BMI Calculator
Calculate your Body Mass Index from height and weight, in metric or imperial, and see the category, the formula, worked examples and what BMI does and does not measure.
What BMI is
Body Mass Index is a single screening number worked out from your height and weight. It sorts adults into broad weight categories and is used as a quick, population-level indicator — the same starting point a doctor might glance at before a fuller assessment. It was devised in the nineteenth century by the mathematician Adolphe Quetelet, which is why it is sometimes still called the Quetelet index. The calculator above gives your figure and category instantly; the sections below explain where the number comes from and, just as importantly, what it cannot tell you.
The BMI formula
BMI divides your weight by the square of your height. There are two equivalent versions depending on your units:
Metric: BMI = weight(kg) / height(m)^2Imperial: BMI = weight(lb) x 703 / height(in)^2The constant 703 in the imperial version simply rescales pounds and inches onto the same metric scale, so both formulas give an identical result for the same person. It comes from the unit conversion between pounds, inches and the metric kilograms-per-metre-squared that BMI is defined in.
Worked examples
Seeing the arithmetic makes the formula concrete:
The categories
The standard adult categories used by the World Health Organization are:
| Category | BMI range |
|---|---|
| Underweight | Below 18.5 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese (Class I) | 30.0 – 34.9 |
| Obese (Class II) | 35.0 – 39.9 |
| Obese (Class III) | 40.0 and above |
What BMI does and does not measure
BMI is genuinely useful as a rough screen, but it is widely over-interpreted. The distinction below is the most important thing to understand about it:
| BMI reflects | BMI ignores |
|---|---|
| Weight relative to height | Body fat percentage |
| A broad weight category | Muscle versus fat |
| Population-level trends | Where fat is carried |
| A quick screening figure | Bone density and frame size |
| — | Age, sex and ethnicity differences |
Because it cannot tell muscle from fat, BMI misclassifies people at both ends: a muscular athlete can score as overweight while carrying very little fat, and an older person with low muscle can sit in the normal range while carrying more fat than is healthy. For a fuller picture, body fat percentage, waist circumference and the waist-to-hip ratio all add information BMI cannot.
Normal-weight ranges by height
These weight ranges correspond to a normal BMI of 18.5 to 24.9 at several heights, derived straight from the formula rather than from any health recommendation:
| Height | Normal-BMI weight range |
|---|---|
| 155 cm / 5'1" | 44–60 kg / 97–132 lb |
| 165 cm / 5'5" | 50–68 kg / 111–150 lb |
| 175 cm / 5'9" | 57–76 kg / 125–169 lb |
| 185 cm / 6'1" | 63–85 kg / 140–188 lb |
| 195 cm / 6'5" | 70–95 kg / 155–209 lb |
BMI is a starting point, not a diagnosis. It does not account for body composition, age, sex or ethnicity, and it is not a substitute for professional medical evaluation. For anything about your own health, a doctor who can weigh the whole picture is the right source.
BMI Calculator FAQ
What is BMI?
Body Mass Index is a number calculated from height and weight, using BMI = weight(kg) / height(m) squared. It was developed in the 19th century by the mathematician Adolphe Quetelet and is used as a general screening tool to categorise weight status.
How do I calculate BMI?
In metric, divide your weight in kilograms by your height in metres squared. In imperial, multiply your weight in pounds by 703 and divide by your height in inches squared. For example, 70 kg and 1.75 m gives 70 / (1.75 x 1.75) = 22.9.
What are the BMI categories?
The standard adult categories are underweight below 18.5, normal weight 18.5 to 24.9, overweight 25 to 29.9, and obese 30 and above. The obese range is sometimes split into Class I, II and III.
Why is 703 used in the imperial formula?
It rescales pounds and inches onto the metric kilograms-per-metre-squared scale, so imperial and metric give the same result. It comes from the conversions between pounds, inches and metres.
Is BMI accurate for athletes or muscular people?
Often not — BMI cannot distinguish muscle from fat, so a muscular person can score as overweight or obese while having low body fat. Body composition measurements are more meaningful in those cases.
Does BMI apply to children?
The formula is the same, but for ages 2 to 19 it is read against age- and sex-specific percentile charts rather than the fixed adult thresholds, which a paediatric professional can interpret.
Does this calculator store my data?
No. Everything is calculated in your browser — your height, weight and result are never sent anywhere or stored.