How to Calculate Your BMI (and What the Number Actually Means)
Body Mass Index shows up on medical charts, insurance forms, and fitness apps alike. Here's what it actually measures, how to calculate it by hand, and why doctors treat it as a starting point rather than a verdict.
What BMI is trying to measure
BMI was never designed as a diagnostic tool for individuals. It was created in the 1830s by the Belgian mathematician Adolphe Quetelet as a way to describe the average build of a population, not to assess any one person's health. It caught on decades later as a fast, cheap screening number because it only needs two inputs — height and weight — and correlates reasonably well with body fat percentage across large groups of people.
The formula, step by step
BMI = weight (kg) ÷ height (m)²
Take your weight in kilograms, divide it by your height in meters, then divide again by your height in meters a second time (equivalent to dividing by height squared). For example, someone 1.70 m tall weighing 65 kg gets:
65 ÷ (1.70 × 1.70) = 65 ÷ 2.89 = 22.5
If you're working in pounds and inches instead, the equivalent formula multiplies by 703: BMI = (weight in lb ÷ height in inches²) × 703.
Where the category cutoffs come from
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
These thresholds come from the World Health Organization and are based on population-level studies linking BMI to health risks like cardiovascular disease and type 2 diabetes. Some regional health bodies use adjusted cutoffs — for example, WHO's Western Pacific guidance lowers the overweight threshold to 23 for Asian populations, reflecting evidence that health risk rises at lower BMI values in that group.
Why BMI can be misleading
BMI can't tell the difference between muscle and fat, because it only knows total weight and height. A muscular athlete can register as "overweight" despite having low body fat, while someone with a "normal" BMI but very little muscle mass and high visceral fat can carry real health risk that the number doesn't capture. BMI also doesn't account for age, sex-based differences in body composition, bone density, or where fat is distributed on the body — all of which affect health risk independently of the BMI number itself.
For these reasons, most clinicians treat BMI as a quick first-pass screening tool, to be followed up with more direct measures (waist circumference, body composition scans, blood work) when something looks off, rather than as a standalone diagnosis.
Calculate yours
Want the number without doing the arithmetic by hand? Use the BMI Calculator to get your result and category instantly.