other backgrounds.
This calculator is for adults aged 18 and over. For children and teenagers, BMI is interpreted against growth charts for their age and sex, so ask a doctor or nurse. If you are worried about your weight, a healthcare professional can look at the whole picture.
Frequently asked questions
- What is a healthy BMI?
- For most adults a BMI from 18.5 to 24.9 is classed as a healthy weight. A doctor may interpret your number differently depending on your age, muscle mass and health.
- How do I calculate BMI in pounds and inches?
- Divide your weight in pounds by your height in inches squared, then multiply by 703. For 154 lb and 67 inches: 154 ÷ 4,489 × 703 = 24.1.
- Is BMI accurate for athletes?
- Not always. Muscle weighs more than fat, so a muscular person can have a high BMI without carrying excess body fat.
- Does BMI work for children?
- Not with these adult ranges. Children's BMI is compared with growth charts for their age and sex, so speak to a doctor or nurse.