Body mass index, or BMI, is a quick way to compare your weight with your height. Doctors and health organisations use it as a first screening tool because it’s cheap, fast and needs nothing more than a scale and a tape measure. Enter your height and weight, and this calculator gives you your BMI and the range it falls into. Treat it as a starting point for a conversation about health, not a verdict on it.
Choose your units: metric (kilograms and centimetres) or imperial (pounds and feet/inches).
Enter your height and your weight.
Read your BMI and the category it falls into.
For the most reliable number, measure your height without shoes and weigh yourself at a similar time of day, ideally in the morning.
Metric: BMI = weight in kilograms ÷ (height in metres)²
Imperial: BMI = 703 × weight in pounds ÷ (height in inches)²
Metric. A person weighs 70 kg and is 1.75 m tall.
1.75 × 1.75 = 3.0625, and 70 ÷ 3.0625 = 22.9
Imperial. A person weighs 160 lb and is 5 ft 9 in (69 inches) tall.
69 × 69 = 4,761, and 703 × 160 ÷ 4,761 = 23.6
The World Health Organization groups adult BMI like this:
BMI
Category
Below 18.5
Underweight
18.5 to 24.9
Healthy weight
25.0 to 29.9
Overweight
30.0 and above
Obesity
Obesity is often split further into class 1 (30 to 34.9), class 2 (35 to 39.9) and class 3 (40 and above).
For a person who is 1.75 m tall, the healthy range of 18.5 to 24.9 corresponds to roughly 57 to 76 kg. Doing the same sum for your own height shows what the range means in everyday terms.
BMI is a ratio of weight to height, and nothing more. It doesn’t know what that weight is made of, so it has real blind spots:
Muscle. Muscle is denser than fat. A muscular athlete at 1.80 m and 100 kg gets a BMI of 30.9, which falls in the obesity range, even with a low body fat percentage.
Where fat is stored. Fat around the waist and organs carries more health risk than fat elsewhere, and BMI can’t distinguish between them. Waist measurement is often used alongside it.
Age. Older adults naturally lose muscle and bone, so a BMI in the “healthy” range can still hide low muscle mass. Younger people are still growing.
Ethnicity. Health risks linked to body fat can appear at lower BMI values in some populations, particularly people of South and East Asian descent. Health bodies in some countries use lower cut-offs for them.
Pregnancy and breastfeeding. BMI isn’t meaningful in these periods.
Children and teenagers. The adult categories don’t apply. Under 18, BMI is compared with growth charts for the child’s age and sex, and a doctor or nurse should interpret it.
A single number says very little about how healthy you are. If you want a fuller view, look at BMI alongside:
Waist circumference
Blood pressure
Blood sugar and cholesterol levels
How active you are, how well you sleep and how you feel day to day
Your family history
A doctor can put these together in a way a calculator can’t.
Treating the result as a diagnosis. BMI is a screening tool. It indicates, it doesn’t diagnose.
Using the wrong units. Typing pounds into a kilogram field, or centimetres where metres are expected, gives a nonsense result. Check the unit boxes.
Using the adult chart for a child. It won’t give a meaningful answer.
Fixating on the number. A BMI of 24.9 isn’t “healthy” and 25.1 isn’t “unhealthy”. The categories are broad guidelines, not sharp lines.
Making big changes based on this alone. If your result worries you, speak to a doctor before changing your diet or exercise routine drastically.
Is BMI accurate?
It’s accurate as a measure of weight relative to height, but only a rough guide to health. It’s useful for looking at populations and for a first check, and less useful for individuals with unusual body composition.
Why is my BMI “overweight” when I exercise a lot?
Muscle weighs more than fat, so people with a lot of muscle can have a high BMI without carrying excess body fat. If that sounds like you, other measures such as waist size and body fat percentage give a clearer picture.
What’s a healthy BMI?
For most adults, 18.5 to 24.9 is considered the healthy range. Your ideal weight within or outside it depends on your build, age and health, so it’s worth asking a healthcare professional.
Does BMI differ for men and women?
The formula and adult categories are the same. Women generally carry more body fat than men at the same BMI, which is one more reason the number should be read with care.
Can I use this calculator for my child?
No. Children’s BMI has to be compared with age- and sex-specific growth charts. A paediatrician or school nurse can do this.
How often should I check my BMI?
There’s no need to check it often. Weight naturally shifts from day to day, so looking at the trend over months tells you far more than a single reading.
This calculator is for general information only and is not medical advice. BMI is a screening tool and doesn’t diagnose any condition. Please speak to a doctor or other qualified healthcare professional about your weight and health.