The BMI Formula Explained

Body Mass Index (BMI) is a quick screening number that estimates whether your weight is in a healthy range for your height. It's not a body-fat measurement and it's not a diagnosis — but it's a useful starting point in about three seconds of math.

The BMI formula

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

BMI = weight (kg) / height (m)²

So a person who is 1.75 m tall and weighs 70 kg has a BMI of 70 / (1.75 × 1.75) = 70 / 3.0625 ≈ 22.9.

In imperial units, you use pounds and inches and multiply by a conversion constant so the answer lands on the same scale:

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

The 703 comes from converting pounds to kilograms and inches to meters inside the formula: it equals (1 / 0.0254²) × 0.453592, which collapses to roughly 703. The result is the same number you'd get in metric.

The WHO category ranges

BMI results fall into standard screening categories set by the World Health Organization:

• Under 18.5 — Underweight
• 18.5 to 24.9 — Normal (healthy range)
• 25.0 to 29.9 — Overweight
• 30.0 or above — Obese

These are population-level screening bands, not individual diagnoses. A doctor reads them alongside waist circumference, blood pressure, family history, and lifestyle.

What BMI does and doesn't measure

BMI uses only height and weight, so it can't tell muscle from fat. That creates two well-known blind spots:

• It overestimates body fat for very muscular people. Muscle is denser than fat, so a lean athlete can register as "overweight" with low body fat.
• It underestimates body fat in some older adults who have lost muscle mass without losing much weight — their BMI may look normal while body-fat percentage is high.

BMI also ignores where fat sits on the body. Visceral fat around the organs carries more metabolic risk than the same amount of fat on the hips and thighs. For that reason, BMI is best treated as a starting point, not a verdict.

BMI vs BMR vs calorie targets

These three numbers get confused but answer different questions:

• BMI asks "is my weight in a typical range for my height?" — a screening number.
• BMR (basal metabolic rate) asks "how many calories does my body burn doing nothing?" — it depends on weight, height, age, and sex.
• A calorie target multiplies BMR by an activity factor to estimate daily needs, then adjusts for a goal (lose, maintain, gain).

So BMI tells you about your current weight, while BMR and a calorie target help you change it.

Try it yourself

The BMI Calculator computes your BMI and category from metric or imperial inputs. The BMI + Calorie Target tool goes one step further, combining BMI with a Mifflin-St Jeor BMR estimate and an activity factor to suggest a daily calorie target. Pair it with the BMR Calculator or the Macro Calculator to turn that target into protein, carb, and fat grams.