BMI Calculator

BMI calculator at a glance#

What is BMI?#

Body mass index (BMI) is your weight divided by the square of your height, given in kg/m². It compares weight with height in one quick number. The CDC calls it a screening measure and notes it doesn't tell fat, muscle and bone apart.

How do you calculate BMI?#

Divide your weight in kilograms by your height in meters squared. With pounds and inches, divide pounds by inches squared and multiply by 703. Enter your age, sex, height and weight above, in feet, inches and pounds or in centimeters and kilograms. The calculator shows your BMI, its category, the healthy weight range for your height and how far you are from that range.

What is a healthy BMI for adults?#

For adults 20 and older, the CDC puts a healthy weight at a BMI of 18.5 to less than 25. Under 18.5 is underweight, 25 to less than 30 is overweight, and 30 or more is obesity. These cutoffs are the same for men and women of every age from 20 up.

What BMI measures#

BMI turns two numbers you already know into one that can be compared across people of different heights. A tall person and a short person can both weigh 150 lb, but that weight sits very differently on their frames. Dividing by height squared puts everyone on the same scale.

Take a 35-year-old woman who is 5 ft 6 in (167.64 cm) tall and weighs 150 lb (68.04 kg). Her BMI is 24.2, which sits in the healthy weight range, close to its upper end at 25. We'll use her numbers through this page.

What BMI can't see is what the weight is made of. The CDC page on BMI says BMI does not distinguish between fat, muscle and bone mass, and it doesn't show where fat is carried. A muscular athlete and a sedentary person with the same height and weight get the same BMI. That's why the CDC calls it a screening measure and says that, for an individual, it should be weighed together with medical history, health behaviors, a physical exam and lab results.

How to calculate BMI by hand#

You need only a calculator and your height and weight. Use whichever units you measured in.

BMI with pounds and inches#

The CDC formula for US units is BMI = weight (lb) ÷ height (in)² × 703.

1. Convert height to inches: 5 ft 6 in = 5 × 12 + 6 = 66 inches.
2. Square it: 66 × 66 = 4,356.
3. Divide the weight by that: 150 ÷ 4,356 = 0.034435.
4. Multiply by 703: 0.034435 × 703 = 24.2.

The 703 is there only to convert units. A pound is 0.45359237 kg and an inch is 0.0254 m, and 0.45359237 ÷ (0.0254 × 0.0254) = 703.07, so the pound and inch version gives the same answer as the metric one.

BMI with kilograms and meters#

The metric formula is BMI = weight (kg) ÷ height (m)². Height in centimeters goes to meters by dividing by 100, so 167.64 cm is 1.6764 m.

1. Square the height: 1.6764 × 1.6764 = 2.8103.
2. Divide the weight by it: 68.04 ÷ 2.8103 = 24.2.

The calculator does the same math and rounds the result to one decimal place, so both versions show 24.2 kg/m².

Adult BMI categories#

The CDC's adult BMI categories apply to everyone 20 and older, based on BMI alone, regardless of age, sex or race. Obesity is split into three classes.

The calculator reports the four main categories (underweight, healthy weight, overweight and obese) and doesn't split obesity into classes. It also marks your BMI on a scale under the result.

On that scale, the woman in our example lands at 24.2, inside the healthy band. At 5 ft 6 in, a BMI of 25 is reached at 154.9 lb, so she's about 5 lb under the overweight line.

What the calculator shows#

Besides your BMI and its category, the result table gives you four more numbers.

• Healthy BMI range: 18.5 to 25 kg/m² for adults, or the age and sex range for children.
• Healthy weight for the height: the weights that give a BMI of 18.5 and 25 at your height. For 5 ft 6 in, that's 114.6 to 154.9 lb, or 52.0 to 70.3 kg.
• Gain or lose to reach the range: how many pounds or kilograms separate you from the nearest end of the healthy range. Inside the range, both rows show a dash.
• Ponderal index: weight divided by height cubed instead of squared, in kg/m³. For the example it's 68.04 ÷ (1.6764 × 1.6764 × 1.6764) = 14.44.

The healthy weight range comes from running the BMI formula backward. Multiply 18.5 and 25 by your height in meters squared: 18.5 × 2.8103 = 52.0 kg and 25 × 2.8103 = 70.3 kg. To see where several named ideal weight formulas put you, try the ideal weight calculator, which works from your height and sex.

This gives an estimate, not medical advice. A doctor can tell you what the number means for you.

BMI for children and teens#

Children's BMI is calculated with the same formula, but the result is read differently. The CDC compares a child's BMI with other children of the same age and sex, using sex-specific BMI-for-age percentiles. Its child and teen categories cover ages 2 through 19.

• Underweight: below the 5th percentile.
• Healthy weight: the 5th percentile to less than the 85th.
• Overweight: the 85th percentile to less than the 95th.
• Obesity: the 95th percentile or greater.

When you enter an age from 2 to 20, the calculator switches to these percentile cutoffs, read from the CDC growth charts at each whole year of age. At 20 the calculator still uses the age-20 percentiles, while the CDC uses the adult categories from 20, so at 20 read your BMI against the adult table above. For a 10-year-old girl, for example, it uses a BMI of 14.0 for the 5th percentile, 19.9 for the 85th and 22.9 for the 95th. The healthy BMI range and healthy weight rows then show the 5th to 85th percentile range for that age and sex.

Healthy weight range by height#

Here's the range the calculator shows for common heights, from a BMI of 18.5 to a BMI of 25. The last column is the weight where a BMI of 30, the CDC's obesity cutoff, begins.

Sources#

• CDC, Adult BMI Categories
• CDC, About Body Mass Index (BMI)
• NHLBI, NIH, Calculate Your BMI

Frequently asked questions#

Is BMI different for men and women?#

Not for adults. The CDC uses the same cutoffs for men and women 20 and older, so a man and a woman with the same height and weight get the same BMI and the same category. Sex matters only for ages 2 to 19, where children are compared with others of the same age and sex. That's why the calculator asks for it, and it uses sex up to age 20.

What does a BMI of 30 mean?#

A BMI of 30 is where the CDC's obesity category begins. From 30 to less than 35 is class 1 obesity, 35 to less than 40 is class 2, and 40 or more is class 3. At 5 ft 6 in, a BMI of 30 is reached at 185.9 lb.

Why is my BMI high when I have a lot of muscle?#

BMI only knows your weight and height. It can't tell whether the weight is muscle or fat, and the CDC says it doesn't distinguish between fat, muscle and bone. A very muscular person can land in the overweight range without carrying extra fat. A body fat estimate, like the body fat percentage calculator, looks at a different measure.

How much should I weigh for my height?#

In BMI terms, a healthy adult weight is anything that puts your BMI between 18.5 and 25. For 5 ft 6 in, that's 114.6 to 154.9 lb. The table above lists the range for other heights, and the calculator gives it for your exact height. If you're working toward a weight, the calorie calculator estimates daily calories for losing, keeping or gaining weight.

What is the BMI formula with pounds?#

BMI = weight in pounds ÷ height in inches squared × 703. For 150 lb at 66 inches: 150 ÷ 4,356 × 703 = 24.2.

Is BMI a diagnosis?#

No. The CDC describes BMI as a screening measure and one possible health indicator, to be read alongside other information. The NHLBI notes that BMI doesn't account for muscle mass, bone density or body composition, and that your healthcare provider will consider whether your BMI is too high or too low for you.