How to Calculate BMI: Step-by-Step Guide (Metric & Imperial)

Body Mass Index (BMI) is a simple calculation that uses your height and weight to produce a number that helps classify your weight status. Doctors, nutritionists, and public health organizations use it as a quick screening tool. Whether you want to check your own BMI or understand how the number is derived, this guide walks you through the math step by step.

For a broader understanding of what BMI means, its history, and its limitations, see our companion article: What Is BMI? Everything You Need to Know.

The Two BMI Formulas

There are two versions of the BMI formula depending on whether you use metric or imperial units. Both produce the same result.

Metric Formula

BMI = weight (kg) / [height (m)]²

Weight in kilograms, height in meters

Imperial Formula

BMI = [weight (lbs) / [height (in)]²] x 703

Weight in pounds, height in inches

The multiplication factor of 703 in the imperial formula is a unit conversion constant that accounts for the difference between pounds/inches and kilograms/meters.

Step-by-Step: Metric Calculation

Let us calculate the BMI for a person who weighs 82 kg and is 1.78 m tall.

Example: 82 kg, 1.78 m

Square the height: 1.78 x 1.78 = 3.1684
Divide weight by height squared: 82 / 3.1684 = 25.88

Result: BMI = 25.9 (rounded to one decimal place)

This falls in the "Overweight" category (25.0 - 29.9).

Example: 58 kg, 1.63 m

Square the height: 1.63 x 1.63 = 2.6569
Divide weight by height squared: 58 / 2.6569 = 21.83

Result: BMI = 21.8

This falls in the "Normal weight" category (18.5 - 24.9).

Common pitfall: If your height is in centimeters, you must convert to meters first by dividing by 100. So 178 cm becomes 1.78 m. If you use centimeters directly in the formula, your result will be off by a factor of 10,000.

Step-by-Step: Imperial Calculation

Now let us calculate the BMI for a person who weighs 180 lbs and is 5 feet 10 inches tall.

Example: 180 lbs, 5'10"

Convert height to inches: (5 x 12) + 10 = 70 inches
Square the height: 70 x 70 = 4,900
Divide weight by height squared: 180 / 4,900 = 0.03673
Multiply by 703: 0.03673 x 703 = 25.82

Result: BMI = 25.8

This falls in the "Overweight" category (25.0 - 29.9).

Example: 135 lbs, 5'4"

Convert height to inches: (5 x 12) + 4 = 64 inches
Square the height: 64 x 64 = 4,096
Divide weight by height squared: 135 / 4,096 = 0.03296
Multiply by 703: 0.03296 x 703 = 23.17

Result: BMI = 23.2

This falls in the "Normal weight" category (18.5 - 24.9).

Important: The entire height must be in inches. Do not mix feet and inches in the formula. A person who is 5'4" is 64 inches, not 5.4 feet. If you make this mistake, your BMI will be wildly inaccurate.

BMI Category Quick-Reference Chart

After calculating your BMI, use this chart to see which category you fall into:

BMI	Category
Below 18.5	Underweight
18.5 - 24.9	Normal weight
25.0 - 29.9	Overweight
30.0 - 34.9	Obese Class I
35.0 - 39.9	Obese Class II
40.0 and above	Obese Class III

Sample BMI Values by Height and Weight

This table shows BMI values for common height-weight combinations (imperial units). Find your height on the left, then scan across to your approximate weight:

Height	120 lbs	140 lbs	160 lbs	180 lbs	200 lbs	220 lbs
5'0"	23.4	27.3	31.2	35.2	39.1	43.0
5'4"	20.6	24.0	27.5	30.9	34.3	37.8
5'8"	18.2	21.3	24.3	27.4	30.4	33.5
6'0"	16.3	19.0	21.7	24.4	27.1	29.8
6'4"	14.6	17.1	19.5	21.9	24.4	26.8

Green-range values (18.5-24.9) indicate normal weight. Yellow-highlighted values are in the overweight range, orange in obese class I, and red in obese class II or III.

Converting Between Units

If your measurements are in mixed units, here are the conversions you need:

Pounds to kilograms: Divide by 2.205. Example: 180 lbs / 2.205 = 81.6 kg
Kilograms to pounds: Multiply by 2.205. Example: 75 kg x 2.205 = 165.4 lbs
Feet and inches to meters: Convert everything to inches first, then multiply by 0.0254. Example: 5'10" = 70 inches x 0.0254 = 1.778 m
Centimeters to meters: Divide by 100. Example: 175 cm / 100 = 1.75 m
Meters to inches: Multiply by 39.37. Example: 1.75 m x 39.37 = 68.9 inches

When BMI Calculation May Be Misleading

While BMI is a useful screening tool for most adults, the number can be misleading in certain situations:

Athletes and very muscular people: Muscle is denser than fat. A person with significant muscle mass may have a BMI in the "overweight" or "obese" range despite having very low body fat. Our body fat calculator gives a more accurate picture in these cases.
Elderly individuals: Older adults tend to lose muscle mass and gain fat. A "normal" BMI in a 75-year-old may mask an unhealthy amount of body fat.
Pregnant women: BMI is not applicable during pregnancy. Weight gain during pregnancy is expected and healthy.
Children and teens: Use age-and-sex-specific percentile charts rather than adult BMI categories.

For a more complete assessment, combine your BMI with other metrics. Your basal metabolic rate (BMR) tells you how many calories your body burns at rest, and a calorie calculator can help you determine daily intake targets based on your goals.

Quick Mental Estimation

If you do not have a calculator handy, here is a rough mental shortcut for imperial units: take your weight in pounds, double it, then add 10% of that result. Divide by the square of your height in inches. If the result is between 25 and 34, multiply by 28 to get an approximate BMI. This is imprecise but can give you a ballpark figure.

For an exact result with no math required, our free BMI calculator produces your number instantly and shows it on a visual gauge with your WHO weight classification.