What is the BMI formula?

The BMI formula relates a person's weight and height to produce a single index value used to screen body size categories. In metric units it is weight in kilograms divided by height in meters squared, and in imperial units it is weight in pounds divided by height in inches squared, then multiplied by 703. The resulting number is classified using the standard WHO ranges for underweight, normal, overweight, and obese.

How is the BMI formula calculated in metric units?

In metric units, BMI equals the weight in kilograms divided by the square of the height in meters, written as kg/m2. For example, a person weighing 70 kg with a height of 1.75 m has a BMI of 70 divided by 3.0625, which equals about 22.9. This places them in the normal weight category. The metric formula is the international standard and is what the WHO classification tables are based on.

How is the BMI formula calculated in imperial units?

In imperial units, BMI equals the weight in pounds divided by the square of the height in inches, multiplied by 703, written as (lbs/in2) x 703. The factor of 703 converts the pound-inch measurements into the equivalent kg/m2 scale so results are directly comparable to the metric values. For example, a person weighing 154 lb at 69 inches tall has a BMI of about 22.7, consistent with the metric calculation.

What do the variables in the BMI formula represent?

The variables are body weight and height, measured in kilograms or pounds and meters or inches respectively. Weight is the numerator and height squared is the denominator, which is why height has an outsized effect on the result. Squaring height normalizes the index so that it behaves consistently across different body sizes, allowing the same cutoffs to apply to adults of very different statures.

BMI Formula — Metric Calculation

The Body Mass Index (BMI) is a measure of body fat based on height and weight that applies to adult men and women. To calculate BMI, you divide weight by the square of height. This value categorizes individuals into groups: underweight, normal weight, overweight, or obese.

While BMI does not measure body fat directly, it is a highly reliable indicator of body fatness for most people. Using our BMI calculator or the formulas below, you can quickly assess weight-related health risks using either metric or imperial units.

BMI Formulas: Metric vs. Imperial

The mathematical principle is identical for both systems: weight divided by height squared. However, the imperial system requires a conversion factor to align with the standard metric scale.

Metric System: Uses kilograms (kg) and meters (m). No conversion factor is needed.
Imperial System: Uses pounds (lbs) and inches (in). A conversion factor of 703 is required to standardize the result.

Metric BMI Formula (kg/m²)

The standard metric formula is the most direct way to calculate BMI:

If you are also planning a home gym for your fitness journey, our guide on How to Calculate Square Footage (Room, Wall & Lot) can help you measure the space — see How to Calculate Square Footage (Room, Wall & Lot).

BMI = weight (kg) / [height (m)]²

Step-by-Step Example (Metric)

For an adult weighing 70 kg with a height of 1.75 meters:

Height Squared: 1.75 × 1.75 = 3.0625
Divide Weight: 70 / 3.0625 = 22.86
Result: BMI is 22.86 (Normal Weight).

Imperial BMI Formula (lbs/in²)

When using pounds and inches, apply the following conversion:

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

Step-by-Step Example (Imperial)

For an adult weighing 150 lbs with a height of 5’8″ (68 inches):

Height Squared: 68 × 68 = 4624
Divide Weight: 150 / 4624 = 0.032439
Multiply by 703: 0.032439 × 703 = 22.80
Result: BMI is 22.80 (Normal Weight).

For more details on the math, see How BMI Is Calculated or learn How to Count BMI for a deeper dive into its history.

Understanding BMI Categories

After calculating your score, use these standard World Health Organization (WHO) categories for adults:

Underweight: Below 18.5
Normal weight: 18.5 – 24.9
Overweight: 25.0 – 29.9
Obese: 30.0 or higher

Note: BMI is a screening tool. It does not account for muscle mass, bone density, or overall body composition. Athletes often have high BMIs due to muscle weight, not excess body fat.

BMI Calculation Reference Table

The following table provides examples across different weight classes using both measurement systems.

Weight (kg)	Height (m)	Weight (lbs)	Height (in)	BMI Result	Classification
45 kg	1.60 m	99.2 lbs	63.0 in	17.58	Underweight
70 kg	1.75 m	154.3 lbs	68.9 in	22.86	Normal Weight
80 kg	1.70 m	176.4 lbs	66.9 in	27.68	Overweight
100 kg	1.70 m	220.5 lbs	66.9 in	34.60	Obesity

Frequently Asked Questions

What is the most accurate BMI formula?

Both the metric and imperial formulas are equally accurate. The imperial formula simply includes the 703 conversion factor to ensure the final number matches the metric standard used worldwide.

Does BMI apply to children?

While the formula is the same, the interpretation is different. For children and teens, BMI is plotted on growth charts as a “BMI-for-age” percentile because body fat changes as they grow.

Why does BMI use height squared?

Squaring the height scales the measurement to two-dimensional space, which better correlates weight to surface area and body volume than height alone.

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Disclaimer: These tools and formulas are for educational purposes only and do not constitute medical advice. Always consult a healthcare professional before making health-related decisions.

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BMI Formula: How to Calculate Body Mass Index (Metric &am…