BMI Calculator

How to use it

Enter your height and weight — in metric (centimetres and kilograms) or imperial (feet/inches and pounds) — and BMI is calculated instantly as you type. Switch units with the toggle at any time; your numbers convert automatically. Body Mass Index is your weight divided by the square of your height. In metric terms: weight in kilograms divided by height in metres, squared. In imperial terms, the calculator converts your feet and inches to metres and your pounds to kilograms first, then applies the same formula, so the result is identical either way. For example, someone who is 175 cm tall and weighs 70 kg has a height of 1.75 m. Squaring that gives 3.0625 m². Dividing 70 by 3.0625 gives a BMI of about 22.9 — inside the normal weight range. The result is sorted into one of four standard adult categories: under 18.5 is underweight, 18.5 to 24.9 is normal weight, 25 to 29.9 is overweight, and 30 or above is obese. These thresholds come from World Health Organization guidance and apply to adults; they aren't appropriate for children or teenagers, who need age- and sex-specific growth charts instead. BMI is a useful screening number, not a diagnosis. It doesn't distinguish muscle from fat, so a muscular athlete can show as 'overweight' despite low body fat, while someone with little muscle mass can fall in the 'normal' range while carrying more fat than is healthy. It also says nothing about where fat is stored — fat carried around the waist is linked to more health risk than fat carried elsewhere, even at the same BMI. Pregnancy, older age, and certain ethnic backgrounds also shift how well the standard categories apply. Use your BMI as a starting point, not a final answer. If your result is outside the normal range, or you have concerns about your weight or health, talk to a doctor or registered dietitian — they can look at your whole picture, not just one number.

Frequently asked questions

What is BMI?

Body Mass Index is your weight in kilograms divided by the square of your height in metres. It's a simple screening tool for weight categories.

What are the BMI categories?

Under 18.5 is underweight, 18.5–24.9 is normal, 25–29.9 is overweight, and 30 or more is obese (for adults).

Is BMI accurate for everyone?

No. BMI doesn't distinguish muscle from fat, so it can misclassify athletes and others. It's a starting point, not a diagnosis.

Does BMI apply to children and teenagers?

No. The 18.5–24.9 'normal' range only applies to adults. Children and teens are assessed with age- and sex-specific BMI-for-age percentile charts, since healthy body composition changes a lot as kids grow. Ask a pediatrician if you need to check a child's growth.