BMI Calculator
Calculate your Body Mass Index • Enter in any unit - automatic conversion
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Tip: Enter your measurements in any unit. The calculator automatically converts between metric and imperial.
BMI Categories
What is BMI? Complete Guide to Body Mass Index
BMI (Body Mass Index) is a numerical value derived from your weight and height. It's used as a screening tool to categorize individuals into different weight categories: underweight, normal weight, overweight, and obese. BMI is calculated by dividing your weight in kilograms by the square of your height in meters (kg/m²), or for imperial units, weight in pounds divided by height in inches squared, multiplied by 703.
Developed by Belgian mathematician Adolphe Quetelet in the 1830s, BMI has become a standard measure used by healthcare professionals worldwide to assess whether a person has a healthy body weight for their height. While BMI doesn't directly measure body fat, research has shown that it correlates with more direct measures of body fat and can identify people at increased risk for health problems related to excess weight.
Our free BMI calculator makes it easy to calculate your Body Mass Index instantly. Simply enter your weight and height in either metric or imperial units, and get immediate results showing your BMI value, category, and personalized health recommendations. Whether you're tracking your fitness journey, monitoring your health, or just curious about your BMI, this calculator provides accurate results in seconds.
How to Calculate BMI: Formula and Examples
BMI Formula (Metric System)
BMI = Weight (kg) ÷ Height² (m²)
Example:
Weight: 70 kg, Height: 1.75 m (175 cm)
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9
This BMI of 22.9 falls in the "Normal Weight" category (18.5-25).
BMI Formula (Imperial System)
BMI = (Weight (lbs) ÷ Height² (inches²)) × 703
Example:
Weight: 154 lbs, Height: 5 feet 9 inches (69 inches)
BMI = (154 ÷ (69 × 69)) × 703 = (154 ÷ 4761) × 703 = 22.8
This BMI of 22.8 falls in the "Normal Weight" category.
BMI Categories and What They Mean
Severe Underweight (BMI < 16)
A BMI under 16 indicates severe underweight, which can be a sign of malnutrition, eating disorders, or other serious health conditions. This requires immediate medical attention.
Health Risks: Weakened immune system, osteoporosis, anemia, fertility issues, organ damage.
Underweight (BMI 16 - 18.5)
A BMI between 16 and 18.5 indicates underweight. While not as critical as severe underweight, it may still pose health risks and should be addressed with a healthcare provider.
Health Risks: Nutritional deficiencies, weakened immune function, decreased muscle mass, fatigue.
Normal Weight (BMI 18.5 - 25)
A BMI between 18.5 and 25 is considered healthy and normal. People in this range typically have the lowest risk of weight-related health problems.
Recommendation: Maintain current weight through balanced diet and regular physical activity.
Overweight (BMI 25 - 30)
A BMI between 25 and 30 indicates overweight. This increases the risk of developing health problems, though the risk is generally lower than for obesity.
Health Risks: Increased risk of high blood pressure, type 2 diabetes, heart disease, sleep apnea.
Obese Class I (BMI 30 - 35)
A BMI between 30 and 35 indicates obesity. This significantly increases the risk of serious health conditions.
Health Risks: Type 2 diabetes, high blood pressure, heart disease, stroke, certain cancers, osteoarthritis.
Obese Class II (BMI 35 - 40)
A BMI between 35 and 40 indicates severe obesity with substantially increased health risks.
Recommendation: Medical intervention and comprehensive weight management plan recommended.
Obese Class III (BMI > 40)
A BMI over 40 indicates very severe (morbid) obesity with extremely high health risks.
Recommendation: Immediate medical consultation strongly recommended. Treatment may include lifestyle changes, medication, or bariatric surgery.
Limitations of BMI: What BMI Doesn't Tell You
While BMI is a useful screening tool, it has important limitations that you should be aware of:
Doesn't Measure Body Fat Directly
BMI only uses height and weight, so it can't distinguish between muscle mass and fat mass. A muscular athlete might have a high BMI but low body fat.
Doesn't Account for Fat Distribution
BMI doesn't show where fat is stored. Abdominal fat (visceral fat) is more dangerous than fat stored in hips or thighs, but BMI treats them the same.
May Not Apply to All Populations
BMI standards were developed primarily using Caucasian populations. Different ethnic groups may have different body compositions at the same BMI. Asian populations, for example, may have higher health risks at lower BMI values.
Age and Gender Differences
BMI doesn't account for age or gender. Older adults naturally have more body fat, and women typically have more body fat than men at the same BMI.
Not Suitable for Certain Groups
BMI is not accurate for: pregnant or breastfeeding women, competitive athletes and bodybuilders, children and teenagers (use age and sex-specific BMI percentiles instead), elderly individuals, people with certain medical conditions affecting weight.
How to Achieve and Maintain a Healthy BMI
Healthy Eating Habits
- • Eat plenty of fruits and vegetables (5+ servings daily)
- • Choose whole grains over refined grains
- • Include lean proteins (fish, poultry, beans, nuts)
- • Limit saturated fats, added sugars, and sodium
- • Control portion sizes
- • Stay hydrated with water
- • Limit processed and fast foods
Physical Activity
- • Aim for 150 minutes of moderate aerobic activity weekly
- • Include strength training 2+ times per week
- • Start slowly and gradually increase intensity
- • Find activities you enjoy to stay motivated
- • Take the stairs, walk more, stay active throughout the day
- • Mix cardio, strength, and flexibility exercises
- • Get adequate sleep (7-9 hours for adults)
Frequently Asked Questions About BMI
What is a healthy BMI?
A healthy BMI is between 18.5 and 25. This range is associated with the lowest risk of weight-related health problems for most adults.
How accurate is BMI?
BMI is a useful screening tool but not a diagnostic measure. It doesn't directly measure body fat and may not be accurate for athletes, elderly, pregnant women, or certain ethnic groups. Use it as one indicator alongside other health measures.
Is BMI different for men and women?
The BMI formula and categories are the same for adult men and women. However, women typically have more body fat than men at the same BMI. Some health professionals consider this when evaluating health risks.
Can BMI be used for children?
BMI can be used for children aged 2-19, but it's interpreted differently using age and sex-specific percentile charts. Children's BMI changes as they grow, so percentiles are used instead of fixed categories.
What BMI is considered obese?
A BMI of 30 or higher is classified as obese. This is further divided into Class I (30-35), Class II (35-40), and Class III (over 40) obesity, with increasing health risks at higher levels.
Does muscle mass affect BMI?
Yes. Since muscle weighs more than fat, very muscular people (like athletes and bodybuilders) may have a high BMI despite having low body fat. BMI doesn't distinguish between muscle and fat mass.
How often should I check my BMI?
Check your BMI monthly or quarterly if you're actively working on weight management. For general health monitoring, checking once or twice a year is sufficient for most people.
Is BMI the same worldwide?
The BMI calculation is universal, but some countries use different category cutoffs. For example, Asian countries often use lower thresholds (overweight at 23, obese at 27.5) due to higher health risks at lower BMI values in Asian populations.
What if my BMI is on the border between categories?
BMI categories are guidelines, not absolute thresholds. If you're on a border (like 24.9 or 25.1), your health risk is similar. Focus on overall health indicators like waist circumference, blood pressure, cholesterol, and fitness level.
Can I use BMI if I'm pregnant?
No, BMI is not accurate during pregnancy due to natural weight gain. Your healthcare provider will monitor your weight gain based on your pre-pregnancy BMI and recommend appropriate ranges for healthy pregnancy.