The BMI Formula
Body Mass Index was devised in the 1830s by Belgian statistician Adolphe Quetelet and popularized for health screening in the 1970s. The math has not changed since: BMI = weight in kilograms divided by height in meters squared.
If you think in pounds and inches: BMI = (weight in pounds / height in inches squared) x 703. The 703 is just the unit-conversion constant that makes the imperial version match the metric one.
Worked example, metric: 73 kg at 1.75 m. Square the height: 1.75 x 1.75 = 3.0625. Divide: 73 / 3.0625 = 23.8. Worked example, imperial: 160 lb at 5'9" (69 in). Square the height: 69 x 69 = 4,761. Divide: 160 / 4,761 = 0.0336. Multiply by 703: 23.6. Same person, same answer, rounding aside.
The BMI Categories
The World Health Organization cutoffs for adults:
Under 18.5 — Underweight. May indicate undernutrition or an underlying condition worth discussing with a doctor.
18.5 to 24.9 — Healthy weight. Associated with the lowest population-level health risk.
25 to 29.9 — Overweight. Elevated risk for several conditions; the range where lifestyle changes have the most leverage.
30 and above — Obese (30-34.9 class I, 35-39.9 class II, 40+ class III). Used as a screening threshold for further evaluation.
More useful than the single number is your healthy weight range: the weights that put your BMI between 18.5 and 24.9. For someone 5'9", that is roughly 125 to 169 lb. A range gives you a target zone instead of a pass/fail, and our BMI calculator computes it automatically from your height.
What BMI Gets Wrong
BMI measures mass against height. It cannot see composition, and composition is most of the story. A lifter at 12% body fat can land in the overweight band. An older adult who has lost muscle can land in the healthy band while carrying risky visceral fat. Same number, opposite realities.
It also ignores distribution. Fat around the abdomen carries more metabolic risk than fat on the hips and thighs, and BMI treats them identically. Waist circumference exists precisely to fill this gap, which is why clinicians pair the two.
The cutoffs themselves carry history. They were built mainly from European-descended populations, and several Asian health authorities use lower thresholds (for example, 23 instead of 25 for overweight) because risk rises earlier. Children and teens do not use fixed cutoffs at all; they use BMI-for-age percentiles. Pregnancy, elite training, and significant age-related muscle loss each bend the interpretation further.
None of this makes BMI useless. As a cheap, instant screen it is excellent at flagging who deserves a closer look. The error is treating the screen as the diagnosis. If your number surprised you, the right next step is a conversation with a professional who can measure what BMI cannot see, not a crash diet or a victory lap.
When BMI Is Actually Useful
As a personal baseline. Take it at the start of a fitness or weight plan, then re-take it monthly under similar conditions. The trend matters more than any single reading.
As a triage tool. Insurers, workplace wellness programs, and researchers use it because it is fast, free, and correlates with population risk even while misfiring on individuals.
As a reality check before a doctor visit. Walking in knowing your number, your waist measurement, and your recent trend makes the appointment more productive than walking in with a vague worry.
Frequently Asked Questions
What is the BMI formula?
BMI equals weight in kilograms divided by height in meters squared. In imperial units: divide weight in pounds by height in inches squared, then multiply by 703.
What is a healthy BMI?
For adults, the World Health Organization defines 18.5 to 24.9 as the healthy range. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese.
Why is BMI criticized?
BMI cannot distinguish muscle from fat, ignores fat distribution, and was built mainly from European populations. Athletes, older adults, pregnant people, and children all need different interpretation. It is a screening tool, not a diagnosis.
How do I find my healthy weight range?
Multiply 18.5 and 24.9 each by your height in meters squared (or use the imperial equivalent). The two results are the weights that put your BMI at the bottom and top of the healthy range. Our free BMI calculator shows this range automatically.