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BMI Calculator

Calculate your BMI (body mass index) and see how it compares to WHO recommendations.

✓ Last reviewed: Sources: WHO — Body mass index (BMI)

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What the BMI calculator measures

The BMI calculator turns two numbers — your weight and your height — into a single figure that shows whether your weight sits in a healthy range for your height. BMI stands for Body Mass Index. It’s the first screening measure health services worldwide reach for, because it needs no lab test: just a scale and a tape measure.

This tool is built for adults (18 and over). Enter your weight in kilograms and your height in centimetres, or switch the units to pounds and feet/inches. You get three things at once:

  • your BMI number,
  • the World Health Organization (WHO) weight category it falls into,
  • the weight range that would put you in the healthy band for your height.

You also see how your result compares with other people who have used the calculator, so a bare number turns into context.

BMI is a starting point, not a diagnosis. It doesn’t measure body fat directly and can’t tell muscle from fat — more on that in the FAQ.

How to use it

  1. Enter your weight. Use your current weight, ideally measured in the morning. The default unit is kilograms; you can switch to pounds.
  2. Enter your height. In centimetres (e.g. 175) or feet and inches. Height matters as much as weight — a 2 cm difference shifts the result.
  3. Read the result. Your BMI appears instantly, colour-coded by WHO category, next to the healthy weight range for your height.

There are no advanced options to configure — BMI is deliberately simple. The whole point is a reliable answer in a few seconds.

How the BMI calculation works

BMI is your weight divided by the square of your height in metres:

BMI = weight (kg) ÷ height (m)²

Height goes in as metres, so 175 cm becomes 1.75 m. If you prefer imperial units, the equivalent formula is BMI = 703 × weight (lb) ÷ height (in)² — the result is identical.

The calculator rounds BMI to one decimal place, then reads off the WHO category. It also works the formula backwards to show your healthy weight range: it multiplies 18.5 and 24.9 by your height squared to get the lightest and heaviest weights that keep you in the healthy band.

Step-by-step example

Take someone who weighs 70 kg and is 175 cm tall.

  1. Convert height: 175 cm = 1.75 m.
  2. Square it: 1.75 × 1.75 = 3.0625 m².
  3. Divide: 70 ÷ 3.0625 = 22.9.

A BMI of 22.9 sits in the healthy range. For that height, the healthy weight range runs from 56.7 kg to 76.3 kg — so 70 kg is comfortably inside it.

A heavier example: someone weighing 110 kg at 170 cm. Height squared is 1.70 × 1.70 = 2.89 m², and 110 ÷ 2.89 = 38.1, which falls in the obesity range.

Reading your result: the WHO categories

The World Health Organization uses the same four adult cut-offs everywhere, which is what makes BMI comparable across countries:

BMI Category
below 18.5 underweight
18.5 – 24.9 healthy weight
25.0 – 29.9 overweight
30.0 and above obesity

The healthy weight range the calculator shows is simply the 18.5–24.9 band converted into kilograms for your exact height. Two people of different heights can both weigh 70 kg and land in different categories — that’s the whole reason BMI adjusts for height.

Frequently asked questions

How do I calculate BMI by hand?

Divide your weight in kilograms by your height in metres squared. For 70 kg and 1.75 m: square the height (1.75 × 1.75 = 3.0625), then divide (70 ÷ 3.0625 = 22.9). In imperial units, multiply weight in pounds by 703 and divide by height in inches squared. The calculator above does this instantly and adds the WHO category.

What is a healthy BMI?

According to the WHO, a healthy BMI for adults is between 18.5 and 24.9. Below 18.5 is classed as underweight, 25.0–29.9 as overweight, and 30.0 or higher as obesity. These cut-offs are the same worldwide, which lets you compare results across countries and over time.

Is BMI accurate for athletes and muscular people?

Not always. BMI uses only weight and height, so it can’t tell muscle from fat. Muscle is denser than fat, so very muscular people — bodybuilders, some athletes — can show a high BMI while carrying little fat. For them, BMI overstates risk, and a body-fat measurement gives a truer picture.

Does the BMI calculator work for children?

No. This calculator uses adult cut-offs and is meant for people aged 18 and over. Children and teenagers grow at different rates, so their BMI is read against age- and sex-specific percentile charts rather than fixed numbers. A child’s result should be interpreted by a paediatrician.

What’s the difference between BMI and body fat percentage?

BMI estimates whether your weight is healthy for your height using only two measurements. Body fat percentage measures how much of your body is actually fat, usually via calipers, bioimpedance scales or a scan. BMI is faster and free but cruder; body fat percentage is more precise but needs equipment. They answer related but different questions.

What is a healthy weight for my height?

Multiply 18.5 and 24.9 by your height in metres squared to get the range. At 1.75 m that’s 18.5 × 3.0625 = 56.7 kg up to 24.9 × 3.0625 = 76.3 kg. The calculator shows this range automatically once you enter your height, so you can see how far any target weight is from the healthy band.

Sources