BMI Calculator

BMI Calculator

Calculate your Body Mass Index (BMI) to assess if you’re at a healthy weight for your height using our free online calculator.

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

Use Cases

This BMI calculator is commonly used by individuals monitoring their health, fitness enthusiasts tracking weight goals, healthcare professionals screening patients, nutritionists assessing clients, and anyone wanting to understand their weight status.

What is BMI?

Body Mass Index (BMI) is a calculation using height and weight to estimate body fat and determine if you’re underweight, normal weight, overweight, or obese. It’s a widely used screening tool for health risk assessment.

BMI Formula

The BMI calculation varies by unit system:

Metric Formula (kg and cm):

$$\text{BMI} = \frac{\text{Weight (kg)}}{\left(\text{Height (m)}\right)^2}$$

Imperial Formula (lbs and inches):

$$\text{BMI} = \frac{\text{Weight (lbs)}}{\left(\text{Height (in)}\right)^2} \times 703$$

Math.js Expression (Metric):

weight_kg = 75;
height_cm = 175;
height_m = height_cm / 100;

bmi = weight_kg / (height_m^2);
bmi  # 24.49

Math.js Expression (Imperial):

weight_lbs = 165;
height_inches = 69;

bmi = (weight_lbs / (height_inches^2)) * 703;
bmi  # 24.36

BMI Categories

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

WHO classification for adults 18 years and older

Example Calculations

Example 1: Metric System

Person Details:

• Weight: 75 kg
• Height: 175 cm

weight = 75;
height_cm = 175;
height_m = height_cm / 100;  # 1.75 m

bmi = weight / (height_m^2);
bmi  # 24.49

Result: BMI of 24.49 = Normal Weight

Example 2: Imperial System

Person Details:

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

weight = 180;
height = 70;

bmi = (weight / (height^2)) * 703;
bmi  # 25.82

Result: BMI of 25.82 = Overweight

Example 3: Assessing Health Risk

Person Details:

• Weight: 90 kg
• Height: 170 cm

weight = 90;
height_m = 1.70;

bmi = weight / (height_m^2);
bmi  # 31.14

Result: BMI of 31.14 = Obese Class I (High health risk)

Examples

• Weight 75 kg, Height 175 cm → BMI = 24.5 (Normal Weight)
• Weight 180 lbs, Height 70 inches → BMI = 25.8 (Overweight)
• Weight 90 kg, Height 170 cm → BMI = 31.1 (Obese Class I)
• Weight 120 lbs, Height 64 inches → BMI = 20.6 (Normal Weight)
• Weight 100 kg, Height 180 cm → BMI = 30.9 (Obese Class I)

Health Implications by BMI Category

Underweight (BMI < 18.5)

Health Risks: Nutrient deficiencies, weakened immune system, osteoporosis, anemia, fertility issues

Recommendations: Consult healthcare provider, increase calorie intake with nutrient-dense foods, strength training

Normal Weight (BMI 18.5-24.9)

Health Risks: Lowest risk for weight-related health problems

Recommendations: Maintain healthy diet and regular exercise, monitor weight periodically

Overweight (BMI 25.0-29.9)

Health Risks: Increased risk of type 2 diabetes, high blood pressure, heart disease, sleep apnea

Recommendations: 5-10% weight loss can significantly reduce health risks, increase physical activity, improve diet

Obese (BMI ≥ 30.0)

Health Risks: High risk of cardiovascular disease, diabetes, certain cancers, joint problems, reduced life expectancy

Recommendations: Medical supervision for weight loss, diet and exercise program, possible medication or surgery for severe cases

BMI Limitations

Doesn’t Measure Body Fat Directly

BMI doesn’t distinguish between muscle and fat. Bodybuilders may have high BMI with low body fat.

Doesn’t Show Fat Distribution

Abdominal fat (apple shape) is riskier than hip/thigh fat (pear shape). Waist circumference provides additional information.

Not Accurate for All Groups

• Athletes: High muscle mass can result in high BMI despite low body fat
• Elderly: May underestimate body fat due to muscle loss
• Children/Teens: Requires BMI-for-age percentile charts
• Pregnant Women: BMI standards don’t apply during pregnancy
• Certain Ethnicities: Asian populations may have health risks at lower BMI thresholds

Additional Health Measurements

Waist Circumference

High Risk Thresholds:

• Men: > 40 inches (102 cm)
• Women: > 35 inches (88 cm)

Waist-to-Hip Ratio

Calculation: Waist circumference ÷ Hip circumference

High Risk:

• Men: > 0.90
• Women: > 0.85

Body Fat Percentage

More accurate than BMI for body composition:

• Men: 6-24% (healthy range)
• Women: 14-31% (healthy range)

Common Mistakes & Tips

Using Wrong Units: Ensure you select the correct unit system (metric or imperial). Mixing units leads to incorrect BMI calculations.

BMI Doesn’t Measure Body Fat: BMI doesn’t distinguish between muscle and fat. Athletes with high muscle mass may have high BMI but low body fat.

Not Suitable for Everyone: BMI may not be accurate for athletes, pregnant women, children, elderly individuals, or certain ethnic groups.

Ignoring Waist Circumference: BMI alone doesn’t show fat distribution. Waist circumference is also important for health risk assessment.

Focusing Only on the Number: BMI is one health indicator among many. Consider overall fitness, diet, blood pressure, cholesterol, and family history.

Not Consulting Healthcare Professionals: BMI is a screening tool, not a diagnosis. Consult doctors for personalized health advice.

Frequently Asked Questions

Is BMI accurate?

BMI is a screening tool, not a diagnostic tool. It’s useful for general health assessment but should be used alongside other measurements like waist circumference, body fat percentage, and medical advice.

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, check BMI every 3-6 months. If actively managing weight, monthly checks can help track progress alongside other metrics.

What’s more important: BMI or waist circumference?

Both are important. Waist circumference predicts health risks from abdominal fat, while BMI indicates overall weight status. Use both for comprehensive assessment.

What is a healthy BMI range?

A healthy BMI range is 18.5 to 24.9 for most adults. This range is associated with the lowest health risks, though optimal BMI may vary by ethnicity.

Can BMI be the same for different body types?

Yes. Two people with the same BMI can have very different body compositions. One might be muscular with low fat, the other might have more body fat.

Related Calculators

• Body Mass Index Calculator - Detailed BMI information
• BMI Checker - Quick BMI assessment
• Scientific Calculator - Advanced math calculations
• Unit Converter - Convert weight and height units