Body Mass Index, or BMI, is the number doctors, fitness trackers, and insurance forms use to put a person’s weight into context. It is not a direct measurement of body fat — it is a quick screening estimate based on just two numbers, height and weight — but it is useful because it is fast, standardised, and needs no special equipment. Best of all, you can calculate BMI at home in under a minute once you know the formula, which is why it remains the default screening measure across most of the world.
The BMI Formula
BMI divides your weight in kilograms by the square of your height in metres. The imperial version uses pounds and inches, with a fixed conversion factor built in so the result lands on the same scale. Either way, the output is a single number that falls into one of four categories, and the math itself is simple enough to do on paper. For example, a height of 1.75 m is squared to 3.0625, and then the weight is divided by that figure.
Metric: BMI = weight (kg) / height (m)²
Imperial: BMI = 703 × weight (lb) / height (in)²
Example: 70 kg at 1.75 m
70 / (1.75 × 1.75) = 22.9Find your BMI in seconds
Enter your height and weight to see your BMI and category instantly.
What Your BMI Category Means
- Below 18.5 — underweight: may indicate a need for more calories or nutrient intake.
- 18.5 to 24.9 — healthy range: the target zone for most adults.
- 25 to 29.9 — overweight: a cue to review diet and activity levels.
- 30 and above — obese: a common prompt to consult a health professional.
Know the Limits of BMI
BMI is a screening tool, not a diagnosis. A muscular athlete can show a high BMI with very little body fat, and BMI says nothing about where fat sits on the body or how fit a person actually is. Children, older adults, and pregnant women all need different reference points than the standard adult scale. Use it as one signal among many, track it over time rather than as a single snapshot, and treat a surprising result as a conversation starter with your doctor rather than a verdict. For a fast and accurate figure, a BMI calculator does the division for you so you can focus on what the number means for your health.