Understanding Your BMI: What the Number Really Means

Body Mass Index is your weight in kilograms divided by your height in metres squared, a quick screening number that places you in one of four WHO categories: underweight, normal, overweight or obese. It's the fastest way to see whether your weight sits in a healthy range for your height. The BMI Calculator works it out in metric or imperial units and shows your category with a colour-coded badge and your personal healthy weight range.

What BMI is and who it's for

BMI is a screening measure, not a diagnosis. Doctors, nurses and fitness professionals use it as a first look at whether weight could be a health concern, precisely because it needs only two easy measurements. It won't tell you your body-fat percentage or where fat is stored, but tracking it over weeks and months is a simple, consistent way to monitor progress toward a goal. Anyone curious about where they stand, or filling in a form that asks for it, can get an answer in seconds.

How to calculate your BMI in your browser

1. Pick your unit system — metric (cm and kg) or imperial (feet, inches and pounds).
2. Enter your height.
3. Enter your weight.
4. Read your BMI value and WHO category instantly, along with the healthy weight range for your height.

The WHO BMI categories at a glance

Your BMI number maps to a category using the World Health Organization thresholds. Here's the full scale so you can see where any result lands.

BMI range	Category	What it suggests
Below 18.5	Underweight	May need to gain weight
18.5 – 24.9	Normal weight	Healthy range for most adults
25.0 – 29.9	Overweight	Elevated health risk
30.0 and above	Obese	Higher health risk

One important caveat: BMI can't tell muscle from fat. A muscular athlete may register as overweight while carrying very little body fat, so for highly trained people a body-fat percentage or waist-to-height ratio gives a truer picture.

Key features

• Metric and imperial units with an instant toggle
• WHO categories shown with a colour-coded badge
• Displays the healthy weight range for your specific height
• Live results — no calculate button to press
• 100% private — your height and weight never leave your browser
• Free, with no sign-up required

Try the BMI Calculator now — it's free and runs entirely in your browser.

Frequently asked questions

How is BMI calculated?

Divide weight in kilograms by height in metres squared: BMI = kg ÷ m². In imperial units it's 703 × pounds ÷ inches². A 70 kg person who is 1.75 m tall has a BMI of about 22.9.

What counts as a healthy BMI?

The WHO treats 18.5 to 24.9 as normal weight. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese. The calculator badges your result with the matching category automatically.

Is BMI accurate for athletes?

Not always. Because BMI can't distinguish muscle from fat, very muscular people often read as overweight despite low body fat. For them, body-fat percentage or waist-to-height ratio is more informative.

Does BMI differ for men and women?

The adult formula and categories are identical for both sexes. Women naturally carry slightly more body fat at the same BMI, and for children and teens, age- and sex-specific percentile charts are used instead of the adult scale.

What weight should I be for my height?

Multiply your height in metres squared by 18.5 and by 24.9 for the healthy range. At 1.70 m that's roughly 53.5 kg to 72 kg. This tool displays your personal range automatically alongside your result.

Related free tools

• Age Calculator — your exact age for health forms
• Percentage Calculator — track weight-change percentages
• Unit Converter — switch between kg, lbs, cm and inches
• Area Calculator — quick geometry when you need it

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