BMI Calculator

An essential tool to estimate your body fat based on weight and height.

Your Result

What is BMI (Body Mass Index)?

Body Mass Index (BMI) is a widely used measurement that estimates your body fat based on your weight and height. It provides a simple numeric value that can help determine whether you are in a healthy weight range. The BMI calculation is a screening tool that can indicate potential weight problems for adults. It’s important to remember that while BMI is a useful indicator, it doesn’t measure body fat directly and shouldn’t be the only factor used to assess your health.

The BMI Formula and Explanation

The BMI calculation is straightforward and depends on whether you use the metric or imperial system. The BMI formula uses an individual’s weight in the calculation.

Metric Formula

BMI = weight (kg) / [height (m)]²

When using kilograms for weight and centimeters for height, you must first convert the height into meters by dividing by 100.

Imperial Formula

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

The conversion factor of 703 is used to ensure the result is consistent with the metric formula.

Variables Table

Description of variables used in the BMI calculation.

Variable	Meaning	Unit (Metric / Imperial)	Typical Range
Weight	The mass of the individual.	Kilograms (kg) / Pounds (lbs)	40-150 kg / 90-330 lbs
Height	The vertical measurement of the individual.	Centimeters (cm) / Inches (in)	140-200 cm / 55-79 in

Practical Examples

Example 1: Metric Units

• Input Weight: 75 kg
• Input Height: 180 cm
• Calculation:
  1. Convert height to meters: 180 cm / 100 = 1.8 m
  2. Square the height: 1.8 m * 1.8 m = 3.24 m²
  3. Divide weight by height squared: 75 kg / 3.24 m² = 23.1
• Result: The BMI is 23.1, which is in the “Healthy Weight” range.

Example 2: Imperial Units

• Input Weight: 165 lbs
• Input Height: 68 inches (5’8″)
• Calculation:
  1. Square the height: 68 in * 68 in = 4624 in²
  2. Divide weight by height squared: 165 lbs / 4624 in² = 0.03568
  3. Multiply by the conversion factor: 0.03568 * 703 = 25.1
• Result: The BMI is 25.1, which is in the “Overweight” range.

How to Use This BMI Calculator

1. Select Your Units: Choose between Metric (kg, cm) or Imperial (lbs, in) systems. The input labels will update automatically.
2. Enter Your Weight: Type your current weight into the corresponding field.
3. Enter Your Height: Type your height into the height field.
4. Calculate: Click the “Calculate BMI” button.
5. Interpret Your Results: The calculator will display your BMI number, the category you fall into (e.g., Underweight, Healthy Weight, Overweight), and a visual chart.

Key Factors That Affect Body Mass Index

While the BMI calculation only uses weight and height, several underlying factors can influence these values and the interpretation of the result:

• Body Composition: BMI doesn’t distinguish between fat and muscle mass. An athlete with high muscle mass might have a high BMI without having excess body fat.
• Age: Body composition changes with age. An older adult might have more body fat than a younger adult with the same BMI.
• Sex: On average, adult women have more body fat than men at the same BMI.
• Genetics: Your genetic makeup can influence your body size and where you store fat.
• Diet and Nutrition: The types and amounts of food you consume directly impact your weight and, consequently, your BMI. For a healthy diet, see our Healthy Recipes.
• Physical Activity Level: Regular exercise helps build muscle and burn fat, affecting your weight and body composition. Check out our Fitness Plans for ideas.

Frequently Asked Questions (FAQ)

1. How is BMI calculated?

BMI is calculated by dividing an individual’s weight by the square of their height. For metric units, it’s weight in kilograms divided by height in meters squared. For imperial units, it’s weight in pounds divided by height in inches squared, then multiplied by 703.

2. What is a healthy BMI range?

For most adults, a healthy BMI is between 18.5 and 24.9. A BMI below 18.5 is considered underweight, 25 to 29.9 is overweight, and 30 or above is classified as obesity.

3. Is BMI the same for men and women?

Yes, the calculation formula is the same for adult men and women. However, body fat percentages can differ between sexes at the same BMI.

4. How accurate is the BMI calculation?

BMI is a reliable screening tool for the general population but can be misleading for certain groups, such as athletes, bodybuilders, and pregnant women. It doesn’t account for muscle mass, bone density, or fat distribution.

5. Why does the imperial formula use a factor of 703?

The factor of 703 is a conversion constant. It’s used to adjust the units from pounds/inches to the standard kg/m² used in the universally accepted BMI formula, ensuring the results are consistent across both systems.

6. Can I use this calculator for a child?

No, this calculator is designed for adults over 18. For children and teens, BMI is interpreted using age- and sex-specific percentile charts because their body composition changes as they grow.

7. What should I do if my BMI is high?

A high BMI can be an indicator of health risks. It’s a good starting point for a conversation with a healthcare provider, who can perform a more comprehensive assessment. For more information on weight management, see our Weight Management Guide.

8. What are the limitations of using BMI?

The main limitation is that the BMI calculation cannot tell the difference between fat and muscle. It also does not consider the location of body fat, which can be an important health factor. A Body Fat Calculator might be a useful related tool.