How to Calculate Your BMI Online for Free

Body Mass Index is one of the most widely used health metrics in the world and also one of the most misunderstood. It's a simple calculation that takes about ten seconds and gives you a number that doctors, insurance companies, and fitness trackers all use — but the number comes with significant limitations that are worth understanding before you put too much weight on it (pun intended).

How BMI is calculated

The formula is your weight in kilograms divided by your height in metres squared. So someone who weighs 70kg and is 1.75m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.

In imperial units, the formula is (weight in pounds × 703) ÷ (height in inches squared). Our free calculator handles both — enter your measurements and you get the result immediately without needing to do the arithmetic yourself.

What the number means

The standard BMI categories established by the World Health Organisation are:

• Under 18.5: Underweight
• 18.5 – 24.9: Normal weight
• 25.0 – 29.9: Overweight
• 30.0 and above: Obese

These categories are population-level averages. They were designed for use in public health research and clinical screening — not as a precise individual health verdict. A BMI that falls in the "overweight" range doesn't mean you're unhealthy, and a BMI in the "normal" range doesn't mean you are.

The limitations you should know about

BMI doesn't distinguish between muscle and fat. A professional athlete with very low body fat percentage can have the same BMI as someone who is sedentary with high body fat. This is why BMI systematically misclassifies muscular individuals as overweight or obese.

BMI also doesn't account for where fat is distributed on your body. Abdominal fat (around the waist) carries significantly higher health risks than fat stored in the hips and thighs — but two people with identical BMIs and different fat distribution patterns get the same number from the formula.

Age and ethnicity also affect how BMI maps to actual health risk. The standard thresholds were developed primarily from data on European populations. Some health organisations use lower thresholds for people of Asian descent, where health risks associated with excess body fat appear at lower BMI values.

How to use BMI sensibly

BMI is a useful screening tool, not a diagnosis. If your BMI puts you in the overweight or obese category, it's a prompt to look more carefully at other health markers — blood pressure, blood sugar, waist circumference, cholesterol — not a definitive statement about your health status.

If you're tracking progress over time, BMI can be a useful relative measure even if the absolute number is imprecise. A downward trend over months of consistent healthy habits is meaningful even if the exact figure isn't medically precise. Use it as one data point among several.