If you’ve ever been to a check-up, filled a health form, or read an article about weight, you’ve probably run into BMI. Body Mass Index is that single number that’s supposed to say something about whether your weight is “healthy” for your height. A lot of people love to point out that BMI is flawed—and they’re not wrong. It doesn’t know the difference between muscle and fat. It doesn’t account for where you carry weight or how old you are. Athletes can land in “overweight” and still be in great shape. So why do doctors, insurers, and researchers still use it? Because for most people, in most situations, it’s a quick, consistent way to screen. The trick is knowing what it’s good for and what it’s not.
BMI is just your weight in kilograms divided by the square of your height in metres. So if you weigh 70 kg and you’re 1.75 m tall, you do 70 ÷ (1.75 × 1.75), which is about 22.9. That number falls in what’s usually called the “normal” or “healthy” range. The World Health Organisation and most health bodies use the same bands: under 18.5 is underweight, 18.5 to 24.9 is normal, 25 to 29.9 is overweight, and 30 or above is obese. Those cut-offs are based on large studies linking BMI to health outcomes. They’re not perfect for every individual, but they’re useful for populations and for tracking your own trend over time.
Nobody expects you to do that maths by hand. A BMI calculator does it in seconds. You enter your height—in centimetres or feet and inches—and your weight in kg or pounds. The tool gives you your BMI and typically tells you which band you’re in. Many also suggest an “ideal weight range” for your height, which can be helpful if you’re trying to set a target. The point isn’t to obsess over a single number. It’s to have a consistent way to see where you stand and whether that’s changing. If your BMI has been creeping up over the years, that might be a nudge to look at diet and activity. If it’s suddenly dropped without you trying, that could be worth a conversation with a doctor.
Who is BMI not great for? Very muscular people often get a high BMI because muscle weighs more than fat. Same height and weight can look completely different on a bodybuilder and someone who’s sedentary—BMI can’t tell them apart. Older adults sometimes lose muscle and hold more fat, so their BMI might look “normal” while their body composition isn’t ideal. Pregnant women, kids, and some ethnic groups are often better served by other measures. So if you’re an athlete or you’ve been told you have a lot of muscle mass, take your BMI with a pinch of salt. For everyone else, it’s a reasonable starting point.
What you do with the result is up to you and, when in doubt, your doctor. Some people use it to set a loose goal: “I’d like to be in the normal range.” Others use it to track: “Last year I was 26, now I’m 24.” It’s not a verdict. It’s one piece of information. Diet, exercise, sleep, stress, and genetics all matter. BMI doesn’t replace a proper health assessment—it just gives you a number you can understand and, if you want, act on.
If you’d like to check your own BMI without signing up anywhere or sending your data to a server, you can use our free BMI calculator. Enter your height and weight, and you’ll get your BMI, your category, and often an ideal weight range. The calculation runs in your browser, so your numbers never leave your device. It’s the same formula doctors use—nothing fancy, just quick and private. Whether you’re curious, tracking a change, or preparing for a check-up, it’s a useful tool to have bookmarked.