BMI & Weight
What Is BMI and Why Does It Matter?
Learn what Body Mass Index (BMI) is, how it's calculated, and why doctors use it as a basic health screening tool.
Body Mass Index, or BMI, is a number calculated from your height and weight. It gives a quick snapshot of whether your weight falls into a range considered healthy for your height. Although it sounds technical, the idea behind it is simple: taller people can carry more weight without health risks, so BMI adjusts for that.
Where Did BMI Come From?
BMI was invented in the 1830s by Belgian mathematician Adolphe Quetelet. He wasn't a doctor — he was a statistician studying population trends. His 'Quetelet Index' wasn't meant to measure individual health; it was designed to describe averages across large groups. Modern medicine later adopted it because it's cheap, fast, and requires no equipment beyond a scale and a tape measure.
How Is BMI Calculated?
The formula is straightforward. In metric units, you divide your weight in kilograms by your height in meters squared. So if you weigh 70 kg and stand 1.75 m tall, your BMI is 70 ÷ (1.75 × 1.75) = 22.9. In imperial units, the formula uses pounds and inches, with a correction factor of 703.
What Do the Numbers Mean?
- Below 18.5 — Underweight
- 18.5 to 24.9 — Healthy weight
- 25.0 to 29.9 — Overweight
- 30.0 and above — Obese
Why Do Doctors Use BMI?
Doctors use BMI as a first screening step because it costs nothing and takes seconds. A person with a BMI above 30 has a statistically higher risk of type 2 diabetes, heart disease, and certain cancers. That doesn't mean every high-BMI person will develop these conditions — but it flags people who may benefit from further testing.
The Bottom Line
BMI is a useful starting point, not a final verdict. Use it alongside other measures — waist circumference, blood pressure, blood sugar, and how you feel day-to-day. Our free BMI calculator gives you your number instantly, along with context about what it means for your health.