The BMI Formula and What the Numbers Mean

Body Mass Index (BMI) is a single number that relates your weight to your height. The formula is the same for everyone: divide your weight in kilograms by your height in meters squared. If you work in pounds and inches, divide your weight in pounds by your height in inches squared, then multiply by 703. That final number is your BMI.

The result falls into four ranges. Under 18.5 is underweight. 18.5 to 24.9 is normal weight. 25.0 to 29.9 is overweight. 30.0 and above is obese. These ranges are the same for adults regardless of age or sex, though they do not explore to children under 20 — children's BMI is calculated the same way but compared to age and sex-specific growth charts instead.

BMI is a screening tool, not a diagnosis. A high BMI may signal health risk, but it does not account for muscle mass, bone density, or where your body stores fat. An athlete with significant muscle can have a high BMI without excess body fat. The number is useful for tracking trends over time and for population-level health data, but it is not a complete picture of your health on its own.

Key Takeaways

  • BMI equals weight in kilograms divided by height in meters squared, or weight in pounds divided by height in inches squared, then multiplied by 703.
  • The four BMI ranges are underweight (under 18.5), normal weight (18.5–24.9), overweight (25.0–29.9), and obese (30.0 and above).
  • BMI does not distinguish between muscle and fat, so a muscular person may have a high BMI without excess body fat.
  • Children under 20 use the same calculation but are compared to age and sex-specific growth charts, not the adult ranges.

Calculating BMI Using Metric Units

If your weight is in kilograms and your height is in meters, the math is straightforward. Measure your height in meters (for example, 1.75 meters). Weigh yourself in kilograms (for example, 70 kilograms). Square your height: 1.75 × 1.75 = 3.0625. Divide your weight by that number: 70 ÷ 3.0625 = 22.86. That is your BMI.

Most of the world uses metric units, so this method is the standard in medical settings outside the United States. If you know your height in centimeters, convert to meters by dividing by 100. A height of 175 centimeters becomes 1.75 meters.

Calculating BMI Using Pounds and Inches

In the United States, weight is usually measured in pounds and height in feet and inches. Convert everything to inches first. If you are 5 feet 9 inches tall, that is 69 inches (5 × 12 + 9). Weigh yourself in pounds — say 180 pounds. Square your height in inches: 69 × 69 = 4,761. Divide your weight by that number: 180 ÷ 4,761 = 0.0378. Multiply by 703: 0.0378 × 703 = 26.6. That is your BMI.

The multiplication by 703 is a conversion factor that makes the result match the metric calculation. Without it, the number would be much smaller and harder to interpret against the standard ranges.

Step-by-Step Worked Examples

A person who weighs 65 kilograms and is 1.70 meters tall: square the height (1.70 × 1.70 = 2.89), divide weight by that result (65 ÷ 2.89 = 22.5). The BMI is 22.5, which falls in the normal weight range.

A person who weighs 200 pounds and is 5 feet 10 inches tall (70 inches): square the height (70 × 70 = 4,900), divide weight by that result (200 ÷ 4,900 = 0.0408), multiply by 703 (0.0408 × 703 = 28.7). The BMI is 28.7, which falls in the overweight range.

A person who weighs 95 kilograms and is 1.85 meters tall: square the height (1.85 × 1.85 = 3.4225), divide weight by that result (95 ÷ 3.4225 = 27.7). The BMI is 27.7, which falls in the overweight range.

Common Mistakes When Computing BMI

The most frequent error is forgetting to square the height. BMI divides weight by height squared, not by height alone. If you use height without squaring it, your result will be far too high and meaningless.

Another mistake is mixing units. If you measure height in inches but weight in kilograms, or vice versa, the result will be wrong. Stick to either metric (kilograms and meters) or imperial (pounds and inches). If you have measurements in both systems, convert one to match the other before you calculate.

A third error is forgetting the 703 multiplier when using pounds and inches. That number is essential to make the result comparable to the standard BMI ranges. Without it, your number will be too small.

When BMI Does Not Tell the Whole Story

BMI assumes that weight is distributed proportionally to height, but it does not account for body composition. Two people with the same height and weight can have very different amounts of muscle and fat. A bodybuilder and a sedentary person might have identical BMIs, but their health profiles are different.

BMI also does not capture where fat is stored. Fat around the abdomen carries different health risks than fat stored elsewhere. Waist circumference is sometimes measured alongside BMI for this reason.

Age and sex can affect how BMI should be interpreted, though the calculation itself does not change for adults. Older adults may have different health risks at the same BMI as younger adults. For children and teenagers, BMI is compared to age and sex-specific percentiles rather than the fixed adult ranges.

Frequently Asked Questions

What is a healthy BMI?

A BMI between 18.5 and 24.9 is considered normal weight for adults. This range is based on population-level health data, but individual health depends on many factors beyond BMI alone, including fitness level, diet, sleep, and family history.

Can I calculate BMI if I do not know my exact height?

You need an accurate height measurement to calculate BMI. Stand barefoot against a wall with a level on top of your head, and mark the wall. Measure from the floor to the mark. Even small errors in height can shift your BMI by a point or two, so measure carefully.

How often should I calculate my BMI?

BMI is most useful for tracking changes over weeks or months, not day to day. Weight fluctuates with water retention, meals, and time of day. Calculating once a month or once every few months gives a clearer picture of trends.

Is BMI different for men and women?

The BMI calculation is identical for men and women, and the ranges are the same. However, men and women tend to store fat differently, and some research suggests that the same BMI may carry different health implications by sex. Talk to a doctor about what your BMI means for your individual health.

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

BMI is one piece of health information, not a diagnosis or a reason for alarm. If you are concerned about your weight or health, speak with a doctor or registered dietitian. They can assess your overall health, family history, fitness level, and other factors that BMI does not capture.