BMI Calculator - Body Mass Index

BMI Calculator: Assess Your Body Weight

Unit System:	Select unit system Metric (kg, cm)
Weight (kg):	kg
Height (cm):	cm
	Calculate

What is BMI?

Body Mass Index (BMI) is a simple calculation using a person’s height and weight. The formula is:

$$\text{BMI} = \frac{\text{weight (kg)}}{\text{height (m)}^2}$$

Or in imperial units:

$$\text{BMI} = \frac{\text{weight (lbs)}}{\text{height (in)}^2} \times 703$$

BMI is used to screen for weight categories that may lead to health problems, but it is not diagnostic of the body fatness or health of an individual.

BMI Categories

According to the World Health Organization (WHO) and CDC, BMI values fall into the following categories:

BMI Range	Category	Health Risk
Below 18.5	Underweight	Possible nutritional deficiency and health issues
18.5 - 24.9	Normal Weight	Lowest health risk
25.0 - 29.9	Overweight	Increased risk of health problems
30.0 - 34.9	Obese (Class I)	High risk of health problems
35.0 - 39.9	Obese (Class II)	Very high risk of health problems
40.0 and above	Obese (Class III)	Extremely high risk of health problems

Formulas

1. BMI Calculation (Metric)

For weight in kilograms and height in centimeters:

$$\text{BMI} = \frac{\text{weight}}{\left(\frac{\text{height}}{100}\right)^2}$$

Math.js Expression:

weight = 70; // kg
height = 170; // cm
height_m = height / 100;

bmi = weight / (height_m^2);
round(bmi, 1)

2. BMI Calculation (Imperial)

For weight in pounds and height in inches:

$$\text{BMI} = \frac{\text{weight} \times 703}{\text{height}^2}$$

Math.js Expression:

weight = 154; // pounds
height = 67; // inches

bmi = (weight * 703) / (height^2);
round(bmi, 1)

3. Healthy Weight Range

Calculate the healthy weight range for a given height (BMI 18.5 to 24.9):

$$\text{Min Weight} = 18.5 \times \left(\frac{\text{height}}{100}\right)^2$$ $$\text{Max Weight} = 24.9 \times \left(\frac{\text{height}}{100}\right)^2$$

Math.js Expression:

height = 170; // cm
height_m = height / 100;

healthy_weight_min = 18.5 * (height_m^2);
healthy_weight_max = 24.9 * (height_m^2);

{
  min: round(healthy_weight_min, 1),
  max: round(healthy_weight_max, 1)
}

4. Weight to Lose/Gain

Calculate how much weight to lose or gain to reach the healthy range:

Weight to Lose:

$$\text{Weight to Lose} = \max(0, \text{Current Weight} - \text{Max Healthy Weight})$$

Weight to Gain:

$$\text{Weight to Gain} = \max(0, \text{Min Healthy Weight} - \text{Current Weight})$$

Math.js Expression:

weight = 90; // kg
height = 170; // cm
height_m = height / 100;

healthy_weight_max = 24.9 * (height_m^2);
healthy_weight_min = 18.5 * (height_m^2);

weight_to_lose = max(0, weight - healthy_weight_max);
weight_to_gain = max(0, healthy_weight_min - weight);

{
  to_lose: round(weight_to_lose, 1),
  to_gain: round(weight_to_gain, 1)
}

Example Calculations

Example 1: Metric Units

Person Details:

• Weight: 75 kg
• Height: 175 cm

Step 1: Convert Height to Meters

height_cm = 175;
height_m = height_cm / 100;
height_m

Step 2: Calculate BMI

weight = 75;
height_m = 1.75;

bmi = weight / (height_m^2);
round(bmi, 1)

Result: BMI = 24.5 (Normal Weight)

Example 2: Imperial Units

Person Details:

• Weight: 180 lbs
• Height: 5’10” (70 inches)

Step 1: Calculate BMI

weight_lbs = 180;
height_inches = 70;

bmi = (weight_lbs * 703) / (height_inches^2);
round(bmi, 1)

Result: BMI = 25.8 (Overweight)

Example 3: Finding Healthy Weight Range

For height: 165 cm

height = 165;
height_m = height / 100;

min_weight = 18.5 * (height_m^2);
max_weight = 24.9 * (height_m^2);

{
  min: round(min_weight, 1),
  max: round(max_weight, 1)
}

Result: Healthy weight range = 50.3 kg to 67.7 kg

Important Limitations of BMI

While BMI is a useful screening tool, it has several limitations:

1. Doesn’t Measure Body Fat Directly

BMI doesn’t distinguish between muscle mass and fat mass. Athletes with high muscle mass may have a high BMI but low body fat.

2. Doesn’t Account for Distribution

BMI doesn’t consider where fat is distributed. Visceral fat (around organs) is more harmful than subcutaneous fat.

3. Age and Gender Differences

• Women typically have more body fat than men at the same BMI
• Older adults may have more body fat than younger adults at the same BMI

4. Not Suitable for Everyone

BMI may not be accurate for:

• Athletes and bodybuilders
• Pregnant or breastfeeding women
• Children and adolescents (use age-specific charts)
• Elderly individuals
• People with certain medical conditions

5. Ethnicity Considerations

Different ethnic groups may have different health risks at the same BMI level. For example:

• Asian populations may have health risks at lower BMI levels
• Pacific Islander populations may be healthier at higher BMI levels

Alternative Measurements

For a more comprehensive health assessment, consider these additional metrics:

1. Waist Circumference - Measures abdominal fat
2. Waist-to-Hip Ratio - Indicates fat distribution
3. Body Fat Percentage - Direct measure of body composition
4. Waist-to-Height Ratio - Another indicator of central obesity

Health Recommendations by BMI Category

Underweight (BMI < 18.5)

• Consult a healthcare professional or nutritionist
• Focus on nutrient-dense, calorie-rich foods
• Consider strength training to build muscle mass
• Rule out underlying medical conditions

Normal Weight (BMI 18.5-24.9)

• Maintain current healthy habits
• Continue balanced diet and regular exercise
• Regular health check-ups
• Monitor for any significant weight changes

Overweight (BMI 25-29.9)

• Aim for gradual weight loss (1-2 lbs per week)
• Increase physical activity to 150+ minutes per week
• Focus on whole foods and portion control
• Consider consulting a dietitian

Obese (BMI ≥ 30)

• Consult healthcare professional for comprehensive evaluation
• Develop supervised weight loss plan
• Address any obesity-related health conditions
• Consider combination of diet, exercise, and possibly medical intervention

Frequently Asked Questions

How accurate is BMI?

BMI is a screening tool, not a diagnostic tool. It’s about 80% accurate in identifying people with obesity-related health risks, but should be used alongside other measurements and medical assessments.

Can I use BMI for children?

No. Children and teens should use BMI-for-age growth charts that account for developmental changes. Consult a pediatrician for children’s BMI assessment.

How often should I check my BMI?

For general health monitoring, checking BMI every 3-6 months is sufficient. If you’re actively trying to lose or gain weight, monthly checks can help track progress.

What’s more important: BMI or waist circumference?

Both are important. Waist circumference can better predict health risks related to abdominal fat, while BMI gives a general indication of overall body weight status.

Related Calculators

• Basal Metabolic Rate (BMR) - Calculate daily calorie needs
• Body Fat Percentage - More accurate body composition assessment
• Ideal Weight Calculator - Find your ideal weight range
• Calorie Calculator - Plan your daily calorie intake

Medical Disclaimer

This BMI calculator is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding your weight or health status.

References

1. World Health Organization (WHO) - BMI Classification
2. Centers for Disease Control and Prevention (CDC) - About Adult BMI
3. National Institutes of Health (NIH) - Calculate Your BMI
4. American Heart Association - Body Composition Tests