BMI: what it is and how to calculate

BMI (Body Mass Index) is your weight divided by the square of your height. It has been around since the nineteenth century, it is used worldwide, and it solves a real problem — but it solves a population problem, not a personal one. Understanding that distinction is what separates using BMI from being used by it.

The formula

BMI = 703 × weight (lb) ÷ height (in)²

In metric units it is simply kg ÷ m²; the 703 is just the conversion factor. Note that height enters squared. That is not a detail — it is the origin of almost every criticism of the index, as you will see below.

Worked example

Someone weighing 172 lb at 5'9" (69 inches):

1. Height squared: 69 × 69 = 4,761
2. Multiply and divide: 703 × 172 ÷ 4,761 = 25.40

The result, BMI 25.4, lands in the overweight range — 3.4 lb above the top of the normal band, which for 5'9" ends at 168.6 lb.

The WHO categories

BMI	Category	Weight at 5'9"
under 18.5	Underweight	under 125.3 lb
18.5 to 24.9	Normal weight	125.3 to 168.6 lb
25.0 to 29.9	Overweight	169.3 to 202.5 lb
30.0 to 34.9	Obesity class I	203.2 to 236.4 lb
35.0 to 39.9	Obesity class II	237.0 to 270.2 lb
40 or above	Obesity class III	270.9 lb or more

The "normal" band is deliberately wide: at 5'9" it spans more than 43 lb. That alone tells you the index was never designed to name an ideal weight.

What BMI cannot see

The index uses two numbers — weight and height — and ignores everything else. In practice that leaves four important blind spots:

• It cannot tell muscle from fat. An athlete at 220 lb and 5'11" has a BMI of 30.7, technically "obesity class I", possibly with body fat under 10%. The index measures mass, not composition.
• It cannot see where the fat is. Visceral (abdominal) fat carries far higher cardiometabolic risk than peripheral fat, and two people with identical BMI can have opposite distributions.
• It loses accuracy at the extremes of height. Because height enters squared while the human body does not scale exactly as a square, BMI tends to overstate in very tall people and understate in very short ones.
• It does not apply equally to everyone. Older adults lose lean mass and can hold a "normal" BMI with a high body-fat percentage. Children and teenagers are not assessed on adult bands at all, but on age- and sex-specific percentile curves. And the WHO recognises that for Asian populations metabolic risk begins rising from BMI 23, not 25.

So why is it still used?

Because it is cheap, reproducible and works well in aggregate. Across a population of thousands, BMI correlates well with cardiovascular disease and type 2 diabetes risk, and it requires nothing but a scale and a tape measure. No other method with that correlation costs so little.

The error is not using BMI. It is using it alone, on one person, as though it were a diagnosis.

A complement that takes 30 seconds

The single most useful measure to track alongside BMI is the waist-to-height ratio: measure your waist circumference at the navel and divide by your height, in the same units.

The benchmark is easy to remember: keep your waist under half your height. At 5'9", that means a waist under 34.5 inches. This ratio captures exactly what BMI misses — abdominal fat — and several studies find it a better predictor of cardiometabolic risk than BMI on its own.

Frequently asked questions

What is the ideal BMI? The question has no good answer, because there is no ideal point — there is a wide band of normality. Tracking the trend over time is far more useful than chasing a number.

Does BMI apply during pregnancy? Not during pregnancy itself. Expected weight gain is tracked on dedicated curves, based on your pre-pregnancy BMI.

Do men and women use the same ranges? Yes, the WHO bands are identical. But women carry a higher average body-fat percentage at the same BMI — one more reason not to read the number in isolation.

My BMI came out in the overweight range. Now what? A BMI between 25 and 30 is a reason to look closer, not to panic. Measure your waist, and check blood pressure, blood glucose and a lipid panel. Much of the risk BMI hints at shows up directly in those numbers — and they measure what actually matters.

Sources: World Health Organization — BMI classification for adults and the specific recommendations for Asian populations; CDC — Adult BMI. This content is informational and does not replace medical assessment.