The BMI formula can look confusing at first. It includes squared height, different measurement systems, and a mysterious number called 703. Fortunately, the calculation is much easier to understand once each part is broken down.
BMI stands for Body Mass Index. It compares your body weight with your height and produces a number that can be placed within a general weight category.
Healthcare professionals commonly use it as a quick screening tool because measuring height and weight is inexpensive, simple, and widely accessible.
However, BMI is not a direct measurement of body fat, fitness, or overall health. It cannot tell whether your weight comes mainly from muscle, fat, bone, or another type of body tissue.
This guide offers the BMI formula explained in simple terms, with clear examples using kilograms and meters as well as pounds and inches.
You will also learn why height is squared, what the final number means, which mistakes to avoid, and why BMI should always be interpreted with additional health information.
What Is the BMI Formula?
The standard metric BMI formula is:
BMI = weight in kilograms ÷ height in meters squared
It can also be written as:
BMI = kg ÷ m²
The phrase “height squared” simply means multiplying your height by itself. Someone who is 1.70 meters tall would square that measurement like this:
1.70 × 1.70 = 2.89
Their weight would then be divided by 2.89. The result is their Body Mass Index.
The formula does not directly calculate how much body fat someone has. It creates a weight-to-height ratio that can be compared with established adult BMI categories.
The CDC describes BMI as a calculated measure based on body weight in kilograms divided by height in meters squared.
Why Does the Formula Square Your Height?
Weight and height do not increase at the same rate as people grow. A taller person generally has more body tissue and naturally weighs more than a shorter person.
Simply dividing weight by height would not adjust sufficiently for the difference between shorter and taller bodies. Squaring height makes the index more useful when comparing body weight across people of different heights.
You do not need to understand advanced mathematics to use the formula. Just remember that height must be multiplied by itself before weight is divided by it.
For example, imagine two adults who both weigh 75 kilograms. One is 1.60 meters tall, while the other is 1.85 meters tall. Their BMI results will be different because the same amount of weight is distributed across bodies of different heights.
The formula is not a perfect model of human shape or body composition. It is a practical mathematical tool designed to standardize weight relative to height.
How to Calculate BMI Using Kilograms and Meters
Suppose an adult weighs 70 kilograms and is 1.75 meters tall. The calculation can be completed in three simple stages.
Step 1: Square the Height
Multiply 1.75 by itself:
1.75 × 1.75 = 3.0625
Step 2: Divide Weight by Squared Height
Divide 70 by 3.0625:
70 ÷ 3.0625 = 22.86
Step 3: Round the Result
The final BMI is approximately:
BMI = 22.9
A BMI of 22.9 falls within the standard healthy-weight category for adults. That category covers results from 18.5 to 24.9.
When height is recorded in centimeters, convert it to meters by dividing by 100. For example, 175 centimeters becomes 1.75 meters.
An alternative formula allows you to keep the height in centimeters:
BMI = weight in kg ÷ height in cm ÷ height in cm × 10,000
Using the same measurements:
70 ÷ 175 ÷ 175 × 10,000 = 22.9
Both methods produce the same answer. The CDC recognizes both versions of the metric formula.
How to Calculate BMI Using Pounds and Inches
People who measure weight in pounds and height in feet and inches can use the U.S. customary BMI formula:
BMI = weight in pounds ÷ height in inches squared × 703
The number 703 is a conversion factor. It adjusts the pounds-and-inches calculation so that the answer matches the standard BMI scale used with kilograms and meters.
Suppose an adult weighs 160 pounds and is 5 feet 8 inches tall.
First, convert the full height into inches. Five feet equals 60 inches, so adding the remaining 8 inches gives a total height of 68 inches.
Next, square the height:
68 × 68 = 4,624
Divide the weight by the squared height:
160 ÷ 4,624 = 0.0346
Finally, multiply by 703:
0.0346 × 703 = 24.3
The person’s BMI is approximately 24.3, which falls within the standard healthy-weight range for adults.
The most common error with this version is entering feet and inches as a decimal. For example, 5 feet 8 inches should not be entered as 5.8. Convert the entire measurement into inches first.
What Does Your BMI Number Mean?
Standard BMI categories for adults are commonly interpreted as follows:
| BMI Range | Adult Weight Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5–24.9 | Healthy Weight |
| 25.0–29.9 | Overweight |
| 30.0–34.9 | Class 1 Obesity |
| 35.0–39.9 | Class 2 Obesity |
| 40.0 or Higher | Class 3 Obesity |
The World Health Organization defines overweight in adults as a BMI of at least 25 and obesity as a BMI of at least 30.
These categories are screening thresholds, not precise borders between healthy and unhealthy bodies. A person with a BMI of 24.9 is not dramatically different from someone with a BMI of 25.0.
BMI should also be interpreted differently for children and teenagers. For people aged 2 through 19, the CDC uses sex-specific BMI-for-age percentiles because children’s body composition changes as they grow.
Standard adult categories may also require additional context during pregnancy, in older age, or among people with unusually high muscle mass.
Common Mistakes When Using the BMI Formula
One frequent mistake is forgetting to square the height. Dividing weight by height only once will not produce a valid BMI.
Another is mixing measurement systems. Kilograms must be paired with meters or centimeters, while pounds must be paired with inches. Combining kilograms with inches will produce an incorrect result.
Entering centimeters into the meter-based formula is another common problem. A height of 170 centimeters must be written as 1.70 meters unless you are using the alternative formula that includes multiplication by 10,000.
It is also better to avoid rounding too early. Keep several decimal places during the calculation, then round the final result to one decimal place.
Finally, use reasonably accurate measurements. Remove shoes when recording height, wear light clothing when checking weight, and place the scale on a firm, level surface.
What the BMI Formula Cannot Tell You
BMI does not distinguish body fat from muscle. A muscular athlete may have a high result even when their body-fat percentage is relatively low.
The formula also cannot show where fat is stored. Abdominal fat can be particularly relevant to metabolic health, which is why waist circumference may provide useful additional information.
NIDDK recommends considering both BMI and waist size when assessing weight-related health risks. BMI cannot measure blood pressure, cholesterol, blood glucose, physical fitness, nutrition quality, sleep, stress, or family medical history.
Someone with a BMI in the healthy range can still have important health risks, while a person with a higher result may have favorable laboratory results and strong physical fitness.
For that reason, BMI is best viewed as a screening tool rather than a diagnosis. A healthcare professional may combine it with waist size, medical history, physical examinations, laboratory tests, and lifestyle information.
How to Use Your BMI Result Sensibly
Start by treating the result as one piece of information. Avoid using it as a personal grade or a complete description of your body.
Look for broader patterns. Consider whether your weight has changed unexpectedly, how your clothes fit around the waist, how active you are, and whether measurements such as blood pressure or blood glucose are within appropriate ranges.
A BMI result that sits slightly outside a category should not trigger an extreme diet or exercise plan. Sustainable habits, including balanced eating, regular movement, sufficient sleep, and routine healthcare, are more valuable than chasing a particular number.
Consult a qualified healthcare professional when you have unexplained weight changes, concerning symptoms, an unusually high or low result, or questions about how BMI applies to your individual circumstances.
The BMI formula compares your weight with the square of your height. In metric units, divide kilograms by meters squared. When using pounds and inches, divide pounds by inches squared and multiply the answer by 703.
The calculation is simple, but its meaning requires context. BMI can place adults into general weight categories and help screen for potential health concerns, yet it cannot directly measure body fat, muscle mass, fitness, or overall well-being.
Calculate your BMI with accurate measurements, check that you are using compatible units, and interpret the result alongside waist circumference, health tests, lifestyle, and medical history. Use the number to begin a thoughtful health conversation-not as the final verdict on your body.