Body Mass Index Calculator

Body Mass Index Calculator

Calculate your Body Mass Index and understand what your BMI means for your health with comprehensive categories, risk assessments, and personalized recommendations.

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

Use Cases

This Body Mass Index calculator helps doctors assess patient health risks, fitness trainers design weight management programs, individuals set realistic health goals, researchers study population health trends, and insurance companies evaluate health factors.

What is Body Mass Index?

Body Mass Index (BMI) is a numerical value derived from a person’s weight and height that indicates whether they have a healthy body weight. It’s a widely accepted screening method used by healthcare professionals worldwide to categorize weight status.

How Body Mass Index Works

BMI provides a reliable indicator of body fatness for most people and is used to screen for weight categories that may lead to health problems.

Metric Formula:

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

Imperial Formula:

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

Math.js Expression:

# Metric example
weight_kg = 70;
height_cm = 168;
height_m = height_cm / 100;

bmi_metric = weight_kg / (height_m^2);
bmi_metric  # 24.80

# Imperial example
weight_lbs = 154;
height_inches = 66;

bmi_imperial = (weight_lbs * 703) / (height_inches^2);
bmi_imperial  # 24.85

BMI Classification System

World Health Organization (WHO) Categories

BMI Range	Classification	Health Status
< 16.0	Severe Thinness	Severely underweight
16.0 - 16.9	Moderate Thinness	Moderately underweight
17.0 - 18.4	Mild Thinness	Mildly underweight
18.5 - 24.9	Normal Range	Healthy weight
25.0 - 29.9	Overweight	Pre-obese
30.0 - 34.9	Obese Class I	Moderately obese
35.0 - 39.9	Obese Class II	Severely obese
≥ 40.0	Obese Class III	Very severely obese

Asian-Pacific BMI Classifications

Due to different body composition, Asian populations use modified thresholds:

BMI Range	Classification
< 18.5	Underweight
18.5 - 22.9	Normal
23.0 - 24.9	Overweight
25.0 - 29.9	Obese Class I
≥ 30.0	Obese Class II

Health Implications

Underweight (BMI < 18.5)

Associated Health Risks:

• Malnutrition and nutrient deficiencies
• Weakened immune system
• Osteoporosis and bone fragility
• Anemia (iron, B12, folate deficiency)
• Fertility problems
• Increased surgical complications

Recommended Actions:

• Consult healthcare provider to rule out underlying conditions
• Work with nutritionist for healthy weight gain plan
• Increase calorie intake with nutrient-dense foods
• Incorporate resistance training

Normal Weight (BMI 18.5-24.9)

Health Status:

• Optimal health risk profile
• Lower risk of chronic diseases
• Better cardiovascular health
• Improved metabolic function

Maintenance Strategies:

• Continue balanced diet
• Regular physical activity (150 min/week moderate exercise)
• Annual health checkups
• Monitor weight quarterly

Overweight (BMI 25.0-29.9)

Associated Health Risks:

• Increased risk of type 2 diabetes
• High blood pressure (hypertension)
• Cardiovascular disease
• Sleep apnea
• Joint problems and osteoarthritis
• Certain cancers

Recommended Actions:

• Aim for 5-10% weight loss (significant health benefits)
• Increase physical activity to 200-300 min/week
• Reduce calorie intake by 500-750 calories/day
• Medical supervision recommended

Obese (BMI ≥ 30.0)

Associated Health Risks:

• High risk of type 2 diabetes
• Heart disease and stroke
• Certain cancers (breast, colon, kidney)
• Gallbladder disease
• Respiratory problems
• Reduced life expectancy
• Mental health issues

Recommended Actions:

• Medical evaluation and supervision essential
• Structured weight loss program
• Possible medication or bariatric surgery for Class II/III
• Behavioral therapy and support groups
• Monitor comorbid conditions

Example Calculations

Example 1: Normal BMI

Person Profile:

• Height: 170 cm (5’7”)
• Weight: 65 kg (143 lbs)

weight = 65;
height_m = 1.70;

bmi = weight / (height_m^2);
bmi  # 22.49

Assessment: BMI of 22.49 = Normal Weight (healthy range)

Example 2: Overweight Category

Person Profile:

• Height: 175 cm (5’9”)
• Weight: 85 kg (187 lbs)

weight = 85;
height_m = 1.75;

bmi = weight / (height_m^2);
bmi  # 27.76

Assessment: BMI of 27.76 = Overweight (5-10% weight loss recommended)

Example 3: Target Weight Calculation

Question: What should I weigh for healthy BMI?

Given:

• Height: 165 cm
• Target BMI: 22 (mid-normal range)

target_bmi = 22;
height_m = 1.65;

target_weight = target_bmi * (height_m^2);
target_weight  # 59.9 kg (132 lbs)

Examples

• Height 160 cm, Weight 55 kg → BMI = 21.5 (Normal)
• Height 180 cm, Weight 95 kg → BMI = 29.3 (Overweight)
• Height 5’4”, Weight 110 lbs → BMI = 18.9 (Normal)
• Height 6’0”, Weight 220 lbs → BMI = 29.8 (Overweight)
• Height 172 cm, Weight 105 kg → BMI = 35.5 (Obese Class II)

Calculating Healthy Weight Range

For a healthy BMI (18.5-24.9), calculate your ideal weight range:

Formula:

$$\text{Weight Range} = \text{BMI} \times \text{Height}^2$$

Math.js Expression:

height_m = 1.75;  # 175 cm

min_healthy_weight = 18.5 * (height_m^2);
max_healthy_weight = 24.9 * (height_m^2);

min_healthy_weight  # 56.6 kg (125 lbs)
max_healthy_weight  # 76.2 kg (168 lbs)

Result: For 175 cm height, healthy weight range is 56.6-76.2 kg (125-168 lbs)

BMI vs Other Measurements

BMI vs Body Fat Percentage

BMI: Weight-to-height ratio (doesn’t measure body composition)

Body Fat %: Actual fat tissue percentage

• Men healthy range: 10-20%
• Women healthy range: 18-28%

Advantage: Body fat % more accurate for athletes and muscular individuals

BMI vs Waist-to-Height Ratio

Formula: Waist circumference ÷ Height (same units)

Healthy: < 0.50 (waist should be less than half your height)

Advantage: Better predictor of cardiovascular risk

BMI vs Body Composition Analysis

Methods: DEXA scan, bioelectrical impedance, skin fold calipers

Provides: Muscle mass, bone density, visceral fat, body water

Advantage: Most comprehensive body assessment

Common Mistakes & Tips

Treating BMI as Body Fat Percentage: BMI estimates body fat but doesn’t measure it directly. Two people with identical BMI can have vastly different body compositions.

Ignoring Muscle Mass: Bodybuilders and athletes often have “overweight” or “obese” BMI classifications despite having very low body fat due to high muscle mass.

Using BMI for Children: Children require BMI-for-age percentile charts. Adult BMI cutoffs don’t apply to developing bodies.

Forgetting Ethnic Variations: Asian, Pacific Islander, and other populations have different health risk thresholds. Use population-specific guidelines when available.

Focusing Only on BMI: Combine BMI with waist circumference, body fat percentage, blood pressure, cholesterol, and blood sugar for complete health picture.

Not Considering Age: Older adults may have slightly higher healthy BMI ranges due to muscle loss and bone density changes.

Frequently Asked Questions

What does Body Mass Index tell me?

BMI indicates whether your weight is in a healthy range for your height. It screens for weight categories associated with health risks but doesn’t diagnose body fatness or individual health.

Is a BMI of 25 overweight?

Yes, BMI of 25.0-29.9 is classified as overweight. However, this is a threshold, and individual health risks depend on many factors including body composition, age, and ethnicity.

Why is BMI not always accurate?

BMI doesn’t distinguish between muscle and fat, doesn’t account for bone density, and doesn’t show fat distribution. Athletes, elderly, children, and pregnant women need alternative assessments.

Can I have a high BMI but be healthy?

Possibly. If you’re very muscular, your high BMI may not indicate excess body fat. Check waist circumference, body fat percentage, and metabolic health markers for complete assessment.

What BMI is considered skinny?

BMI below 18.5 is classified as underweight. BMI below 17.0 is considered thin, and below 16.0 is severely thin, all carrying health risks.

How do I lower my BMI safely?

Create a moderate calorie deficit (500-750 cal/day) through balanced diet and increased physical activity. Aim for 0.5-1 kg (1-2 lbs) weight loss per week under medical supervision.

Related Calculators

• BMI Calculator - Quick BMI calculation
• BMI Checker - Fast BMI assessment tool
• Unit Converter - Convert weight and height units
• Scientific Calculator - Mathematical calculations