Body Mass Index is a simple ratio of weight to height that sorts adults into weight categories. It costs nothing, needs only a scale and a height measurement, and is calculated the same way in clinics worldwide — but "correctly" matters twice over: the arithmetic must be right, and so must the measurements, because small errors in height are squared in the formula and move the result more than most people expect.
The Formula
In metric units, BMI is weight in kilograms divided by height in meters squared:
In imperial units, the same ratio needs a conversion factor of 703:
Worked Example 1: Metric
An adult weighs 74 kg and is 1.78 m tall. Height squared: 1.78 × 1.78 = 3.1684. BMI = 74 ÷ 3.1684 = 23.36 — inside the normal range (18.5 to 24.9).
Worked Example 2: Imperial
An adult weighs 185 lb and stands 5 ft 10 in — 70 inches in total. Height squared: 70 × 70 = 4,900. BMI = (185 × 703) ÷ 4,900 = 130,055 ÷ 4,900 = 26.54 — inside the overweight range (25.0 to 29.9).
The Adult Categories (WHO)
| BMI | Category | |---|---| | Below 18.5 | Underweight | | 18.5 – 24.9 | Normal / healthy weight | | 25.0 – 29.9 | Overweight | | 30.0 – 34.9 | Obese, class I | | 35.0 – 39.9 | Obese, class II | | 40.0 and above | Obese, class III |
Health authorities in many Asian countries apply lower action thresholds — overweight from 23.0 and obese from 27.5 — because metabolic risk rises at lower BMI in those populations. These bands apply to adults; children and teenagers are assessed on age- and sex-specific growth percentiles instead, where the same BMI number means different things at different ages.
Measuring Correctly: Where the Errors Come From
Height is the fragile input. Because height is squared, mis-measuring by 2 cm at 1.78 m moves BMI by about half a point — enough to cross a category boundary near a threshold. Measure barefoot, standing straight against a wall or stadiometer, ideally in the morning (you are up to 2 cm taller than in the evening). Old errors to avoid: using your remembered height from a decade ago, and entering centimeters into a meters field, which produces absurd results like a BMI of 0.2 or 2,300 — an instant sign the units are wrong.
Weight should be measured, not remembered. Weigh in the morning, after the bathroom and before breakfast, in light clothing, and use a recent figure rather than a remembered one. Then sanity-check the output: adult BMI values outside roughly 12 to 60 almost always indicate a unit error, not a medical finding.
What BMI Cannot Tell You
BMI counts mass, not composition. It cannot distinguish muscle from fat, so a lean rugby player or weightlifter can score "obese" with low body fat, while an older adult with muscle loss (sarcopenia) can score "normal" with high body fat and real metabolic risk. It says nothing about where fat is carried — visceral abdominal fat is the dangerous kind — and it should not be used as-is during pregnancy. Clinicians therefore pair it with waist circumference (risk rises above 102 cm for men and 88 cm for women), waist-to-height ratio (keep your waist under half your height), or a direct body-fat measurement before drawing conclusions about an individual.
Frequently Asked Questions
How do I calculate BMI by hand in imperial units?
Multiply your weight in pounds by 703, and divide by your height in inches squared. For 185 lb and 70 inches: 185 × 703 = 130,055, divided by 4,900 = 26.54. If you forget the 703, the answer comes out near 0.038 — the factor is what makes imperial units produce the same scale as metric.
Is BMI different for men and women?
The formula and the adult category bands are the same for both sexes. What differs is the interpretation context: at the same BMI, women on average carry a higher body-fat percentage than men, which is one reason BMI is a screening tool rather than a diagnosis.
My BMI is just above 25. Should I worry?
A single reading just inside a boundary is the weakest possible signal — measurement error alone covers it. Retake the measurements carefully, look at your waist measurement alongside, and consider the trend over months rather than one number. If several indicators point the same way, that is a conversation for your doctor, not a verdict from a ratio.