BMI Calculator

Calculate your BMI instantly with our free online calculator. Enter weight in kg or lbs and height in cm or ft. Get your BMI score, WHO weight category (underweight, normal, overweight, obese), healthy weight range, and personalized tips. Works on any device. No sign-up.

What is BMI Calculator?

Body Mass Index (BMI) is a simple, widely used screening tool that estimates whether a person has a healthy body weight for their height. Developed by Belgian mathematician Adolphe Quetelet between 1830 and 1850, the Quetelet Index (as it was originally known) was designed for population-level statistics — not individual health diagnosis. Today, BMI is used by healthcare professionals worldwide as a quick, non-invasive way to identify potential weight-related health risks. Healthcare providers use the clinical BMI waist-to-height ratio obesity risk calculator for metabolic syndrome screening and treatment planning. Dietitians and clinical nutritionists apply the WHO obesity classification and metabolic health screening tool for patient assessment and intervention tracking. Fitness professionals leverage the personal trainer body composition assessment tool for goal setting and progress monitoring alongside body fat percentage measurements.

BMI is calculated by dividing your weight in kilograms by the square of your height in meters. The resulting number places you into a weight category: underweight (below 18.5), normal weight (18.5–24.9), overweight (25.0–29.9), or obese (30.0 and above). According to the World Health Organization, over 2.5 billion adults worldwide are overweight, including 890 million living with obesity — nearly triple the rate since 1975.

While BMI is a useful starting point for health assessment, it has important limitations. It does not distinguish between muscle and fat mass, does not account for where fat is distributed in the body, and may not be equally accurate across all ethnic groups. For example, athletes with high muscle mass may be classified as overweight despite being in excellent health, while some individuals with a normal BMI may have unhealthy levels of visceral fat. For this reason, doctors often use BMI alongside waist circumference, blood tests, and other measurements for a complete health picture.

Formula

BMI = weight (kg) / [height (m)]²

Metric: BMI = kg / m²

Imperial: BMI = [weight (lbs) / height (in)²] × 703

Example: A person weighing 75 kg with a height of 1.75 m:

BMI = 75 / (1.75 × 1.75) = 75 / 3.0625 = 24.5 kg/m²

How to Calculate

Enter your current weight using kilograms (kg) or pounds (lbs). For accuracy, weigh yourself in the morning without clothes.
Enter your height in centimeters (cm), meters (m), or feet and inches. Stand straight without shoes for most accurate measurement.
Click the calculate button — your BMI value appears instantly along with your weight category.
Review the healthy BMI range (18.5–24.9) shown on the result to understand where you fall on the scale.
Use your BMI as a conversation starter with your healthcare provider. Combine it with waist measurement and other health metrics for a full assessment.

Example

Maria weighs 68 kg and is 165 cm (1.65 m) tall. Her BMI is calculated as 68 ÷ (1.65 × 1.65) = 68 ÷ 2.7225 = 24.98. This places Maria at the upper end of the normal weight category (18.5–24.9). She is just 0.02 away from the overweight threshold. If Maria loses 3 kg, her BMI would drop to 23.9 — comfortably within the healthy range. However, if Maria is an athlete with significant muscle mass, her BMI may overestimate body fat, and a body composition test would give a more accurate picture.

Key Benefits

Instant health screening — get your Body Mass Index in seconds from just weight and height
Track fitness progress objectively with a standardized numerical measure
Identify potential health risks early — both underweight and overweight categories carry health concerns
Free and private — all calculations happen in your browser with zero data uploaded
World Health Organization validated — uses the same categories as healthcare professionals

Common Mistakes to Avoid

Treating BMI as a diagnostic tool instead of a screening tool — it cannot diagnose health conditions
Not accounting for muscle mass — athletes and bodybuilders often get false 'overweight' readings
Using adult BMI ranges for children and teenagers (must use age/sex percentiles instead)
Ignoring ethnic differences — Asian populations may need adjusted BMI cutoffs for accurate risk assessment
Measuring height or weight inaccurately — even small errors significantly change the BMI result

Pro Tips

Weigh yourself at the same time each day (morning is best) for consistent tracking
Combine BMI with waist circumference measurement — a waist over 88 cm (35 in) for women or 102 cm (40 in) for men indicates increased health risk regardless of BMI
Don't rely on BMI alone if you're an athlete, elderly, pregnant, or have a muscular build — ask your doctor about body fat testing
Track your BMI trend over time rather than a single measurement — consistent upward or downward trends matter more than the number itself

Key Terms Explained

Body Mass Index (BMI): A numerical value calculated from weight and height, used as a screening tool for weight categories
Quetelet Index: The original name for BMI, named after Belgian mathematician Adolphe Quetelet who developed the formula in the 1830s
Underweight: BMI below 18.5 — may indicate undernutrition, eating disorders, or underlying medical conditions
Normal Weight: BMI 18.5–24.9 — associated with lowest risk of weight-related health problems for most adults
Overweight: BMI 25.0–29.9 — increased risk of cardiovascular disease, type 2 diabetes, and other conditions
Obese: BMI 30.0 and above — significantly elevated risk for numerous health conditions
Visceral Fat: Fat stored around internal organs in the abdominal cavity — more dangerous than subcutaneous fat and not measured by BMI
Waist-to-Hip Ratio (WHR): A measurement comparing waist circumference to hip circumference, often considered a better health risk predictor than BMI

Common Use Cases

Annual health checkups and insurance medical screenings
Tracking weight changes during fitness or weight loss programs
Pre-surgical assessments to determine anesthesia requirements
School and workplace wellness programs for population health monitoring
Research studies analyzing weight trends across populations

Interpreting Your Results

BMI below 18.5 = Underweight: You may need to gain weight for better health. Consult a doctor if unintentional. | BMI 18.5–24.9 = Normal weight: This is the healthy range associated with the lowest risk of weight-related health problems. | BMI 25.0–29.9 = Overweight: Increased risk of heart disease, type 2 diabetes, and high blood pressure. Diet and exercise are recommended. | BMI 30.0+ = Obese: Significantly elevated health risks. Medical guidance is strongly recommended. Note: BMI is a screening tool, not a diagnostic test. Athletes and older adults may get misleading results.

Frequently Asked Questions

What is a healthy BMI range for adults?

For most adults aged 18–65, a healthy BMI ranges from 18.5 to 24.9. Below 18.5 is considered underweight, 25.0–29.9 is overweight, and 30.0 or above is obese. However, healthy ranges may differ for athletes, older adults, and certain ethnic groups — particularly Asian populations, where health risks increase at lower BMI thresholds.

Is BMI accurate for measuring body fat?

BMI is a screening tool, not a direct measure of body fat. It correlates moderately with direct body fat measurements at the population level, but cannot distinguish between muscle, bone, fat, or water weight. Athletes, bodybuilders, pregnant women, and the elderly may get misleading results. For a more accurate body composition assessment, consider DEXA scans, bioelectrical impedance, or skinfold measurements.

Does BMI differ for men and women?

The BMI formula and categories are the same for both men and women. However, women naturally tend to have higher body fat percentages than men at the same BMI level. Despite this difference, the standard BMI categories are statistically associated with similar health outcomes for both genders according to WHO research.

What are the main limitations of BMI?

BMI has several limitations: it doesn't measure body fat percentage or fat distribution; it doesn't account for muscle mass, bone density, age, sex, or ethnicity; it was designed for population studies, not individual diagnosis; and it may misclassify muscular individuals as overweight while missing unhealthy fat in people with 'normal' BMI. Always consult a healthcare professional for a complete health assessment.

What is the difference between BMI and body fat percentage?

BMI is a simple calculation using only height and weight, while body fat percentage directly measures the proportion of fat tissue in your body. Two people with the same BMI can have very different body fat percentages — for example, a bodybuilder and a sedentary person may both have a BMI of 27, but with vastly different body compositions. Body fat percentage requires specialized equipment to measure but provides more useful health information.

Can children use the same BMI chart as adults?

No. Children and teenagers aged 2–19 require age- and sex-specific BMI percentiles that account for growth and development. A child's BMI is plotted on a growth chart and compared to peers of the same age and sex. A BMI above the 85th percentile is considered overweight, and above the 95th percentile is considered obese for children.

How often should I check my BMI?

For most adults, checking BMI once a month is sufficient to track trends. If you're actively trying to lose or gain weight, weekly checks can help monitor progress. The most important factor is consistency — use the same scale, measure at the same time of day, and track the trend rather than focusing on individual readings.

How do I calculate BMI in kg and cm?

To calculate BMI in kg and cm, divide your weight in kilograms by your height in meters squared. Since height is usually given in cm, convert it first: divide by 100. Formula: BMI = weight(kg) ÷ (height(m))². Example: A person who is 170 cm tall and weighs 70 kg has a BMI of 70 ÷ (1.70)² = 70 ÷ 2.89 = 24.2, which is in the normal weight range.

What is a normal BMI range for adults?

According to the World Health Organization (WHO), a normal (healthy) BMI for adults is between 18.5 and 24.9. Below 18.5 is underweight; 25.0–29.9 is overweight; 30.0 and above is obese. These ranges apply to adults aged 18 and over regardless of sex. For children and teenagers, BMI is interpreted using age- and sex-specific growth charts.

Is BMI the same for men and women?

The BMI formula and thresholds are the same for men and women. However, BMI does not distinguish between fat and muscle mass, which means it can overestimate body fat in muscular individuals and underestimate it in others. For this reason, BMI is best used as a screening tool alongside other health measurements such as waist circumference and body fat percentage.