Calculate your BMI in a few seconds
Body Mass Index (BMI) is a quick screening number that compares your weight to your height. It won't tell you everything about your health, but it's a fast way to see roughly which range you fall into: underweight, normal, overweight, or obese. Doctors use it as a starting point, and it's handy any time you want a baseline before setting a weight goal.
The math is simple enough to do by hand, but it's easy to fumble the unit conversions. The fastest route is to plug your numbers into a calculator that handles metric and imperial for you. Below you'll find both: the step-by-step tool method, and the formula so you can sanity-check the result anywhere.
How to calculate your BMI
- Open the BMI calculator. It loads with sample values (170 cm / 70 kg) so you can see how it behaves before entering your own.
- Pick your unit system at the top. Tap Metric (kg / cm) or Imperial (lb / ft). The input fields switch automatically, so you never have to convert anything yourself.
- Enter your height. In metric that's a single field in centimetres; in imperial it splits into feet and inches.
- Enter your weight in kilograms or pounds. The moment both height and weight are filled in, your BMI appears below, rounded to one decimal, with a colour-coded category label.
- Read the result. A large number shows your BMI, the category beneath it (for example "Normal" or "Overweight"), and a coloured scale with a marker showing where you land. The tool also lists the healthy weight range for your height and how much you'd need to gain or lose to reach it.
- (Optional) Expand Advanced Options to add age, sex, and waist circumference. With age and sex, you get an estimated body-fat percentage; adding your waist unlocks a waist-to-height ratio and a fuller health-risk read-out.
- (Optional) Use Save to log the result, Copy to grab a text summary, or Export CSV if you're tracking changes over time in a spreadsheet.
Everything runs in your browser as a local calculation. The numbers you type stay on your device, nothing is uploaded, and there's no sign-up. The optional history is stored only in your own browser.
The formula behind it
BMI uses metric units:
BMI = weight (kg) ÷ height (m)²
So for 70 kg at 1.70 m: 70 ÷ (1.70 × 1.70) = 70 ÷ 2.89 ≈ 24.2.
If you only know pounds and inches, the imperial version is:
BMI = (weight in lb ÷ height in inches²) × 703
The 703 factor converts the result back to the same scale as the metric formula. The calculator does this conversion internally, which is why you can switch units without re-entering anything.
The standard adult categories (WHO) are:
- Under 18.5 — underweight
- 18.5 to 24.9 — normal
- 25 to 29.9 — overweight
- 30 and above — obese
Tips
- Measure consistently. Weigh yourself in the morning, before eating, in light clothing. Day-to-day swings of a kilo or two are normal and will move your BMI slightly.
- One decimal is plenty. BMI is a screening tool, not a precise measurement. Rounding to 24.2 vs 24.18 makes no practical difference.
- Add your waist for a better picture. Waist-to-height ratio (keeping your waist under half your height) is often a stronger signal of cardiovascular risk than BMI alone. The advanced section calculates it for you.
- Track the trend, not one reading. Use the Save button across weeks to see direction. A single number matters far less than whether it's moving toward or away from the healthy range.
- Know when BMI breaks down. Very muscular people often read "overweight" despite low body fat, and older adults can carry more fat than their BMI suggests. If that's you, an ideal weight estimate or a body-fat calculation gives useful context.
FAQ
Is BMI accurate for everyone? No. BMI doesn't distinguish muscle from fat, and it doesn't account for age, sex, ethnicity, or bone density. It's a population-level screening tool. Athletes, pregnant people, and the elderly are common cases where the number can mislead. Treat it as a starting point, not a diagnosis.
What's a "healthy" BMI? For most adults, 18.5 to 24.9 is considered the normal range. The calculator also shows the exact weight range that keeps you in that band for your specific height, which is more actionable than the BMI number itself. Some research suggests a slightly higher range (around 22 to 27) may be fine for adults over 65.
Do I need to enter age and sex? Not for the basic BMI — height and weight alone produce it. Age and sex are only needed for the extra estimates like body-fat percentage and ideal-weight formulas. You can skip the advanced section entirely.
How is BMI different from body-fat percentage? BMI is just weight relative to height; it can't see inside you. Body-fat percentage estimates how much of your weight is actually fat. Two people with the same BMI can have very different body compositions, which is why the body-fat estimate is a useful companion measure.
Want to go further? Pair your result with the ideal weight calculator to set a target, or estimate your daily energy needs with the BMR calculator.