The Basic Formula for Body Mass Index

Body Mass Index (BMI) is a number that relates your weight to your height. The formula is the same everywhere: divide your weight in kilograms by your height in meters squared. If you use pounds and inches instead, you divide your weight in pounds by your height in inches squared, then multiply the result by 703.

The math itself takes about 30 seconds. You need two measurements—your weight and your height—and a calculator. The result is a single number, usually between 15 and 40 for most adults. That number falls into a category: underweight, normal weight, overweight, or obese. BMI does not measure body fat directly; it estimates it based on the relationship between those two measurements.

Key Takeaways

  • BMI uses only weight and height; you calculate it by dividing weight by height squared, then explore a conversion factor if you use imperial units.
  • The metric formula is weight in kilograms ÷ (height in meters)², and the imperial formula is (weight in pounds ÷ (height in inches)²) × 703.
  • A BMI under 18.5 is considered underweight, 18.5 to 24.9 is normal weight, 25 to 29.9 is overweight, and 30 or higher is obese.
  • BMI does not distinguish between muscle and fat, so athletes or very muscular people may have a high BMI despite low body fat.
  • Many health providers use BMI as a screening tool alongside other measurements like waist circumference and blood pressure.

Calculating BMI Using the Metric System

If you know your weight in kilograms and height in meters, the calculation is straightforward. Measure your height in meters (for example, 1.75 m), then square that number by multiplying it by itself. So 1.75 × 1.75 = 3.0625. Then divide your weight in kilograms by that result.

Example: You weigh 70 kilograms and are 1.75 meters tall. Your height squared is 3.0625. Divide 70 by 3.0625, and you get 22.9. That is your BMI. This number falls in the normal weight range (18.5 to 24.9).

The metric system is simpler because the conversion factor is already built into the units. You do not need to multiply by anything at the end—just divide weight by height squared, and you have your answer.

Calculating BMI Using Pounds and Inches

If you measure yourself in pounds and inches, the formula has an extra step because the units do not line up as neatly. Divide your weight in pounds by your height in inches squared, then multiply the result by 703. That 703 is a conversion constant that makes the final number match the metric result.

Example: You weigh 154 pounds and are 69 inches tall (5 feet 9 inches). Your height squared is 69 × 69 = 4,761. Divide 154 by 4,761, which gives you 0.0323. Multiply 0.0323 by 703, and you get 22.7. That is your BMI.

The 703 multiplier is not arbitrary—it exists so that a person who weighs 154 pounds at 69 inches gets the same BMI whether you calculate in metric or imperial units. Without it, the numbers would be much smaller and harder to interpret.

What Your BMI Number Means

Once you have your number, compare it to the standard ranges used by the Centers for Disease Control and Prevention and most health providers. A BMI under 18.5 is underweight. From 18.5 to 24.9 is normal weight. From 25 to 29.9 is overweight. A BMI of 30 or higher is obese.

These ranges are the same for all adults, regardless of age or sex, with one exception: children and teens use different charts because their body composition changes as they grow. If you are calculating BMI for someone under 20, you need age-specific growth charts, not the adult ranges.

The categories are broad because BMI is a screening tool, not a diagnosis. A single number cannot tell you whether you are healthy. Two people with the same BMI can have very different amounts of body fat, muscle, and overall health.

Why BMI Has Limits as a Health Measure

BMI does not measure body fat directly—it estimates it. The formula works well for most people, but it breaks down in specific groups. Athletes and very muscular people often have a high BMI because muscle weighs more than fat. A bodybuilder might have a BMI of 28 and very little body fat, yet fall into the overweight category.

Older adults often have more body fat at the same BMI as younger people, because muscle naturally decreases with age. Pregnant people and people with certain medical conditions also have BMI readings that do not reflect their actual body composition. For these reasons, doctors usually look at BMI alongside other measurements: waist circumference, blood pressure, cholesterol levels, and physical fitness.

BMI is useful as a quick screening tool that takes seconds to calculate. It is less useful as a standalone measure of health or fitness. If your BMI falls outside the normal range and you want to understand what that means for your health, a conversation with your doctor is more informative than the number itself.

Common Mistakes When Computing BMI

The most common error is mixing units. If you use pounds for weight but meters for height, or kilograms for weight but inches for height, your answer will be wrong. Write down which system you are using before you start, and stick with it all the way through.

Another frequent mistake is forgetting to square the height. The formula requires height squared—height multiplied by itself. If you just divide weight by height without squaring, you will get a number that is too high. Double-check that step before you multiply by 703 (if using imperial) or move to the final answer (if using metric).

Rounding too early can also throw off your result. If you round your height squared to the nearest whole number before dividing, you may be off by a point or two. Keep at least two decimal places through the entire calculation, then round your final BMI to one decimal place.

Tools and Alternatives to Hand Calculation

Many online BMI calculators let you enter your weight and height, and they compute the number when ready. These are accurate as long as you enter your measurements correctly. A calculator is faster than doing the math by hand, especially if you are checking your BMI multiple times or calculating it for different people.

Some bathroom scales and fitness trackers display BMI if you enter your height into them. These are convenient but only as accurate as your weight measurement. If the scale is not calibrated, the BMI will be off.

If you want to understand your body composition beyond what BMI tells you, other methods exist. Waist circumference is straightforward to measure and correlates with health risk. Bioelectrical impedance scales estimate body fat percentage. DEXA scans and hydrostatic weighing are more precise but require a visit to a clinic or lab. For most people, BMI plus waist circumference gives a clearer picture than BMI alone.

Frequently Asked Questions

Is BMI the same for men and women?

Yes. The BMI formula and the standard ranges (underweight, normal, overweight, obese) are identical for men and women. However, men and women typically carry body fat differently—men tend to store more around the abdomen, women around the hips and thighs. BMI does not account for this difference, which is why waist circumference is sometimes measured alongside BMI.

What if I am very tall or very short?

The BMI formula works across all heights, but it may be less accurate at the extremes. Very tall people sometimes have a higher BMI than their body composition suggests, and very short people sometimes have a lower one. If you are significantly taller or shorter than average, discuss your BMI with your doctor rather than relying on the standard ranges alone.

How often should I calculate my BMI?

There is no set schedule. If you are tracking changes in your weight or health, calculating it once a month gives you enough time to see meaningful shifts without obsessing over day-to-day fluctuations. Weight naturally varies by a few pounds depending on hydration, time of day, and other factors, so a single calculation is less useful than a trend over weeks or months.

Can BMI be used for children?

The adult BMI formula works for children, but the interpretation is different. Children's BMI is compared to age and sex-specific growth charts rather than the fixed adult ranges. If you need to calculate BMI for a child, use the same formula but consult your pediatrician or the CDC growth charts to understand what the number means.

What should I do if my BMI is outside the normal range?

A BMI outside the normal range is a signal to talk with your doctor, not a diagnosis of a health problem. Your doctor can look at your overall health, family history, fitness level, and other factors to determine whether your weight poses any health concerns. They may suggest changes to diet or exercise, or they may tell you that your current weight is healthy for you.