BMI 계산기

What BMI actually measures — and what it doesn't

Body Mass Index is a single number that relates your weight to your height. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet as a way to describe populations, not to diagnose individuals — which is exactly why it works well as a quick screen but poorly as a verdict. BMI does not measure body fat directly. It measures mass relative to height, and then assumes that, across a typical population, higher mass-for-height usually means more fat. That assumption holds for most sedentary adults and breaks down for anyone with unusual body composition.

The formula, exactly

BMI is weight divided by height squared. In metric units the formula is:

BMI = weight(kg) / height(m)²

This calculator uses that formula directly. When you switch to imperial units it first converts to metric internally — multiplying inches by 0.0254 to get metres and pounds by 0.453592 to get kilograms — then applies the same division. That means the metric and imperial modes give identical results for the same real body; there is no separate "imperial formula" with a 703 factor doing anything different under the hood, just a unit conversion followed by the standard calculation.

A worked example

Take an adult who is 170 cm tall and weighs 65 kg. First convert height to metres: 170 cm = 1.70 m. Square it: 1.70 × 1.70 = 2.89. Now divide: 65 ÷ 2.89 = 22.49, which rounds to 22.5. That falls in the 18.5–24.9 band, so this tool labels it Healthy. If the same person gained 10 kg to reach 75 kg, the new BMI would be 75 ÷ 2.89 = 25.9 — now in the Overweight band. Notice how the same absolute weight change moves a tall person less than a short one, because dividing by a larger height-squared dampens the effect.

The categories this tool uses

These thresholds are the World Health Organization's cut-offs for adults aged 18 and over. They are the same regardless of sex, which is one of BMI's limitations — men and women carry fat differently, but the index does not adjust for that.

Where BMI misleads, and what to do about it

• Muscular athletes read as "overweight." Muscle is denser than fat, so a rugby player or powerlifter can have very low body fat yet a BMI of 28. The number is not wrong — they genuinely have high mass-for-height — but interpreting it as excess fat would be.
• It ignores where fat sits. Visceral fat around the organs carries far more health risk than fat on the hips, yet BMI cannot tell the two apart. A waist measurement or waist-to-height ratio adds the information BMI is missing.
• It can understate risk in older adults. People lose muscle with age, so a "normal" BMI can hide a high fat percentage. Some guidelines suggest a slightly higher target band (roughly 22–27) is protective after 65.
• It does not apply to children. Growing bodies use age- and sex-specific percentile charts, not the fixed adult bands. This tool is for adults only.
• It is not valid in pregnancy. Weight gain during pregnancy is expected and tracked differently.

Using the result well

Treat BMI as a starting conversation, not a conclusion. If your number sits well inside the healthy band and you feel well, it is reassuring. If it sits near a boundary, remember the categories are arbitrary lines on a continuous scale — 24.9 and 25.1 are not meaningfully different bodies. The most useful next steps are pairing BMI with at least one other measure: a tape-measure waist reading, a body-composition scan, or simply tracking the trend over months rather than fixating on a single value. A BMI that is slowly climbing tells you more than one snapshot ever can.

Why the result is instant and private

The calculation is pure arithmetic — one division and a unit conversion — so it runs entirely in your browser the moment you type. Your height and weight are never sent anywhere; they are only encoded into the page's own URL so you can bookmark or share a particular calculation if you choose. Nothing about your body is transmitted to or stored on a server.