How to Calculate BMI: The Metric and Imperial Formulas, Explained

BMI (Body Mass Index) is the number your doctor writes down at every checkup, the number your insurance company looks up, and the number people quote badly on the internet. It’s two measurements — weight and height — turned into a single number. This guide covers exactly how to calculate it, what the number means, and — importantly — the situations where BMI gives you a wrong answer.

The two formulas

Metric (kilograms and metres)

Example: someone who weighs 70 kg and is 1.75 m tall — BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.86.

The most common mistake here is using centimetres instead of metres. If your height is in cm, divide by 100 first. 175 cm becomes 1.75 m before you square it.

Imperial (pounds and inches)

Example: someone who weighs 155 lb and is 5′9″ (69 inches) — BMI = (155 × 703) ÷ (69 × 69) = 108,965 ÷ 4,761 = 22.89.

The 703 is a conversion constant that makes the imperial formula produce the same scale as the metric one — so 22.89 imperial and 22.86 metric mean the same body.

For height, first convert feet-and-inches to inches: (feet × 12) + inches. 5′9″ = 60 + 9 = 69 inches.

What the number means

The WHO categories for adults (18+):

• Under 18.5 — underweight
• 18.5 to 24.9 — normal / healthy weight
• 25.0 to 29.9 — overweight
• 30.0 to 34.9 — obese class I
• 35.0 to 39.9 — obese class II
• 40.0 and above — obese class III (severe)

These cutoffs are the same worldwide — a BMI of 27 is “overweight” whether you measured it in Tokyo, Toronto, or Tallinn. Some Asian health guidelines use slightly lower thresholds (23 for overweight, 27.5 for obese) because population studies suggest health risks appear at lower BMIs in some Asian populations. Check what your country’s health authority uses.

What BMI does NOT measure

Adolphe Quetelet invented BMI in the 1830s to describe populations, not to diagnose individuals. Two centuries later that’s still the honest use case. The formula has known blind spots:

• Muscle mass. Muscle is denser than fat. A trained athlete with a BMI of 27 is not “overweight” in any meaningful health sense — they’re just muscular. Rugby players, weightlifters, dancers, and CrossFitters routinely land in the “overweight” or “obese” brackets while being metabolically healthy.
• Fat distribution. Fat around the waist (visceral fat) carries far more health risk than fat elsewhere. Two people with identical BMIs can have very different health outcomes based on where their fat sits. Waist circumference is a better indicator here.
• Age. Muscle mass declines from about age 30 onwards. A 65-year-old with a BMI of 22 may actually have more fat and less muscle than a 25-year-old with the same BMI.
• Bone density and frame size. A person with a large frame gets penalized by BMI; a person with a small frame gets flattered by it. Doctors used to reference “small/medium/large frame” corrections — those largely fell out of use, unfortunately.
• Children and adolescents. The adult categories don’t apply. Children use age- and sex-specific percentiles instead — a pediatrician will interpret those.

Better metrics if you want to look deeper

• Waist circumference — measured at the belly button. Above 40″ (102 cm) for men or 35″ (88 cm) for women is associated with elevated risk regardless of BMI.
• Waist-to-hip ratio — waist ÷ hip. Above 0.90 for men or 0.85 for women flags an apple-shaped fat distribution.
• Body fat percentage — measured via calipers, bioelectric impedance, or DEXA scan. More directly meaningful than BMI, harder to measure.
• TDEE (Total Daily Energy Expenditure) — the calories your body burns. Useful for planning weight change; our TDEE Calculator uses the Mifflin-St Jeor formula.
• Cardiovascular fitness — VO₂ max, resting heart rate, or just how far you can walk in a minute. Fitness independently predicts health outcomes better than weight does.

Quick calculation

For a one-shot answer without doing the math, use our BMI Calculator — it supports both metric and imperial input and shows the category interpretation. For target-weight planning, try the Ideal Body Weight calculator (which uses four different formulas because there’s no single “ideal”).