BMI Calculator

Body mass index is weight divided by height squared. In US units, BMI = 703 × weight (lb) ÷ height (in)²; in metric, BMI = weight (kg) ÷ height (m)². For an adult who is 5 ft 9 in (69 in) and weighs 160 lb, 703 × 160 ÷ 4,761 ≈ 23.6, which the CDC places in its Healthy Weight category. The BMI calculator below does the arithmetic for you.

How to use the BMI calculator

1. Choose Adult, 20+ or Child or teen, 2 to 19. The adult categories only apply from age 20.
2. Pick US units (feet, inches and pounds) or metric units (centimeters and kilograms). Switching converts what you typed.
3. Enter your height and weight. The result updates as you type, or press Enter or Calculate BMI.
4. Read your BMI, the CDC category and the color scale, then the healthy-weight range for your height and your BMI Prime.
5. Open Show steps for the formula worked with your numbers, and Weight ranges for every CDC category at your height.

BMI is a screening number, not a diagnosis, and these results are estimates, not medical advice. A healthcare provider can tell you what your BMI means for you.

BMI formula in metric and US units

The CDC gives the formula two ways:

• Metric: BMI = weight (kg) ÷ [height (m)]²
• US customary: BMI = weight (lb) ÷ [height (in)]² × 703

BMI is always reported in kilograms per square meter (kg/m²). Height is in meters, not centimeters, so 175 cm becomes 1.75 m before you square it.

Where the 703 comes from

The US formula starts with pounds per square inch and converts the answer to kilograms per square meter. One pound is exactly 0.45359237 kg and one inch is exactly 0.0254 m, so 1 lb/in² = 0.45359237 ÷ 0.0254² ≈ 703.07 kg/m². The CDC rounds this factor to 703. The difference is about 0.01%, which almost never changes a BMI rounded to one decimal.

CDC adult BMI categories

For adults 20 and older, the CDC uses the same categories regardless of age, sex or race. The ranges are published as “18.5 to less than 25” and so on; with BMI rounded to one decimal, that is 18.5 to 24.9. The last column is the CDC’s own example for an adult 5 feet 9 inches tall, and the calculator reproduces it exactly for that height.

Obesity as a whole is a BMI of 30.0 or greater. The BMI Prime column is simple arithmetic (BMI ÷ 25), not a separate CDC scale.

Worked BMI examples

Example 1: US units

An adult is 5 ft 9 in tall and weighs 160 lb. Height in inches: 5 × 12 + 9 = 69 in, and 69² = 4,761. Multiply the weight by 703: 160 × 703 = 112,480. Divide: 112,480 ÷ 4,761 ≈ 23.625, which rounds to a BMI of 23.6. That is between 18.5 and 24.9, so the CDC category is Healthy Weight.

Example 2: metric units

An adult is 165 cm tall and weighs 70 kg. Convert the height to meters: 165 ÷ 100 = 1.65 m, and 1.65² = 2.7225. Divide: 70 ÷ 2.7225 ≈ 25.71, so the BMI is 25.7. That falls in the CDC Overweight range (25.0 to 29.9), and the BMI Prime is 25.7 ÷ 25 = 1.028.

Example 3: healthy-weight range for a height

Rearrange the formula to find weight: weight (lb) = BMI × height (in)² ÷ 703. For 5 ft 4 in (64 in), 64² = 4,096. A BMI rounds to 18.5 through 24.9 when it is at least 18.45 and below 24.95, so 18.45 × 4,096 ÷ 703 ≈ 107.5 lb and 24.95 × 4,096 ÷ 703 ≈ 145.4 lb. In whole pounds, the healthy-weight range at 5 ft 4 in is 108 to 145 lb. In metric, the same method gives 50.3 to 67.9 kg for a height of 165 cm.

Healthy weight range by height

This chart lists the weights that give an adult BMI of 18.5 to 24.9 at each height, worked out with the same rounding rule as the CDC example above.

What is BMI Prime?

BMI Prime is your BMI divided by 25, the point where the CDC healthy-weight range ends. It turns BMI into a ratio: 1.000 means a BMI of exactly 25, 0.900 means 10% below that cutoff, and 1.200 means 20% above it (a BMI of 30). In Example 1, 23.6 ÷ 25 = 0.944. Dividing the rounded BMI keeps the cutoffs in line with the CDC categories.

BMI for children and teens

For ages 2 through 19, the adult categories do not apply. Because children are still growing, the CDC compares a child’s BMI with other children of the same sex and age and reports a BMI-for-age percentile. The CDC child and teen categories are:

• Underweight: less than the 5th percentile
• Healthy Weight: 5th percentile to less than the 85th percentile
• Overweight: 85th percentile to less than the 95th percentile
• Obesity: 95th percentile or greater
• Severe Obesity: 120% of the 95th percentile or greater, or a BMI of 35 or greater

The calculator above works out a child’s BMI number with the same formula, but it does not estimate a percentile. For that, use the CDC Child and Teen BMI Calculator, which asks for age and sex and plots the result on a growth chart. Our percentile calculator explains what a percentile means.

Limitations of BMI

The CDC calls BMI a quick, safe and reliable screening measure, and NHLBI notes that it applies to adult men and women. Both also describe what BMI cannot tell you:

• BMI is not a direct measure of body fat.
• It cannot tell fat from muscle and bone. People with a lot of muscle, such as many athletes, can have a BMI in the overweight range.
• It does not take muscle mass, bone density or body composition into account.
• It does not show what types of fat a person has or where on the body the fat is carried.
• It is one measure of health. For an individual, the CDC says BMI should be considered together with medical history, health behaviors, a physical exam and lab results.

BMI is useful for spotting a possible issue and tracking change over time; a healthcare provider decides whether a BMI is too high or too low for you.

Common BMI calculation mistakes

• Not squaring the height. 70 ÷ 1.65 is about 42.4, not a BMI. You must divide by 1.65² = 2.7225.
• Using centimeters in the metric formula. 70 ÷ 165² gives about 0.0026. Change 165 cm to 1.65 m first.
• Using 703 with kilograms and meters. The 703 factor is only for pounds and inches.
• Writing 5’9″ as 5.9 feet. 9 inches is 0.75 ft, so 5 ft 9 in is 5.75 ft or 69 in.

Sources

Checked November 2026.

• CDC: Adult BMI Categories
• CDC: Frequently Asked Questions about BMI
• CDC: About Body Mass Index (BMI)
• CDC: BMI Calculator for Children and Teens
• CDC: Adult BMI Calculator
• NHLBI: Calculate Your BMI

BMI practice quiz: 10 questions

Test yourself with 10 multiple-choice questions. Choose an answer and press Submit to see if you are right and why. Your score appears at the end.

Frequently asked questions

What is a healthy BMI for adults?

The CDC Healthy Weight range for adults 20 and older is a BMI of 18.5 to less than 25, which is 18.5 to 24.9 when BMI is rounded to one decimal. Below 18.5 is Underweight, 25.0 to 29.9 is Overweight and 30.0 or greater is Obesity.

How do I calculate BMI by hand?

In pounds and inches, multiply your weight by 703 and divide by your height in inches squared. In kilograms and meters, divide your weight by your height squared. For 150 lb at 5 ft 9 in: 150 × 703 ÷ 4,761 ≈ 22.1.

Is BMI accurate for muscular people?

Not always. The CDC notes that BMI cannot tell fat from muscle and bone, so people with a lot of muscle can have a BMI in the overweight category without having excess body fat. A healthcare provider can use other measures to look closer.

What BMI is considered obesity?

For adults, the CDC defines obesity as a BMI of 30.0 or greater. It splits this into Class 1 (30.0 to 34.9), Class 2 (35.0 to 39.9) and Class 3, also called severe obesity (40.0 or greater).

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