BMI Calculator
Calculate your Body Mass Index (BMI) instantly. Supports both metric and imperial units. Get your BMI category and health recommendations.
Table of Contents
What is BMI?
**Body Mass Index (BMI)** is a measure of body fat based on height and weight. It's a simple, non-invasive screening tool used to identify potential weight problems in adults.
BMI is calculated by dividing a person's weight (in kilograms) by the square of their height (in meters). The result is a number that falls into one of several categories: underweight, normal weight, overweight, or obese.
While BMI doesn't directly measure body fat, it correlates with more direct measures of body fat and provides a quick, inexpensive screening method for weight categories that may lead to health problems.
BMI is widely used by healthcare providers, researchers, and individuals to assess weight status and identify potential health risks associated with being underweight, overweight, or obese.
How it Works
The BMI calculation is straightforward, but the formula differs slightly depending on the unit system you use:
Metric Formula
For metric units (kilograms and centimeters), BMI is calculated as:
Note: Height must be converted from centimeters to meters (divide by 100).
Imperial Formula
For imperial units (pounds and inches), BMI is calculated as:
The 703 is a conversion factor to account for the difference between metric and imperial units.
Our calculator automatically handles all conversions and calculations, giving you instant results as you enter your measurements. Simply select your preferred unit system, enter your weight and height, and get your BMI along with its category.
BMI Categories
BMI values are categorized into four main groups. These categories are based on statistical associations between BMI and health outcomes:
Underweight (BMI < 18.5)
Being underweight may indicate insufficient nutrition, underlying health conditions, or other factors. It can be associated with health risks including weakened immune function, bone loss, and fertility issues.
If you're underweight, consult with a healthcare provider to determine the cause and develop a safe plan to reach a healthy weight.
Normal Weight (BMI 18.5 - 24.9)
This range is generally associated with the lowest health risks. People in this category typically have a healthy weight for their height.
Maintaining a normal weight through a balanced diet and regular physical activity is recommended for overall health.
Overweight (BMI 25.0 - 29.9)
Being overweight may increase the risk of health conditions such as heart disease, type 2 diabetes, high blood pressure, and certain cancers.
If you're overweight, consider consulting with a healthcare provider to develop a safe and effective weight management plan.
Obese (BMI ≥ 30.0)
Obesity is associated with increased risk of numerous health conditions including heart disease, stroke, type 2 diabetes, certain cancers, sleep apnea, and more.
If you're in the obese category, it's important to consult with a healthcare provider to develop a comprehensive weight management plan tailored to your needs.
BMI Limitations
While BMI is a useful screening tool, it has limitations and should not be used as the sole indicator of health:
- Doesn't measure body fat directly — BMI is based on weight and height, not body composition. It doesn't distinguish between muscle and fat.
- May overestimate body fat in muscular individuals — Athletes or people with high muscle mass may have a high BMI but low body fat.
- May underestimate body fat in older adults — As people age, they may lose muscle mass, which can result in a normal BMI despite having excess body fat.
- Doesn't account for fat distribution — Where fat is stored (visceral vs. subcutaneous) affects health risks, but BMI doesn't measure this.
- May not be accurate for all ethnicities — Some research suggests BMI cutoffs may need adjustment for different ethnic groups.
- Not suitable for children — BMI calculations for children and teens use different formulas that account for age and sex.
BMI should be used as a starting point for health assessment, not a definitive diagnosis. Always consult with a healthcare provider for a comprehensive health evaluation.
Common Use Cases
- Health Screening: Quick assessment of weight status and potential health risks
- Fitness Tracking: Monitor changes in BMI as part of a fitness or weight management program
- Medical Research: Used in studies to assess population health and weight trends
- Insurance Underwriting: Some insurance companies use BMI as a factor in health insurance assessments
- Personal Health Awareness: Understanding your BMI category can motivate healthy lifestyle changes
- Weight Management Programs: Setting goals and tracking progress in weight loss or gain programs
- Nutrition Planning: Healthcare providers may use BMI to guide nutritional recommendations
Examples
Example 1: Metric Calculation
Weight: 70 kg
Height: 175 cm (1.75 m)
BMI Calculation: 70 / (1.75)² = 70 / 3.0625 = 22.9
Result: BMI of 22.9 falls in the Normal Weight category (18.5 - 24.9)
Example 2: Imperial Calculation
Weight: 180 lbs
Height: 5 feet 10 inches (70 inches)
BMI Calculation: (180 / 70²) × 703 = (180 / 4900) × 703 = 25.8
Result: BMI of 25.8 falls in the Overweight category (25.0 - 29.9)
Example 3: Different Categories
Person A: 60 kg, 170 cm → BMI = 20.8 (Normal Weight)
Person B: 90 kg, 170 cm → BMI = 31.1 (Obese)
Person C: 50 kg, 170 cm → BMI = 17.3 (Underweight)
This shows how the same height can result in different BMI categories depending on weight.
Frequently Asked Questions
A healthy BMI typically falls in the normal weight range of 18.5 to 24.9. However, BMI is just one indicator of health. Other factors like body composition, diet, physical activity, and medical history are also important.
BMI has limitations and may not be accurate for everyone. It may overestimate body fat in muscular individuals and underestimate it in older adults who have lost muscle mass. BMI also doesn't account for fat distribution or body composition. It's best used as a screening tool alongside other health assessments.
This calculator is designed for adults (18 years and older). BMI calculations for children and teens use different formulas that account for age and sex, and results are interpreted using growth charts. Consult with a pediatrician for accurate BMI assessment for children.
If your BMI indicates you're overweight or obese, it's important to consult with a healthcare provider. They can help you develop a safe and effective weight management plan that may include dietary changes, increased physical activity, and other lifestyle modifications. Remember, small, sustainable changes are often more effective than drastic measures.
If your BMI indicates you're underweight, consult with a healthcare provider to determine the cause. Being underweight can be associated with health risks, and a healthcare provider can help you develop a safe plan to reach a healthy weight through proper nutrition and, if needed, medical intervention.
There's no set frequency for checking BMI. Some people check it monthly as part of their health tracking, while others check it less frequently. If you're actively working on weight management, you might check it more often, but remember that weight can fluctuate daily due to various factors. Focus on long-term trends rather than daily changes.
No. All calculations are performed locally in your browser. We don't store, save, or transmit any personal information or measurements. Your privacy is protected.