The BMI formula and how to use it

BMI stands for Body Mass Index, and it is a single number that estimates whether your weight is healthy for your height. The formula is straightforward: divide your weight in kilograms by your height in meters squared. If you use pounds and inches instead, the formula is slightly different but just as straightforward.

The metric version is: BMI = weight (kg) ÷ height (m)². The imperial version is: BMI = [weight (lbs) ÷ height (inches)²] × 703. The 703 is a conversion factor that makes the imperial result match the metric one. Either way, you end up with a single number between roughly 15 and 40 for most adults.

Once you have your BMI number, you compare it to standard ranges. A BMI under 18.5 is considered underweight, 18.5 to 24.9 is normal weight, 25 to 29.9 is overweight, and 30 and above is obese. These ranges are the same for all adults regardless of age or sex, though some health professionals note that they work better for some groups than others.

Key Takeaways

  • The metric formula is weight in kilograms divided by height in meters squared; the imperial formula is weight in pounds divided by height in inches squared, then multiplied by 703.
  • You need only two measurements—your weight and your height—to calculate BMI; no other data is required.
  • A BMI of 18.5 to 24.9 falls in the normal range for adults, while 25 to 29.9 is overweight and 30 or above is obese.
  • BMI is a screening tool only and does not measure body fat directly or account for muscle, bone density, or where weight is distributed on your body.

Step-by-step calculation in metric units

Start by converting your weight to kilograms if you know it only in pounds. Divide your weight in pounds by 2.205 to get kilograms. For example, 180 pounds is roughly 81.6 kilograms.

Next, convert your height to meters. If you know your height in feet and inches, convert the whole thing to inches first, then divide by 39.37 to get meters. A person who is 5 feet 10 inches tall is 70 inches, which is about 1.78 meters. Alternatively, divide feet by 3.281 and inches by 39.37 separately, then add them.

Now square your height in meters. Multiply it by itself. If you are 1.78 meters tall, 1.78 × 1.78 = 3.17.

Finally, divide your weight in kilograms by the squared height. If you weigh 81.6 kilograms and your squared height is 3.17, then 81.6 ÷ 3.17 = 25.8. That is your BMI.

Step-by-step calculation in imperial units

If you prefer to work in pounds and inches, you do not need to convert. Measure your height in inches from the top of your head to the soles of your feet. A person 5 feet 10 inches tall is 70 inches.

Square your height in inches. Multiply it by itself. 70 × 70 = 4,900.

Divide your weight in pounds by the squared height. If you weigh 180 pounds, then 180 ÷ 4,900 = 0.0367.

Multiply the result by 703. So 0.0367 × 703 = 25.8. That is your BMI, and it matches the metric calculation because 703 is the conversion factor that makes the two formulas equivalent.

What your BMI number means

The Centers for Disease Control and Prevention (CDC) uses these ranges for adults 20 and older: BMI under 18.5 is underweight, 18.5 to 24.9 is normal weight, 25 to 29.9 is overweight, and 30 or above is obese. These ranges do not change based on age or sex.

A BMI in the normal range does not mean you are healthy, and a BMI outside the normal range does not mean you are unhealthy. BMI is a screening tool that flags whether further conversation with a doctor might be useful. It does not measure body fat, does not account for muscle mass, does not consider bone density, and does not show where weight sits on your body—all of which matter for actual health.

Common mistakes when calculating BMI

The most common error is forgetting to square the height. Many people divide by height once instead of multiplying height by itself. Check your work: if your height in meters is 1.78, then 1.78 squared is 3.17, not 1.78.

Another mistake is mixing units. Do not divide pounds by height in meters, or kilograms by height in inches. Stick to one system all the way through. If you start in metric, finish in metric. If you start in imperial, finish in imperial and remember the 703 multiplier.

A third error is measuring height incorrectly. Stand barefoot with your back against a wall, heels touching the wall, and have someone mark the top of your head. Measure from the floor to the mark. Shoes, thick hair, or leaning forward will throw off the result.

When to use BMI and when it falls short

BMI works reasonably well as a quick screening tool for average adults. It is straightforward to calculate, requires only two measurements, and the ranges are standardized so you can compare your number to public health data.

BMI does not work well for athletes or very muscular people, because muscle weighs more than fat and will push the BMI number higher even if body fat is low. It also does not account for age—an older adult with the same BMI as a younger adult may have different health risks. Pregnant people, children, and people with certain medical conditions may need different assessment methods.

If your BMI is outside the normal range and you want to understand what that means for your health, talk to a doctor or registered dietitian. They can look at your full picture—family history, fitness level, where weight sits on your body, blood pressure, cholesterol, and other factors—rather than relying on a single number.

Frequently Asked Questions

Do I need to use metric or imperial units?

Either one works. Use whichever you are more comfortable with. The metric formula is simpler (no conversion factor), but the imperial formula works just as well if you remember to multiply by 703 at the end. Both give the same result.

Does BMI change if I measure myself at different times of day?

Your weight can shift by a few pounds depending on when you eat, drink, or use the bathroom, so your BMI may shift slightly too. For the most consistent result, weigh yourself in the morning before eating or drinking. Height can also vary slightly—you are taller in the morning than at night because your spine decompresses while lying down—but the difference is usually less than half an inch.

Is BMI the same for men and women?

Yes. The CDC uses the same BMI ranges for all adults 20 and older, regardless of sex. Some research suggests that women and men carry weight differently and may have different health risks at the same BMI, but the standard formula and ranges are identical.

What if my BMI is exactly on the border between two categories?

A BMI of exactly 25.0 falls into the overweight category, and a BMI of exactly 30.0 falls into the obese category. These are the cutoff points. If you are right on the line, use the higher category. The difference of 0.1 or 0.2 is not meaningful anyway—BMI is a rough estimate, not a precise measurement.

Can I calculate BMI for children using the same formula?

The formula is the same, but the interpretation is different. Children and teens use age and sex-specific BMI percentiles rather than the fixed ranges used for adults. A BMI that is normal for a 10-year-old boy might be overweight for a 16-year-old girl. If you need to assess a child's BMI, use the CDC's BMI calculator for children, which adjusts for age and sex automatically.