What your BMI tells you
Body mass index, or BMI, compares your weight with your height. Doctors and public health agencies use it as a quick first check of whether an adult's weight sits in a range linked with lower health risks. It takes seconds to work out and needs nothing more than a tape measure and a set of scales.
This BMI calculator gives you the number, the weight category the US Centers for Disease Control and Prevention (CDC) uses for adults, and the range of weights that would count as healthy for your height. It works in metric or US units, and nothing you enter is stored.
How to check your BMI
- Choose metric or US units.
- Enter your height. In US units, put the feet and inches in separate boxes.
- Enter your weight, ideally measured in the morning in light clothes.
- Read your BMI, its category and the healthy weight range for your height.
The BMI formula
BMI = weight in kg ÷ (height in metres)²
BMI = 703 × weight in pounds ÷ (height in inches)²The two lines give the same answer; 703 converts pounds and inches into the metric units. The result is rounded to one decimal place before the category is read, the way the CDC's own calculator does it. The formula is the same for women and men, which is why the calculator does not ask.
Worked example: 175 cm and 70 kg
- Height in metres: 1.75, squared: 3.0625
- BMI: 70 ÷ 3.0625 = 22.86, which rounds to 22.9
- Category: Healthy weight (18.5 to under 25)
A BMI of 22.9 sits in the healthy weight range. For someone 175 cm tall, that range runs from about 56.7 kg to 76.3 kg, worked out by multiplying 3.0625 by 18.5 and by 24.9. In US units, someone 5 ft 9 in and 160 lb has a BMI of 23.6.
Adult BMI categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to under 25 | Healthy weight |
| 25 to under 30 | Overweight |
| 30 to under 35 | Obesity, class 1 |
| 35 to under 40 | Obesity, class 2 |
| 40 or above | Obesity, class 3 |
These are the CDC's adult BMI categories. They apply to adults of any age, sex or background. Children and teenagers are measured differently, against growth charts for their age and sex, so this calculator is for adults only.
What BMI can and cannot tell you
- It does not measure body fat directly. A muscular athlete can have a high BMI and very little fat.
- It does not show where fat is carried. Fat around the waist is linked with more health risk than fat elsewhere, which is why waist size is often checked too.
- It is less precise for older adults, who tend to have less muscle at the same weight.
- It is a starting point. If your BMI is outside the healthy range, or you are worried about your weight, a doctor can look at the full picture.
Frequently asked questions
How do I calculate my BMI?
Divide your weight in kilograms by your height in metres squared. At 1.75 m and 70 kg, that is 70 ÷ 3.0625 = 22.9. In pounds and inches, multiply your weight by 703 and divide by your height in inches squared. Enter your height and weight above and the calculator does both, with each step shown.
Is BMI different for women and men?
The formula and the adult categories are the same for women and men, so the calculator does not ask. Women tend to carry more body fat than men at the same BMI, which is one reason BMI is treated as a screening number rather than a full measure of health.
What is a healthy BMI?
For adults, the CDC counts a BMI from 18.5 to under 25 as healthy weight. Below 18.5 is underweight, 25 to under 30 is overweight and 30 or more is obesity. The calculator also shows the weights that would put you in the healthy range for your own height.
Can I use this BMI calculator for a child?
No. Children and teenagers grow at different rates, so their BMI is compared with others of the same age and sex using growth charts, and the result is a percentile rather than a fixed category. This calculator uses the adult categories, which apply from age 20.
Why might my BMI be misleading?
BMI only looks at height and weight. It cannot tell muscle from fat, so very muscular people can read as overweight, and it says nothing about where weight is carried. Treat it as one useful number, and talk to a doctor if you want a fuller picture of your health.
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