BMI Calculator

Enter your height and weight to calculate your Body Mass Index and see your health category.

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Weight

Unit

Height

Unit

What is BMI?

BMI (Body Mass Index) is a simple screening metric that relates weight to height to estimate body fatness for most adults.

How to calculate BMI

Metric: BMI = weight(kg) / (height(m))². Imperial: BMI = 703 × weight(lb) / (height(in))².

Who should use this

Adults who want a quick health screening number
Coaches and clinicians for fast triage (not a diagnosis)

Examples

70 kg and 175 cm → BMI ≈ 22.9 (Normal)
155 lb and 5 ft 6 in → BMI ≈ 25.0 (Overweight)

Understanding BMI

BMI Formula

Body Mass Index (BMI) is calculated using one of two formulas depending on your measurement system:

Metric Formula:

BMI = weight(kg) / (height(m))²

For example, if you weigh 70 kg and are 1.75 m tall: BMI = 70 / (1.75)² = 70 / 3.0625 = 22.9

Imperial Formula:

BMI = 703 × weight(lb) / (height(in))²

For example, if you weigh 155 lb and are 66 inches tall: BMI = 703 × 155 / (66)² = 108,965 / 4,356 = 25.0

BMI Categories for Adults

The World Health Organization (WHO) classifies BMI into the following categories:

BMI Range	Category	Health Implications
Below 18.5	Underweight	May indicate malnutrition, eating disorders, or other health issues
18.5 - 24.9	Normal weight	Generally considered healthy for most adults
25.0 - 29.9	Overweight	Increased risk of health problems like heart disease and diabetes
30.0 and above	Obese	Significantly increased risk of serious health conditions

Healthy BMI Ranges by Age

While BMI categories are generally the same for all adults, ideal BMI may vary slightly with age:

18-24 years: BMI 19-24 is typically ideal
25-34 years: BMI 20-25 is typically ideal
35-44 years: BMI 21-26 is typically ideal
45-54 years: BMI 22-27 is typically ideal
55-64 years: BMI 23-28 is typically ideal
65+ years: BMI 24-29 is typically ideal (slightly higher is acceptable)

Note: These are general guidelines. Individual health factors, muscle mass, and body composition should also be considered.

BMI for Children and Teens

BMI interpretation differs significantly for children and adolescents (ages 2-19). Instead of fixed categories, pediatric BMI uses percentiles based on age and sex:

Underweight: BMI below the 5th percentile
Healthy weight: BMI between 5th and 85th percentile
Overweight: BMI between 85th and 95th percentile
Obese: BMI at or above the 95th percentile

Children's BMI must be compared to growth charts that account for normal growth patterns. A child's BMI can change significantly as they grow, and what's normal varies by age and sex.

How to Calculate BMI Step-by-Step

Here's a detailed walkthrough for calculating BMI manually:

Using Metric Units:

Measure your weight in kilograms (kg)
Measure your height in meters (m). If you have height in centimeters, divide by 100 (e.g., 175 cm = 1.75 m)
Square your height: height × height
Divide your weight by the squared height: weight / (height)²
Round to one decimal place

Using Imperial Units:

Measure your weight in pounds (lb)
Measure your height in inches (in). Convert feet to inches: (feet × 12) + inches
Square your height: height × height
Multiply your weight by 703: weight × 703
Divide the result by your squared height: (weight × 703) / (height)²
Round to one decimal place

Risks of High BMI (Overweight/Obese)

Having a BMI in the overweight or obese range increases your risk for:

Cardiovascular disease: Heart disease, stroke, high blood pressure
Type 2 diabetes: Insulin resistance and elevated blood sugar
Certain cancers: Including breast, colon, and endometrial cancer
Sleep apnea: Breathing interruptions during sleep
Osteoarthritis: Joint pain and degeneration
Fatty liver disease: Liver inflammation and damage
Kidney disease: Reduced kidney function

Risks of Low BMI (Underweight)

Having a BMI below 18.5 may indicate:

Nutritional deficiencies: Inadequate calorie or nutrient intake
Weakened immune system: Increased susceptibility to infections
Osteoporosis: Reduced bone density and fracture risk
Anemia: Low red blood cell count
Fertility issues: Irregular menstrual cycles and reduced fertility
Muscle loss: Reduced muscle mass and strength

Limitations of BMI

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

Doesn't measure body fat directly: BMI is a calculation based on weight and height, not actual body composition
Doesn't distinguish muscle from fat: Athletes with high muscle mass may have high BMI but low body fat
Doesn't account for fat distribution: Visceral fat (around organs) is more dangerous than subcutaneous fat (under skin)
May not be accurate for certain populations: Older adults, pregnant women, and people with certain medical conditions
Ethnic variations: Some ethnic groups may have different health risks at the same BMI
Doesn't consider bone density: People with denser bones may weigh more without being overweight

For a complete health assessment, BMI should be used alongside other measurements like waist circumference, body fat percentage, and overall health markers.

Frequently Asked Questions

What is a healthy BMI range?

For most adults, a BMI between 18.5 and 24.9 is considered healthy. However, ideal BMI may vary slightly with age, and individual factors like muscle mass and body composition should be considered.

Is BMI different for men and women?

BMI categories are the same for adult men and women. However, women typically have higher body fat percentages than men at the same BMI. For children and teens, BMI is interpreted using sex-specific growth charts.

How accurate is BMI for health assessment?

BMI is a useful screening tool but not a perfect measure of health. It doesn't account for muscle mass, body fat distribution, or individual health factors. Use BMI as one indicator alongside other health metrics.

Can BMI be used for kids or teens?

Yes, but BMI for children (ages 2-19) must be interpreted using age and sex-specific percentile charts, not adult categories. A pediatrician can help interpret your child's BMI percentile.

What should I do if my BMI is outside the normal range?

Consult with a healthcare provider for personalized advice. They can help determine if your BMI indicates a health concern and recommend appropriate lifestyle changes, which may include diet, exercise, or medical interventions.

Why do athletes sometimes have high BMI?

Muscle weighs more than fat, so athletes with significant muscle mass may have high BMI despite low body fat. BMI doesn't distinguish between muscle and fat, which is why it's just one screening tool among many.