BMI Calculator
Calculate your body mass index (BMI) from height and weight in metric or imperial units, with the WHO category.
About the BMI Calculator
The BMI Calculator works out your body mass index (BMI) from your height and weight in seconds. Choose metric (centimetres and kilograms) or imperial (feet, inches, and pounds), type in your measurements, and you instantly see your BMI to one decimal place along with the World Health Organization (WHO) weight category it falls into. It's a quick way to see roughly where your weight sits on a standard scale without hunting for a chart or doing the arithmetic yourself.
BMI is a simple ratio of weight to height, and it has been used for decades as a fast, rough screening measure at a population level. It's worth knowing up front that BMI is a general indicator, not a diagnosis. It doesn't account for muscle mass, body composition, bone density, age, sex, or where fat is stored, so a very muscular person and a less active person can share the same number for very different reasons. Treat your result as one data point rather than the whole picture, and speak with a healthcare professional for anything meaningful — this tool is not medical advice. Everything runs directly in your browser, so your measurements are never uploaded, saved, or shared.
How to use
- Pick your units. Tap Metric (cm / kg) or Imperial (ft·in / lb) at the top. The input fields switch to match your choice.
- Enter your height. In metric, type your height in centimetres. In imperial, enter feet and inches in the two boxes.
- Enter your weight in kilograms (metric) or pounds (imperial).
- Read your result. The panel shows your BMI to one decimal place and the WHO category, shown with a colored label so it's easy to read at a glance.
Every value updates the moment you change an input, so you can switch units or adjust a number and watch the result change immediately. If a field is empty or invalid, the tool shows a dash instead of a confusing error — so you'll never see "NaN."
BMI categories
BMI is calculated as weight in kilograms divided by height in metres squared (kg/m²). Imperial measurements are converted to metric internally before the math runs. The WHO uses these standard adult ranges:
- Underweight — BMI below 18.5
- Normal (healthy) weight — BMI from 18.5 to 24.9
- Overweight — BMI from 25.0 to 29.9
- Obese — BMI of 30.0 or above
These bands apply to adults and are not adjusted for age or sex. Children, teenagers, athletes, and pregnant people need different interpretation, so the ranges above should not be applied to them directly.
Frequently asked questions
How is BMI calculated?
BMI equals your weight in kilograms divided by your height in metres squared (kg/m²). For example, someone who is 1.70 m tall and weighs 68 kg has a BMI of 68 ÷ (1.70 × 1.70) ≈ 23.5. If you enter imperial units, the calculator first converts inches to metres and pounds to kilograms, then applies the same formula, so both unit systems give the same result.
What is a healthy BMI range?
The WHO defines a "normal" or healthy adult range as a BMI from 18.5 to 24.9. Below 18.5 is classed as underweight, 25.0 to 29.9 as overweight, and 30.0 and above as obese. These are broad screening categories, not personal targets, and a healthy weight for any individual depends on many factors beyond BMI.
Is BMI accurate for everyone?
No. BMI is a simple height-to-weight ratio and doesn't distinguish muscle from fat or account for age, sex, or how weight is distributed. Very muscular people (such as athletes) can register as "overweight" despite low body fat, while others may carry more fat than the number suggests. It's a useful rough guide at a population level, but it works best alongside other measures and professional advice.
Does BMI work for children?
The adult categories used here do not apply to children and teenagers. For young people, BMI is interpreted using age- and sex-specific percentile charts instead of fixed cut-offs. If you're checking a child's BMI, use a pediatric percentile tool or consult a healthcare provider rather than these adult ranges.
Should I switch between metric and imperial?
Use whichever units you know your measurements in — the result is identical either way, because imperial values are converted to metric internally before the calculation. Switching units simply changes which input boxes appear; your BMI and category stay consistent.
Is my data sent anywhere?
No. The BMI Calculator runs entirely in your browser. Nothing you type is uploaded, saved, or shared — all the math happens locally on your device, and it keeps working offline once the page has loaded. Your height and weight never leave your computer.