What Is Used To Calculate Bmi

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BMI Calculator: Find Out What is Used to Calculate BMI


BMI Calculator: Find Out What is Used to Calculate BMI

This calculator helps you determine your Body Mass Index (BMI) based on your weight and height. Understand what is used to calculate BMI and interpret your results.




Enter your weight in kilograms.


Enter your height in centimeters.



What is BMI (Body Mass Index)?

Body Mass Index (BMI) is a value derived from the mass (weight) and height of a person. It is a convenient rule of thumb used to broadly categorize a person as underweight, normal weight, overweight, or obese based on tissue mass (muscle, fat, and bone) and height. Essentially, what is used to calculate BMI are your weight and height measurements.

The BMI is widely used by health professionals as a quick indicator of potential weight problems for adults. It’s important to remember that BMI is a screening tool, not a diagnostic tool for body fatness or the health of an individual. A high BMI can be an indicator of high body fatness, and high body fatness can be associated with certain health risks, but other factors also play a role.

Who Should Use the BMI Calculator?

Adults (18 years and older) can use this BMI Calculator to get an estimate of their BMI. It is generally applicable to both adult men and women. However, it may not be accurate for athletes or individuals with very muscular builds (as muscle weighs more than fat), pregnant women, the elderly, or growing children. For children and teens (2-19 years), BMI is interpreted differently using BMI-for-age percentiles.

Common Misconceptions About BMI

  • BMI directly measures body fat: It does not. It is an estimate based on weight and height. Other methods like skinfold thickness measurements, bioelectrical impedance, or DEXA scans are needed for direct body fat measurement.
  • A high BMI always means overweight or obese: Muscular individuals might have a high BMI but low body fat.
  • A normal BMI always means healthy: Someone with a normal BMI can still have excess body fat or other health issues.

BMI Formula and Mathematical Explanation

The core components, or what is used to calculate BMI, are a person’s weight and height. The formula differs slightly depending on whether you are using metric or imperial units.

Metric Units (kilograms and meters)

When weight is measured in kilograms (kg) and height in meters (m), the BMI formula is:

BMI = Weight (kg) / (Height (m) * Height (m))

If height is measured in centimeters (cm), it must first be converted to meters by dividing by 100.

BMI = Weight (kg) / ((Height (cm) / 100) * (Height (cm) / 100))

Imperial Units (pounds and inches)

When weight is measured in pounds (lbs) and height in inches (in), the formula includes a conversion factor of 703:

BMI = (Weight (lbs) / (Height (in) * Height (in))) * 703

If height is given in feet and inches, it must first be converted entirely to inches (1 foot = 12 inches).

Variables Table

Variable Meaning Metric Unit Imperial Unit Typical Range (Adult)
Weight Body mass kg lbs 40-150 kg / 90-330 lbs
Height Stature m or cm ft and in, or just in 1.4-2.1 m / 140-210 cm / 4’7″ – 6’11”
BMI Body Mass Index kg/m² kg/m² (after conversion) 15 – 40+
Variables used in BMI calculation.

Practical Examples (Real-World Use Cases)

Example 1: Metric Units

Let’s say a person weighs 68 kg and is 165 cm tall.

  • Weight = 68 kg
  • Height = 165 cm = 1.65 m
  • BMI = 68 / (1.65 * 1.65) = 68 / 2.7225 = 24.97 kg/m²

A BMI of 24.97 falls within the “Normal weight” range.

Example 2: Imperial Units

Consider a person who weighs 170 lbs and is 5 feet 10 inches tall.

  • Weight = 170 lbs
  • Height = 5 feet 10 inches = (5 * 12) + 10 = 60 + 10 = 70 inches
  • BMI = (170 / (70 * 70)) * 703 = (170 / 4900) * 703 = 0.03469 * 703 = 24.39 kg/m²

A BMI of 24.39 also falls within the “Normal weight” range.

How to Use This BMI Calculator

  1. Select Units: Choose between ‘Metric’ (kg, cm) and ‘Imperial’ (lbs, ft, in) using the radio buttons. The input fields will adjust accordingly.
  2. Enter Weight: Input your current weight into the corresponding field (kg or lbs).
  3. Enter Height: Input your height. If using metric, enter it in centimeters. If using imperial, enter feet and inches in their respective boxes.
  4. Calculate: The calculator updates the BMI in real time as you type, or you can click “Calculate BMI”.
  5. View Results: The calculator will display your calculated BMI, the corresponding weight category (e.g., Underweight, Normal weight, Overweight, Obese), and a visual chart. The formula used based on your input will also be shown.
  6. Interpret: Use the BMI value and category as a starting point to understand your weight status. Consult the BMI categories table for reference.
  7. Reset: Click “Reset” to clear the inputs and start over with default values.
  8. Copy: Click “Copy Results” to copy your BMI, category, and input values to your clipboard.

Reading the Results

The primary result is your BMI score. Below it, you’ll see your BMI category based on standard classifications. The chart provides a visual representation of where your BMI falls within the different categories. To understand more about healthy weight, check our healthy weight guide.

Key Factors That Affect BMI Results (and Limitations)

While what is used to calculate BMI is just weight and height, several factors influence these measures and how BMI should be interpreted:

  1. Muscle Mass: Muscle is denser than fat. Individuals with high muscle mass (athletes, bodybuilders) may have a high BMI that classifies them as “overweight” or “obese” even with low body fat.
  2. Age: Body composition changes with age, with a tendency for muscle mass to decrease and body fat to increase, even if weight remains stable. BMI doesn’t account for these changes within the adult population directly, although interpretation might vary slightly for older adults.
  3. Sex: On average, adult women have a higher percentage of body fat than men at the same BMI.
  4. 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 Caucasians.
  5. Body Frame Size: While not directly in the BMI formula, body frame size (small, medium, large) can influence what might be considered a healthy weight for an individual, although BMI ranges are generally broad enough.
  6. Pregnancy: BMI is not used for pregnant women due to the natural weight gain and changes in body composition during pregnancy.
  7. Fluid Retention: Conditions causing significant fluid retention can temporarily increase weight and thus BMI without a change in body fat.

It’s crucial to consider these factors and consult with healthcare professionals rather than relying solely on the BMI Calculator for health assessment. For those concerned about obesity, see our guide on understanding obesity.

Frequently Asked Questions (FAQ)

What is used to calculate BMI?
The primary factors used to calculate BMI are your weight and your height. The formula then relates these two measurements to give the BMI value.
Is the BMI Calculator accurate?
The BMI Calculator accurately performs the mathematical calculation. However, BMI itself is an indirect measure of body fat and may not be accurate for everyone, such as athletes or pregnant women.
What is a healthy BMI range?
A BMI between 18.5 and 24.9 is generally considered to be in the “Normal” or “Healthy Weight” range for adults.
Can I use this BMI Calculator for children?
No, this calculator is designed for adults. For children and teens, BMI is calculated the same way but interpreted using age and sex-specific percentile charts. See our child BMI calculator for more info.
If my BMI is high, am I definitely unhealthy?
Not necessarily. A high BMI indicates a higher risk for certain conditions, but it’s not a definitive diagnosis of health. Muscular individuals may have a high BMI. Consult a healthcare provider for a comprehensive assessment.
What if my BMI is low?
A low BMI (below 18.5) may indicate that you are underweight, which can also pose health risks. It’s advisable to consult a healthcare provider.
How often should I check my BMI?
Checking your BMI periodically (e.g., during annual check-ups or every few months if you are trying to manage your weight) can be useful, but daily fluctuations are normal and less important than the overall trend.
What are the limitations of using a BMI Calculator?
The main limitation is that it doesn’t distinguish between muscle and fat. It also doesn’t consider fat distribution, age, sex, or ethnicity in the calculation itself, although these factors are important for interpretation.

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