How to Calculate BMI and What It Means
4 min read · Last reviewed August 10, 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 US customary units the usual shortcut is weight in pounds divided by height in inches squared, multiplied by 703. That 703 is a rounded constant; converting with the exact definitions (1 lb = 0.45359237 kg, 1 in = 2.54 cm) and using the metric formula is slightly more precise. The categories are the same whichever units you use.
What the categories mean
The CDC's adult bands are: under 18.5 underweight, 18.5 to less than 25 healthy weight, 25 to less than 30 overweight, and 30 or above obesity — which is itself split into class 1 (30 to under 35), class 2 (35 to under 40) and class 3 or severe obesity (40 and above).
You will usually see these written as 18.5–24.9 and 25–29.9. That's a rounding convenience. The actual boundaries are open at the top, so a BMI of exactly 25.0 sits in the overweight band rather than at the top of the healthy one.
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 adults the CDC defines the healthy range as 18.5 to less than 25 — commonly written 18.5–24.9, though the true upper boundary is 25. 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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