BMI Calculator
Calculate your Body Mass Index instantly in metric or imperial units. See your BMI category and healthy weight range on a clear visual scale.
Body Mass Index (BMI) is a quick way to check whether your weight is in a healthy range for your height. It is used worldwide as a screening tool — a first, low-effort signal that can prompt a closer look, not a diagnosis on its own.
How BMI is calculated
BMI is the Quetelet index: your weight divided by the square of your height.
- Metric:
BMI = weight (kg) ÷ height (m)² - Imperial:
BMI = 703 × weight (lb) ÷ height (in)²
For example, someone who weighs 70 kg and is 1.75 m tall has a BMI of
70 ÷ (1.75 × 1.75) = 22.9, which falls in the healthy range.
What your BMI category means
The World Health Organization classifies adult BMI into four bands:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal (healthy) weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
These thresholds apply to most adults aged 20 and over, regardless of sex. Children and teenagers are assessed differently — see below.
BMI Prime and Ponderal Index
Alongside your BMI, this calculator shows two related numbers that add context:
- BMI Prime is your BMI divided by 25 (the upper limit of the healthy range). A BMI Prime of 1.0 sits exactly at the overweight threshold, so 0.9 means you are at 90% of that limit, and 1.2 means 20% above it. It is a quick, unit-free way to see how far you are from the healthy ceiling.
- Ponderal Index (kg/m³) divides weight by height cubed instead of squared. Because BMI slightly over-rates tall people and under-rates short people, the Ponderal Index stays more consistent across extremes of height. Healthy adults typically fall around 11–15 kg/m³.
Health risks of a high BMI
A BMI in the overweight or obese range is linked to a higher risk of several conditions, including type 2 diabetes, high blood pressure, heart disease and stroke, certain cancers, and osteoarthritis. Risk generally rises as BMI increases, but it is also affected by waist size, activity level, diet, and family history — which is why BMI is a starting point, not a verdict.
Health risks of a low BMI
Being underweight carries its own risks: nutritional deficiencies, weakened immunity, reduced bone density (increasing fracture risk), and, in some people, fertility problems. A low BMI can also be a sign of an underlying condition. If your BMI is below 18.5, it is worth discussing with a clinician rather than simply trying to gain weight.
BMI for children and teens
For anyone under 20, BMI is interpreted differently. Because children grow at different rates, their BMI is compared against age- and sex-specific percentile charts from the CDC rather than the fixed adult thresholds above. The same BMI number can be healthy at one age and a concern at another, so a child’s result should always be read from a percentile chart with a paediatrician. A dedicated child & teen BMI percentile calculator is on the way.
The limits of BMI
BMI is popular because it needs only two numbers — but that simplicity is also its weakness. It cannot tell muscle from fat. A muscular athlete may score as “overweight” despite low body fat, while an older adult who has lost muscle may score as “healthy” while carrying excess fat. It also says nothing about where fat is stored, even though abdominal fat carries more health risk.
For a fuller picture, pair BMI with measures like waist circumference, body fat percentage, and your overall health history — and talk to a clinician before making decisions based on any single number.
Frequently asked questions
What is a good BMI?
For most adults, a BMI between 18.5 and 24.9 is classified as a healthy weight by the World Health Organization. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese. BMI is a screening tool, not a diagnosis.
Is BMI accurate for everyone?
No. BMI does not distinguish between muscle and fat, so it can overestimate body fat in athletes and underestimate it in older adults who have lost muscle. It is also less reliable for pregnant people and children, who use different charts. Treat it as a quick screen, not a complete picture of health.
How do I calculate BMI by hand?
In metric units, divide your weight in kilograms by the square of your height in metres (kg/m²). In imperial units, multiply your weight in pounds by 703, then divide by the square of your height in inches.
Does BMI differ for men and women?
The BMI formula and adult category thresholds are the same for men and women. However, at the same BMI, women tend to have more body fat than men, so BMI should be interpreted alongside other measures such as waist circumference.