BMI Calculator
This free BMI calculator works out your Body Mass Index from your height and weight, in metric (kg and cm) or imperial (lb and inches) units, and tells you which World Health Organization category the result falls into. It takes a few seconds and nothing you enter leaves your browser.
How to use the BMI calculator
- Choose metric or imperial units with the buttons at the top.
- Enter your weight and your height.
- Press Calculate BMI to see your BMI value and its category, shown against the four standard ranges.
The BMI formula
BMI is your weight divided by the square of your height:
- Metric: BMI = weight (kg) ÷ height (m)²
- Imperial: BMI = weight (lb) ÷ height (in)² × 703
The factor 703 converts pounds and inches into the same scale as kilograms and metres, so both versions give the same number for the same person.
Worked example
A person who weighs 70 kg and is 175 cm (1.75 m) tall has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 = 22.9, which is in the normal range. In imperial units, 154 lb and 69 inches gives 154 ÷ 4,761 × 703 = 22.7, almost the same result. For that height, the normal range of 18.5 to 24.9 corresponds to roughly 57 kg to 76 kg.
BMI categories for adults
- Below 18.5 – underweight
- 18.5 to 24.9 – normal weight
- 25.0 to 29.9 – overweight
- 30.0 and above – obesity, often split into class I (30–34.9), class II (35–39.9) and class III (40 and above)
These are the cut-off points used by the WHO for adults aged 18 and over. Some health agencies use lower cut-offs for people of Asian descent, because health risks can rise at a lower BMI.
What BMI can and cannot tell you
BMI is a quick screening number, not a diagnosis. It is useful because it is simple and works for large groups of people, but it does not measure body fat directly. A very muscular athlete can have a high BMI while being lean, and an older adult who has lost muscle can have a normal BMI with a higher share of body fat. Waist circumference, body composition and a check-up with a health professional give a more complete picture.
Frequently asked questions about the BMI calculator
What is a healthy BMI?
For most adults a BMI between 18.5 and 24.9 is considered a healthy weight range according to the WHO.
Is this BMI calculator accurate for children?
No. Children and teenagers are assessed with BMI-for-age percentiles that take age and sex into account. Use a pediatric BMI chart or ask a pediatrician.
Why does the calculator ask for height in centimetres?
Centimetres are easier to type. The calculator converts them to metres before applying the formula.
Does BMI work for athletes and bodybuilders?
It can overestimate body fat in people with a lot of muscle, so for them BMI should be read alongside other measurements.
Is my data saved?
No. The calculation runs in your browser and nothing is stored or sent to a server.
Healthy weight range by height
The table below converts the normal BMI range (18.5 to 24.9) into weights for common heights. It is a quick way to see what the BMI calculator result means in kilograms or pounds.
| Height | Height (ft/in) | Normal range (kg) | Normal range (lb) |
|---|---|---|---|
| 150 cm | 4'11" | 41.6 – 56.0 kg | 92 – 124 lb |
| 155 cm | 5'1" | 44.4 – 59.8 kg | 98 – 132 lb |
| 160 cm | 5'3" | 47.4 – 63.7 kg | 104 – 141 lb |
| 165 cm | 5'5" | 50.4 – 67.8 kg | 111 – 149 lb |
| 170 cm | 5'7" | 53.5 – 72.0 kg | 118 – 159 lb |
| 175 cm | 5'9" | 56.7 – 76.3 kg | 125 – 168 lb |
| 180 cm | 5'11" | 59.9 – 80.7 kg | 132 – 178 lb |
| 185 cm | 6'1" | 63.3 – 85.2 kg | 140 – 188 lb |
| 190 cm | 6'3" | 66.8 – 89.9 kg | 147 – 198 lb |
Each row is calculated with the same formula as the calculator: weight = BMI × height². Use it as a reference, not a target, since a healthy weight for you also depends on age, muscle mass and medical history.
Tips for an accurate BMI reading
- Weigh yourself at the same time of day, ideally in the morning before eating, wearing light clothes.
- Measure your height without shoes, standing straight against a wall.
- Look at the trend over weeks rather than one single reading; daily weight can change by 1 to 2 kg because of water.
- If your result is outside the normal range, talk to a doctor before making big changes to diet or exercise.
Related tools
- Age Calculator – find your exact age in years, months and days.
- Percentage Increase Calculator – measure how much a value, like your weight, has changed.
- Guide: BMI explained in plain language
- All free tools
Source for the categories: WHO fact sheet on obesity and overweight. This tool is for general information and is not medical advice.