How to Calculate BMI and What It Means
4 min read · Last reviewed July 1, 2026
Body Mass Index is a simple screening number that compares your weight to your height. It's widely used because it's quick and needs nothing more than a set of scales and a tape measure — but it's often misunderstood.
This guide shows how BMI is calculated, what the standard categories mean, and — just as importantly — where the number falls short. It's general information, not medical advice; a doctor is the right person to interpret your health.
The BMI formula
In metric units, BMI is your weight in kilograms divided by your height in metres squared. Someone who is 1.75 m tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.
In imperial units, the formula is weight in pounds divided by height in inches squared, multiplied by 703. The categories are the same whichever units you use.
What the categories mean
For most adults, a BMI under 18.5 is classed as underweight, 18.5 to 24.9 as a healthy range, 25 to 29.9 as overweight, and 30 or above as obese.
These bands are a starting point for a conversation, not a diagnosis. A result near a boundary doesn't suddenly change your health — it's a signal to look at the fuller picture rather than a verdict on it.
Where BMI falls short
BMI doesn't distinguish muscle from fat, so very muscular people can read as 'overweight' despite low body fat. It also doesn't account for where fat is stored, which matters for health, and it's less reliable for older adults, athletes, pregnant women, and children.
It's best treated as one rough indicator among several. Waist measurement, body-fat percentage, activity levels, and how you actually feel all add context a single number can't.
Frequently asked questions
▶What is a healthy BMI?
For most adults, a BMI between 18.5 and 24.9 is considered the healthy range. It's a screening guide, not a diagnosis — a healthcare professional can tell you what it means for you.
▶Is BMI accurate for athletes?
Not especially. Because muscle weighs more than fat, muscular people often have a high BMI despite being lean. In those cases a body-fat measurement is far more informative.
Try the calculators
Calculate your Body Mass Index from height and weight, in metric or imperial units, with your category.
Open calculator →Estimate a healthy target weight for your height and sex using several established formulas.
Open calculator →Find how many calories you need each day to maintain, lose, or gain weight, based on your body and activity.
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