How to Calculate BMI: A Complete Guide to Body Mass Index
Master BMI calculation, understand your results, and learn why this metric matters for your health decisions.
What is BMI?
Body Mass Index (BMI) is a simple measurement that estimates body fat based on your height and weight. It's calculated using your weight in kilograms divided by your height in meters squared (kg/m²). Healthcare professionals worldwide use BMI as a screening tool to identify potential weight-related health risks.
BMI was developed by Belgian statistician Adolphe Quetelet in the 1830s and remains one of the most widely used health assessment tools today. While it's not perfect, it provides a quick and accessible way to assess whether someone's weight falls within a healthy range for their height.
BMI Formula Explained
BMI = weight (kg) ÷ height (m)²
Where weight is in kilograms and height is in meters
The formula is straightforward but requires you to convert your measurements to metric units if you're using pounds and feet/inches. Don't worry—we'll show you how to handle that conversion in the calculation section below.
How to Calculate BMI: Step-by-Step
Method 1: Using Kilograms and Meters
- Measure your weight in kilograms (if using pounds, divide by 2.205)
- Measure your height in meters (if using feet/inches: feet × 0.3048 + inches × 0.0254)
- Square your height (multiply it by itself)
- Divide your weight by the squared height
- Round to one decimal place
Example Calculation:
Person A weighs 75 kg and is 1.80 m tall
BMI = 75 ÷ (1.80 × 1.80)
BMI = 75 ÷ 3.24
BMI = 23.1
Method 2: Using Pounds and Inches
If you prefer working with imperial measurements, use this formula:
BMI = [weight (lbs) × 703] ÷ height (inches)²
Example Calculation:
Person B weighs 165 lbs and is 70 inches tall
BMI = (165 × 703) ÷ (70 × 70)
BMI = 115,995 ÷ 4,900
BMI = 23.7
BMI Categories and What They Mean
The World Health Organization (WHO) classifies BMI into five categories. Here's what each category typically indicates:
Underweight: BMI less than 18.5
A BMI below 18.5 may indicate insufficient weight for your height. This can sometimes be associated with nutritional deficiencies or other health concerns. If you fall into this category, consult a healthcare provider.
Normal Weight: BMI 18.5–24.9
This is generally considered the healthiest weight range for your height. People in this range typically have lower risk for weight-related health conditions, though genetics and lifestyle also play important roles.
Overweight: BMI 25.0–29.9
A BMI in this range suggests you may be carrying extra weight. This classification doesn't mean you're unhealthy, but it may be worth evaluating your diet and exercise habits with a healthcare provider.
Obese Class 1: BMI 30.0–34.9
This range indicates higher risk for weight-related health conditions such as type 2 diabetes, heart disease, and sleep apnea. Medical intervention and lifestyle changes are typically recommended.
Obese Class 2+: BMI 35.0 and above
BMI values above 35 indicate significantly increased health risks. Healthcare providers usually recommend comprehensive weight management strategies, which may include medical supervision.
Important Limitations of BMI
While BMI is useful, it has several significant limitations you should understand:
- • Doesn't Distinguish Between Muscle and Fat: Muscle weighs more than fat, so athletes and very muscular individuals may have a "high" BMI despite being fit.
- • Ignores Body Composition: Two people with identical BMI can have very different body fat percentages and health profiles.
- • Doesn't Account for Age: BMI calculations don't change with age, even though body composition naturally changes throughout life.
- • Ignores Ethnicity Differences: Different ethnic groups may have varying risk levels at the same BMI values.
- • Doesn't Measure Overall Fitness: Cardiovascular fitness and strength aren't reflected in BMI calculations.
For these reasons, healthcare professionals often use BMI alongside other measurements like waist circumference, body fat percentage, and overall fitness level to get a complete health picture.
Frequently Asked Questions
What's the easiest way to calculate BMI?
Use our free BMI calculator on this website—just enter your height and weight, and it calculates instantly. No manual calculations needed!
Is BMI accurate for children?
BMI calculations are different for children and teenagers. They use age and sex-specific growth charts because children's body composition changes as they grow. Consult a pediatrician for children's BMI assessment.
Can BMI be misleading?
Yes, BMI can be misleading for athletes, very muscular individuals, and older adults with muscle loss. It should be one of several health assessment tools, not the only one.
How often should I check my BMI?
For most people, checking BMI monthly or quarterly is sufficient to track trends. Rapid fluctuations may indicate measurement errors or significant lifestyle changes.
What should I do if my BMI is too high?
Start with lifestyle modifications: eat a balanced diet, increase physical activity, and get adequate sleep. Consult a doctor or nutritionist for personalized guidance. Remember that healthy weight loss is gradual—aim for 1-2 pounds per week.
Ready to Calculate Your BMI?
Use our free, instant BMI calculator to find your Body Mass Index right now. It takes just seconds!
Calculate Your BMI →