How to Calculate BMI Accurately (And When Not to Trust It)
Body Mass Index, or BMI, shows up everywhere — at the doctor's office, on fitness apps, in insurance forms. It's a quick number meant to flag whether someone's weight falls in a typical range for their height. But despite how often it's used, a lot of people calculate it wrong, or trust it more than they should.
The actual formula
BMI is your weight in kilograms divided by the square of your height in meters. If you're working in pounds and inches, the formula adjusts with a conversion factor: multiply your weight in pounds by 703, then divide by your height in inches squared. Either version gives the same result — the unit conversion is just baked into the second formula's constant.
A person who is 1.75 meters tall and weighs 70 kilograms would calculate: 70 divided by (1.75 × 1.75), which comes out to about 22.9. That falls inside the commonly cited 'healthy weight' range of 18.5 to 24.9.
Where the standard ranges come from
- Below 18.5 — classified as underweight
- 18.5 to 24.9 — classified as healthy weight
- 25 to 29.9 — classified as overweight
- 30 and above — classified as obese
These bands were established by population-level studies looking at correlations between BMI and health outcomes across large groups. That phrase, population-level, is the key limitation: BMI was built to describe trends across thousands of people, not to diagnose any one individual.
Why BMI gets things wrong for individuals
The formula only takes in height and weight. It has no way to distinguish muscle from fat, no way to account for bone density, and no way to factor in where fat is distributed on the body — which matters more for health risk than total body weight alone.
This is why a muscular athlete can show up as 'overweight' on a BMI scale despite having very low body fat, while someone with a 'normal' BMI but high visceral fat (the kind around internal organs) might actually be at greater health risk than the number suggests.
Age and sex also change the picture. Older adults tend to carry more fat at the same BMI compared to younger adults, and women's average body fat percentage runs higher than men's at equivalent BMI values. None of this is captured by a single height-and-weight calculation.
What BMI is actually good for
Despite the caveats, BMI isn't useless. It's cheap, fast, requires no special equipment, and correlates reasonably well with health risk across large populations — which is exactly why public health researchers still use it for screening at scale. As a rough first checkpoint, it can flag when a closer look might be worthwhile.
What it shouldn't be used for is a final verdict on anyone's individual health. If a BMI calculation surprises you, or doesn't match how you feel about your own fitness, that's a reasonable moment to bring it up with a doctor rather than draw conclusions from the number alone — they can look at additional measures like waist circumference, body composition, or blood work that tell a fuller story.
Calculating it yourself
If you want to skip the manual math, a BMI calculator handles the formula and unit conversion for you, and shows your weight category alongside the number. It's the same calculation described above, just without the arithmetic.