What this calculator does
Body mass index, or BMI, is a single number that compares your weight to your height. This calculator works it out for adults aged 20 and older. You can enter your height and weight in feet, inches and pounds, or in centimeters and kilograms. It then shows your BMI, the adult category your number falls into, and the range of body weights that match a BMI between 18.5 and 24.9 at your height.
If your BMI is above or below that range, the calculator also shows how far your current weight is from the nearest edge of the range. This gives you a rough sense of scale. It is not a goal that anyone has set for you.
How to use it
- Pick your units. Choose pounds, feet and inches, or kilograms and centimeters. The other fields switch to match.
- Enter your height. In US units, type whole feet in one box and the leftover inches in the other. A person who is 5 feet 9 inches tall enters 5 and 9. Do not type 69 inches in the feet box.
- Enter your weight. Use a recent weight taken without heavy clothing or shoes if you can.
- Read the results. The big number is your BMI, rounded to one decimal place. Below it you will see the category and the weight range for your height.
How BMI is calculated
The Centers for Disease Control and Prevention (CDC) gives BMI as weight in kilograms divided by height in meters, squared. In other words, you multiply your height by itself, then divide your weight by that result.
BMI = weight (kg) ÷ height (m)2
BMI = 703 × weight (lb) ÷ height (in)2
The second line is the same formula for pounds and inches. The number 703 comes from unit conversion: one pound is exactly 0.45359237 kilograms and one inch is exactly 0.0254 meters, and 0.45359237 divided by 0.0254 squared is about 703.07. This calculator uses the exact conversion factors, so it may differ from a rough 703 calculation by a tiny amount in rare cases.
To find the weight range for your height, the calculator reverses the formula. It multiplies the BMI value (18.5 for the low end, 24.9 for the high end) by your height in meters squared. The answer is the weight in kilograms that would produce that BMI. For US units it converts the result back to pounds.
Adult BMI categories
For adults aged 20 and older, the CDC uses the same categories for everyone, no matter their age, sex or race. Here they are:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 to 34.9 | Obesity, Class 1 |
| 35.0 to 39.9 | Obesity, Class 2 |
| 40.0 or higher | Obesity, Class 3 (severe obesity) |
The calculator rounds your BMI to one decimal place first and then picks the category from that rounded number. A result of 24.96 therefore shows as 25.0 and falls in the overweight row.
Worked examples
These results come straight from the calculator.
| Height and weight | BMI | Category | Weight range for BMI 18.5 to 24.9 |
|---|---|---|---|
| 5 ft 9 in, 160 lb | 23.6 | Healthy weight | 125.3 to 168.6 lb |
| 5 ft 4 in, 200 lb | 34.3 | Obesity, Class 1 | 107.8 to 145.1 lb |
| 175 cm, 90 kg | 29.4 | Overweight | 56.7 to 76.3 kg |
| 160 cm, 45 kg | 17.6 | Underweight | 47.4 to 63.7 kg |
Take the first row by hand. 5 feet 9 inches is 69 inches, and 69 squared is 4,761. Multiply 160 by 703 to get 112,480, then divide by 4,761. The answer is about 23.6.
What BMI can and cannot tell you
The CDC describes BMI as a screening measure. It is not a diagnosis. BMI uses only height and weight, so it does not measure body fat directly. It cannot tell the difference between fat, muscle and bone. A very muscular person can have a high BMI without carrying much extra fat. An older adult who has lost muscle can have a BMI in the healthy range and still have a high share of body fat.
BMI also does not show where fat is stored on the body, and it says nothing about blood pressure, cholesterol, fitness, diet or family history. For these reasons the CDC says BMI should be considered together with other factors when assessing a person's health. A health care provider can look at the whole picture and tell you what your number means for you.
Frequently asked questions
- Is this calculator for children and teens?
- No. The adult categories apply to people aged 20 and older. For ages 2 to 19, the CDC compares BMI with growth charts for the same age and sex, using percentiles. Use the CDC child and teen BMI calculator for them.
- Is a BMI of 25 the same as being unhealthy?
- Not by itself. The categories are cutoffs used for screening. A BMI of 24.9 and a BMI of 25.0 are almost the same body size. Treat the category as a prompt to look at other health measures, not as a verdict.
- Why does the calculator ask for feet and inches separately?
- Because that is how most people in the United States know their height. The calculator adds feet times 12 to the inches, then converts the total to meters.
- Does the result change with age or sex?
- For adults, the BMI number and the CDC categories do not change with age or sex. What the number means for your health can still differ from person to person.
- How accurate is the weight range?
- It is simple arithmetic: the weights at BMI 18.5 and 24.9 for your exact height. It is a reference range for comparison, not a target or a prescription.
- Can I use this while pregnant?
- This calculator does not account for pregnancy. Ask your doctor or midwife what weight gain is healthy for you.
Important notes
This calculator is for general information. It does not diagnose any condition and is not medical advice. BMI is only one indicator and should be read with other factors such as blood pressure, cholesterol and a physical exam. If you have questions about your weight or health, or if you plan to change your diet or exercise a lot, please talk to a doctor or another qualified health care provider.