What is BMI?
Body mass index (BMI) is a single number that compares your weight to your height. It gives a quick estimate of whether your weight sits in a range associated with good health for most adults. It’s widely used because it only needs two measurements you already know — your height and your weight.
BMI is a screening tool, not a measure of body fat or overall health on its own. It works well at a population level and as a starting point for a conversation, but the number alone doesn’t tell the whole story for any one person.
BMI categories for adults
These are the standard adult cut-offs from the World Health Organization:
| Category | BMI range |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese | 30.0 and above |
How BMI is calculated
The formula depends on which units you use:
- Metric: BMI = weight (kg) ÷ height (m)². For example, 70 kg at 1.75 m is 70 ÷ (1.75 × 1.75) ≈ 22.9.
- Imperial: BMI = 703 × weight (lb) ÷ height (in)². For example, 154 lb at 69 in is 703 × 154 ÷ (69 × 69) ≈ 22.7.
What BMI doesn’t tell you
BMI treats all weight the same, so it can be misleading in a few common cases. Muscle weighs more than fat, so very athletic people can land in the “overweight” range while carrying little fat. It also doesn’t account for age, sex, ethnicity, or where you carry weight — factors that all affect health risk. Use BMI as one signal among several, alongside measures like waist circumference and how you feel day to day.
Frequently asked questions
How do I calculate my BMI?
BMI is your weight in kilograms divided by the square of your height in metres (kg/m²). In imperial units, multiply your weight in pounds by 703, then divide by your height in inches squared. The calculator above does both for you.
What is a healthy BMI range?
For most adults, the World Health Organization considers a BMI of 18.5 to 24.9 to be the healthy range. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is in the obese range.
Is BMI accurate?
BMI is a quick screening tool, not a diagnosis. It doesn't distinguish muscle from fat, and it doesn't account for age, sex, ethnicity, or where you carry weight. Treat it as a starting point and discuss your results with a clinician.
Does BMI matter for weight management?
BMI is one of several measures clinicians use when discussing weight. The trend over time is usually more useful than a single reading. If you're working toward a weight goal, ask your prescriber or clinician how to interpret your number.
Keep going
BMI labels your weight; for an actionable daily number, use the TDEE calculator to find your calorie target. Then browse our calorie-counted meal plans, find routines in the exercise library, or read the weight-loss blog.