BMI Calculator

What the BMI formula does with your height and weight

Body mass index puts your weight on a scale that accounts for height. One number comes out, and that number is compared with fixed cutoffs. The CDC gives two versions of the same formula, one for metric units and one for pounds and inches.

Metric: BMI = weight in kilograms / (height in meters squared). The CDC also writes it as (kg / cm / cm) x 10,000, which avoids converting centimeters to meters first.

US customary: BMI = 703 x weight in pounds / (height in inches squared).

The 703 is not a magic number. One pound is 0.45359237 kilograms and one inch is 0.0254 meters, so a pound per square inch is 0.45359237 / 0.00064516 = 703.07 kilograms per square meter. The formula is the metric one wearing American clothes.

Both routes land on the same unit, kilograms per square meter, which is why a BMI of 26.6 means the same thing whether you typed pounds or kilos. If a calculator returns 0.038 for a 170 pound adult, it forgot the 703. If it returns 2,660, it forgot to square the height in inches and used a height in centimeters instead.

The CDC categories for adults 20 and older

The cutoffs do not change with age, sex or race once you are 20 or older. The CDC adult BMI categories page lists four bands and splits obesity into three classes.

The bands are ranges of a screening measure, not diagnoses. The CDC says BMI should be considered with other factors, including medical history, lab results and physical exam findings. A person at 27 with normal blood pressure, a 32 inch waist and a steady weight is in a different position from a person at 27 with a 44 inch waist and rising blood sugar, even though the number on this page is identical.

The line between 24.9 and 25.0 is one tenth of a point. At 5 ft 7 in, that tenth is about 0.6 pounds, so a large meal and a glass of water can move you across a category boundary without changing anything that matters. Treat the edges as fuzzy.

Worked example: 5 feet 7 inches and 170 pounds

Height: 5 ft 7 in is 67 inches. Square it: 67 x 67 = 4,489. Multiply weight by 703: 703 x 170 = 119,510. Divide: 119,510 / 4,489 = 26.62, which rounds to 26.6. That sits in the overweight band, 1.6 points above the healthy ceiling.

Now find the weight that would put the same height at 24.9, the top of the healthy range: 24.9 x 4,489 / 703 = 159.0 pounds. So the gap to the healthy band is 170 - 159 = 11 pounds. At 18.5, the bottom of the band, the same height weighs 18.5 x 4,489 / 703 = 118.1 pounds.

That gives a healthy weight range of roughly 118 to 159 pounds for a person 5 ft 7 in tall. The range is 41 pounds wide, which is a reminder that BMI allows a lot of room inside one category. Two people at the same height and the same label can differ by 40 pounds.

For a frame of reference, 200 pounds at 5 ft 9 in gives 703 x 200 / 4,761 = 29.5, still overweight and 0.5 points under the obesity line. The same person reaches a BMI of 30 at 30 x 4,761 / 703 = 203.2 pounds.

Worked example in metric: 75 kg at 1.75 meters

Height in meters: 1.75. Square it: 1.75 x 1.75 = 3.0625. Divide weight by that: 75 / 3.0625 = 24.49. That is inside the healthy band, 0.41 points under the ceiling.

At 90 kg the same height reads 90 / 3.0625 = 29.39, which is overweight and 1.9 kg below the obesity line. The exact weight at BMI 30 is 30 x 3.0625 = 91.875 kg, so 91.9 kg. At the other end, BMI 18.5 starts at 18.5 x 3.0625 = 56.7 kg.

Here is the same arithmetic written the way the CDC writes it, in centimeters: (75 / 175 / 175) x 10,000 = 24.49. Same answer, no decimal meters to type. The 10,000 is there because centimeters are 100 times smaller than meters, and the height is squared, so 100 x 100 = 10,000.

One common error is dividing by height instead of height squared. At 75 kg and 1.75 m that returns 42.9, a number that would look alarming and means nothing. If your result is in the 40s at a normal weight, check the squaring first.

The healthy weight range for your height

The table below converts the 18.5 to 24.9 band into pounds at common heights, using the 703 formula. Round numbers are shown, so the edges can be off by half a pound.

The weight at BMI 25 is where the healthy band ends. The weight at BMI 30 is where obesity begins. Both columns are computed as 25 x inches squared / 703 and 30 x inches squared / 703, so you can fill in a height that is not listed with two multiplications.

Note how fast the numbers rise with height. Going from 5 ft 0 in to 6 ft 4 in adds 45 pounds to the top of the healthy range. That is why comparing your weight with a friend's weight tells you nothing without the height next to it.

Where BMI gives the wrong answer

BMI cannot separate fat from muscle and bone, and it cannot see where fat sits. The CDC's BMI frequently asked questions say this directly. Four groups run into the limit most often.

Muscular adults. A 6 ft 2 in athlete at 220 pounds reads 703 x 220 / 5,476 = 28.2, in the overweight band, with a body fat percentage that may be under 12. The formula sees the weight, not the tissue.

Older adults. Muscle mass falls with age while fat can rise. A person who has lost an inch of height and several pounds of muscle can keep the same BMI while their body composition changes for the worse. The number stays flat and hides the shift.

Pregnancy. BMI is not the tool for pregnancy weight. Providers use a pre-pregnancy BMI to set a gain range and then track gain against it, which is a different calculation with different targets.

Children and teens. From age 2 to 19, BMI is read as a percentile against growth charts for the child's age and sex, not against the adult bands. A 10 year old at 22 is not in the same position as an adult at 22.

Dogs and cats. The same formula applied to a dog produces absurd numbers, because a healthy Labrador lands near 75 kg/m2. Vets score body condition by hand instead. Our dog BMI page covers that separate scale, and the human bands do not transfer to it.

Reading your own number without overreacting

Three extra measurements turn a single number into something you can act on.

Waist circumference. The NHLBI puts higher risk above 35 inches for women and 40 inches for men. Measure at the top of the hip bones, after breathing out, with the tape snug and level. A person with a BMI of 27 and a 33 inch waist is carrying the weight differently from a person at 27 with a 43 inch waist.

Trend over time. One reading is a snapshot. Weigh yourself under the same conditions, once a week, and look at the direction across two or three months. Day to day swings of 2 to 4 pounds are water and food, not fat.

BMI prime. Divide BMI by 25 and the healthy ceiling becomes 1.0. Our BMI of 26.6 is a prime of 1.06, which reads as 6 percent over the top of the healthy band. Some people find that easier to hold in their head than the raw number.

The NHLBI notes that a loss of 5 to 10 percent of starting weight over about six months is the usual first target, and that 3 to 5 percent already improves triglycerides and blood sugar. For a 170 pound adult, 5 percent is 8.5 pounds and 10 percent is 17 pounds. That is a smaller change than jumping from 26.6 to 24.9, and it is the change the guidance asks for first.

What the calculator does, and what it leaves out

The widget rounds BMI to one decimal, because the cutoffs are written to one decimal and two extra digits would suggest precision the formula does not have. It also shows the healthy weight range for your height, the pounds to the nearest edge of that range, and BMI prime.

It does not adjust for frame size, body fat percentage, waist, fitness or age. It does not know whether you are pregnant, whether you are 19 or 20, or whether you are a dog. Those are the questions the rest of this page answers, and the honest answer for most of them is that BMI is one input among several.

If you want the number checked against something else, waist-to-height ratio is the simplest addition: keep your waist under half your height. A 70 inch adult would aim for a waist under 35 inches, which lines up with the NHLBI cutoff for women and is stricter than the one for men. Neither measure replaces a conversation with a clinician who can see the whole picture.

All of the arithmetic runs in your browser. Nothing you type is sent anywhere or stored.