Formula Used to Calculate Body Mass Index (BMI)

A simple tool to assess your body weight relative to your height.

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–Your BMI

The Body Mass Index (BMI) is a value derived from weight and height. It is a convenient rule of thumb to broadly categorize a person’s weight as underweight, normal weight, overweight, or obese.

Visual representation of your BMI value against standard categories.

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What is the Formula Used to Calculate Body Mass Index?

The formula used to calculate Body Mass Index (BMI) is a simple mathematical equation that provides a score to assess if your weight is appropriate for your height. It is a widely used screening tool to identify potential weight-related health problems. The BMI is defined as the body mass divided by the square of the body height. While it’s a useful indicator, it doesn’t directly measure body fat and should be considered alongside other health metrics.

BMI Formula and Explanation

The specific formula for BMI depends on the unit system you use—metric or imperial. Our calculator handles both conversions for you automatically.

Metric Formula

When using kilograms (kg) for weight and meters (m) for height, the formula is:

BMI = weight (kg) / [height (m)]²

Since height is often measured in centimeters (cm), you must first convert it to meters by dividing by 100.

Imperial Formula

When using pounds (lbs) for weight and inches (in) for height, the formula requires a conversion factor:

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

The conversion factor of 703 is necessary to make the units compatible and arrive at the same BMI value as the metric formula.

Variables in the BMI Formula

Variable	Meaning	Common Units	Typical Range
Weight	The mass of the individual.	Kilograms (kg), Pounds (lbs)	40 – 150 kg (90 – 330 lbs)
Height	The stature of the individual from head to foot.	Centimeters (cm), Inches (in), Meters (m)	140 – 200 cm (55 – 79 in)

Practical Examples

Example 1: Metric Units

Let’s calculate the BMI for an adult who weighs 68 kg and is 165 cm tall.

1. Convert height: 165 cm ÷ 100 = 1.65 m
2. Square the height: 1.65 m × 1.65 m = 2.7225 m²
3. Apply the formula: 68 kg / 2.7225 m² = 24.98 BMI

This result falls into the ‘Healthy Weight’ category.

Example 2: Imperial Units

Let’s calculate the BMI for an adult who weighs 150 lbs and is 5 feet 5 inches tall.

1. Convert height to inches: (5 ft × 12 in/ft) + 5 in = 65 inches
2. Square the height: 65 in × 65 in = 4225 in²
3. Apply the formula: (703 × 150 lbs) / 4225 in² = 105,450 / 4225 = 24.96 BMI

This result is also within the ‘Healthy Weight’ range. For more examples, see this BMI calculation formula guide.

How to Use This Body Mass Index Calculator

1. Select Your Units: Choose between ‘Metric’ (kg, cm) or ‘Imperial’ (lbs, inches) from the dropdown menu. The input labels will update automatically.
2. Enter Your Weight: Input your current weight into the corresponding field.
3. Enter Your Height: Input your height in either centimeters or total inches.
4. View Your Result: The calculator updates in real-time to show your BMI, the associated weight category (e.g., ‘Normal weight’), and a visual chart.
5. Interpret the Results: Use the generated BMI to understand where you fall on the standard weight spectrum. The article below provides more context on these categories.

Key Factors That Affect Body Mass Index

While the BMI formula is straightforward, the resulting number is influenced by a complex interplay of factors beyond just height and weight. It’s important to understand these as they provide context to your BMI value.

• Age: Body composition (the ratio of muscle to fat) changes with age. Older adults may have more body fat than younger adults with the same BMI.
• Sex: Adult women typically have a higher percentage of body fat than men with an equivalent BMI.
• Genetics: Research shows that genetics can play a significant role in a person’s predisposition to weight gain and where their body stores fat.
• Muscle Mass: BMI does not distinguish between muscle and fat. Very muscular individuals, like athletes, can have a high BMI and be classified as ‘Overweight’ despite having very low body fat.
• Physical Activity Level: Lack of physical activity is associated with a higher BMI, as it reduces the number of calories your body uses.
• Sleep Quality: Studies have shown a link between poor sleep and a high BMI. Lack of sleep can affect hormones that control hunger.
• Ethnicity: Different ethnic groups can have different body compositions and associated health risks at the same BMI. For example, some Asian populations may have increased health risks at a lower BMI compared to European populations.

Before making conclusions, consider your factors that influence body weight.

Frequently Asked Questions (FAQ)

1. What is a healthy BMI?

For most adults, a healthy BMI is between 18.5 and 24.9. A value below 18.5 is considered underweight, 25.0 to 29.9 is overweight, and 30.0 or above is classified as obese.

2. Is the formula used to calculate body mass index accurate for everyone?

No. The BMI formula is a screening tool, not a diagnostic one. It has limitations, especially for athletes, pregnant women, the elderly, and children, because it doesn’t account for muscle mass or body fat distribution.

3. Why is there a different formula for imperial and metric units?

The core calculation is based on the metric system. The imperial formula includes a conversion factor (703) to adjust for the different units (pounds and inches) to produce the same standardized BMI value.

4. Can I have a high BMI and still be healthy?

Yes, it’s possible. For example, bodybuilders have a lot of muscle, which is very dense. This can result in a high BMI even if they have very low body fat. This is a key limitation of BMI.

5. How is BMI for children calculated?

The calculation is the same, but the interpretation is different. For children and teens, BMI is plotted on age- and sex-specific percentile charts because body fat changes with age and differs between boys and girls.

6. Does BMI measure body fat?

No, BMI is an indirect measure. It is a value derived from your weight and height and does not distinguish between fat, muscle, or bone. However, for most people, BMI is correlated with body fat levels.

7. Where does the ‘703’ in the imperial BMI formula come from?

It is a conversion factor to align the imperial units with the metric units. It is derived from the conversion of pounds to kilograms and inches to meters squared (kg/m²). Specifically, it’s (lbs/in²) * 703 ≈ (kg/m²).

8. What are the main limitations of using the BMI calculation formula?

The main limitations are that it does not account for body composition (muscle vs. fat), fat distribution (visceral vs. subcutaneous fat), or variations across different ages, sexes, and ethnicities.