BMI calculator

How is BMI calculated?

BMI is weight in kilograms divided by height in meters squared. In pounds and inches, the CDC formula is weight ÷ height² × 703: at 5 ft 8 in (68 inches) and 160 lb, BMI = 160 ÷ 4,624 × 703 = 24.3.

What are the BMI categories for adults?

By the CDC: under 18.5 is underweight, 18.5 to under 25 healthy weight, 25 to under 30 overweight, and 30 or more obesity — class 1 from 30, class 2 from 35 and class 3 (severe) from 40.

What is a healthy weight for my height?

The weights that give a BMI from 18.5 to under 25. At 5 ft 8 in that is about 122 to 164 pounds. The calculator shows the range for your height.

Is BMI accurate for everyone?

It is a quick screening number, not a diagnosis. It does not tell muscle from fat, so a muscular athlete can read "overweight"; it also says nothing about where fat is carried. A doctor looks at more than BMI.

Does it work for children?

No — children and teens are judged by BMI percentiles for their age and sex on CDC growth charts. This calculator uses the adult categories, for ages 20 and up.

Why does the pound-and-inch formula multiply by 703?

It converts units. BMI is defined in kilograms per square meter. One pound is 0.45359237 kg and one inch is 0.0254 m, so pounds per square inch become kilograms per square meter after multiplying by 0.45359237 ÷ 0.0254² = 703.07, which is rounded to 703.

Are the adult BMI categories different for men, women or older adults?

No. For adults aged 20 and older the CDC uses the same weight status categories for all ages and for both sexes. Other tools, such as waist measurements, add information that BMI does not have.

How much does a small change in height or weight move BMI?

More than people expect. At 64 inches and 150 lb the BMI is 25.7; measured as 65 inches instead, it would be 25.0. Height errors matter because the formula squares the height.

How the calculation works

BMI is weight divided by the square of height. In metric units that is kilograms per square meter. With US units the calculator uses weight in pounds × 703 ÷ height in inches squared, which gives the same value as the metric formula. The result is then placed in one of the CDC adult categories: below 18.5, 18.5 to under 25, 25 to under 30, and 30 and above, with the obesity classes starting at 30, 35 and 40. The healthy weight range shown is the weight that gives a BMI from 18.5 up to, but not including, 25 at your height.

Worked example

A person is 64 inches tall (5 ft 4 in) and weighs 150 lb. BMI = 150 × 703 ÷ 64² = 105450 ÷ 4096 = 25.7, which is in the overweight range. In metric units the same person is 162.6 cm and 68.0 kg, and 68.0 ÷ 1.626² = 25.7, the same value. The healthy range at this height is from 108 lb (BMI 18.5) to just under 146 lb (BMI 25).

Common mistakes

• Entering feet as one number. A height of 5 ft 4 in is 5 feet and 4 inches, not 5.4 feet. Use the two boxes.
• Using the adult scale for a child. Under age 20, BMI is read against age- and sex-specific percentiles, so the child BMI calculator should be used instead.
• Treating BMI as a diagnosis. It does not separate muscle from fat or show where fat is carried. A result in the overweight range is a reason to look at other measures, not a verdict.

Sources and related calculators

Categories and formula: CDC, "Adult BMI Categories" and "Calculating BMI Using the English System." Related: child BMI calculator, waist-to-height ratio calculator and ideal weight calculator.

For general information only — not medical advice. Talk with your doctor or another health professional about your own health. How we check our calculators.