What is BMI and what does it measure?

BMI (Body Mass Index) is a numerical value calculated from your height and weight that estimates whether your weight falls within a healthy range. It measures body mass relative to height using the formula: weight in kilograms divided by height in meters squared (kg/m²). BMI does not directly measure body fat but serves as a screening tool to identify potential weight-related health risks.

How do I calculate my BMI?

To calculate BMI manually: For metric units, divide your weight in kilograms by your height in meters squared (BMI = kg/m²). For imperial units, divide your weight in pounds by your height in inches squared, then multiply by 703 (BMI = lbs/in² × 703). For example, a person weighing 70 kg at 1.75 m tall has a BMI of 22.9.

What is a healthy BMI range?

According to the WHO and CDC, a healthy BMI for adults ranges from 18.5 to 24.9 kg/m². Below 18.5 is considered underweight, 25-29.9 is overweight, and 30 or higher indicates obesity. These categories help identify potential health risks associated with weight.

Is BMI accurate for everyone?

BMI has limitations and may not be accurate for everyone. It cannot distinguish between muscle and fat mass, so athletes with high muscle mass may be classified as overweight despite being healthy. BMI also doesn't account for fat distribution, age-related body composition changes, or ethnic variations in health risk thresholds. It's best used as one indicator among several health assessments.

Is BMI different for men and women?

The BMI formula and category ranges are the same for men and women according to CDC guidelines. However, women naturally have higher body fat percentages than men at equivalent BMI values. Despite this physiological difference, the same BMI thresholds (underweight below 18.5, healthy 18.5-24.9, overweight 25-29.9, obese 30+) apply to both sexes for adults.

What BMI categories indicate obesity?

Obesity is classified at BMI 30 or higher, with three severity classes: Class 1 Obesity (BMI 30-34.9), Class 2 Obesity (BMI 35-39.9), and Class 3 Severe Obesity (BMI 40 or higher). Each class carries progressively higher health risks for conditions including type 2 diabetes, heart disease, and certain cancers.

Should I use metric or imperial units for BMI?

Either unit system produces the same BMI result when calculated correctly. Use whichever system you're most familiar with. Metric (kilograms and meters) is standard internationally and simpler mathematically. Imperial (pounds and inches) is common in the US and UK. Our calculator supports both systems for your convenience.

How does age affect BMI interpretation?

For adults 20 and older, standard BMI categories apply regardless of age. However, older adults naturally lose muscle mass and may have more body fat at the same BMI. Some research suggests adults over 65 may benefit from a slightly higher BMI (25-27). For children and teens ages 2-19, BMI is interpreted using age-specific percentile charts rather than fixed categories.

What are the health risks of high BMI?

Higher BMI is associated with increased risk of several health conditions including: type 2 diabetes, heart disease and stroke, high blood pressure, high cholesterol, sleep apnea, certain cancers, osteoarthritis, and mental health conditions. The CDC recommends discussing BMI results with a healthcare provider to understand your personal risk factors.

When should I consult a doctor about my BMI?

Consult a healthcare provider if your BMI falls outside the healthy range (below 18.5 or 25 and above), if you have concerns about your weight, or if you have risk factors like family history of diabetes or heart disease. BMI should be evaluated alongside other health indicators including blood pressure, cholesterol levels, medical history, and lifestyle factors.

What are alternatives to BMI for measuring health?

Complementary measurements to BMI include: waist circumference (measuring abdominal fat distribution), waist-to-hip ratio, waist-to-height ratio, body fat percentage (via calipers, bioelectrical impedance, or DEXA scans), and the Relative Fat Mass (RFM) index. These measurements can provide additional context about health risks that BMI alone cannot capture.

Instant Calculator

Free BMI Calculator

BMI Calculator: Calculate Your Body Mass Index Online

Find out if your weight is healthy in under 30 seconds. Our free BMI calculator uses the same formula trusted by the CDC and WHO to instantly determine your body mass index and show where you fall on the health spectrum—from underweight to obese.

Simply enter your height and weight below to get your BMI score, see your health category, and discover your ideal weight range.

BMI Calculator Tool

Enter your measurements below to calculate your BMI and see your weight category

Weight (kg)

Height (cm)

How to use this calculator:

Select your preferred unit system (Metric or Imperial)
Enter your height in centimeters/meters or feet/inches
Enter your weight in kilograms or pounds
Click "Calculate BMI" for instant results

Your result will show your BMI value, health category, and the healthy weight range for your height.

What BMI actually tells you about your health

Body Mass Index is a screening measurement that estimates whether your weight is proportionate to your height. Developed in the 1830s by Belgian mathematician Adolphe Quetelet, BMI remains the most widely used method for categorizing weight status in medical and public health settings.

The calculation produces a single number—typically between 15 and 40 for most adults—that places you into one of four main categories. Healthcare providers use this number alongside other indicators like blood pressure, cholesterol levels, and family history to assess overall health risk.

BMI measures weight relative to height, not body fat directly. This distinction matters because two people with identical BMI values can have very different body compositions. A muscular athlete and a sedentary office worker might share the same BMI while having vastly different health profiles.

Despite this limitation, population studies consistently show that BMI correlates with health outcomes. Higher BMI values are associated with increased risk of type 2 diabetes, cardiovascular disease, certain cancers, and premature mortality. This statistical relationship is why BMI remains a valuable first-pass screening tool.

The BMI formula explained with real examples

BMI uses straightforward arithmetic that anyone can verify. The formula divides your weight by your height squared, producing a standardized value regardless of whether you're tall or short.

Metric Formula (International Standard)

BMI = Weight (kg) ÷ Height (m)²

Example: 65 kg ÷ (1.68 × 1.68) = 23.0

Imperial Formula (US/UK)

BMI = [Weight (lbs) ÷ Height (in)²] × 703

The 703 factor converts units to metric scale.

Real Calculation Examples

Example 1 (Metric): Sarah weighs 65 kg and stands 1.68 m tall.
BMI = 65 ÷ (1.68 × 1.68) = 65 ÷ 2.82 = 23.0 (Healthy weight)
Example 2 (Imperial): Michael weighs 185 lbs and is 5'10" (70 inches) tall.
BMI = (185 ÷ 4,900) × 703 = 0.0378 × 703 = 26.6 (Overweight)
Example 3 (Metric): Priya weighs 52 kg and stands 1.60 m tall.
BMI = 52 ÷ (1.60 × 1.60) = 52 ÷ 2.56 = 20.3 (Healthy weight)

Understanding BMI categories and what they mean

The World Health Organization and Centers for Disease Control use standardized cutoff points to classify BMI results. These thresholds are based on extensive research linking specific BMI ranges to health outcomes.

BMI Range	Category	Health Implication
Below 18.5	Underweight	May indicate nutritional deficiency; increased risk of osteoporosis, weakened immunity
18.5 – 24.9	Healthy Weight	Lowest risk range for weight-related health conditions
25.0 – 29.9	Overweight	Elevated risk for cardiovascular disease, diabetes, and metabolic conditions
30.0 – 34.9	Obesity Class I	High risk for chronic disease; medical intervention often recommended
35.0 – 39.9	Obesity Class II	Very high risk; significant lifestyle modification typically necessary
40.0 and above	Obesity Class III	Severe obesity; highest risk category requiring comprehensive medical support

The healthy weight zone between 18.5 and 24.9 represents where statistical health risks are lowest. However, optimal BMI varies individually—some research suggests slightly higher BMIs (up to 27) may be protective for adults over 65.

Step-by-step guide to calculating your BMI

Getting an accurate BMI reading requires precise measurements. Here's how to measure yourself correctly at home:

Measuring Height

• Stand straight against a wall without shoes
• Look straight ahead with chin parallel to the floor
• Have someone mark the highest point of your head
• Measure from floor to mark

Measuring Weight

• Weigh yourself in the morning before eating
• Wear light clothing or none at all
• Use the same scale consistently
• Place scale on a hard, flat surface

Using Our Tool

Enter measurements into the calculator. For imperial users, input feet and inches separately. The calculator handles all conversions automatically.

Why BMI doesn't tell the whole story

BMI earned its place in medicine through simplicity and predictive power. But individual health assessment requires nuance that a single number cannot provide.

Athletes and muscular individuals

Often register as "overweight" or "obese" despite low body fat. Muscle is denser than fat. A 2005 study found that 50% of NFL players classified as obese by BMI had healthy body fat percentages.

Body fat distribution

Visceral fat (around organs) poses greater risks than subcutaneous fat. Two people with identical BMIs can have different health profiles depending on where fat accumulates.

Age & Body composition

Adults lose muscle mass after age 30. An older person at a "healthy" BMI may carry more fat than a younger person. Some experts suggest optimal BMI shifts slightly higher for seniors.

Ethnicity

Asian populations experience health risks at lower BMI values. The WHO recommends action points starting at BMI 23 for Asian populations, vs 25 generally.

Common mistakes when using BMI calculators

Confusing unitsMixing metric and imperial measurements produces inaccurate results. Always use consistent units.
Inconsistent timingWeight fluctuates 2-6 lbs daily. Measure at the same time (morning) for accuracy.
Ignoring contextA single reading is a snapshot. Trends over time provide more useful info than one measurement.
Using for childrenChildren need age-specific percentile charts. A BMI of 22 is high for a 10-year-old but healthy for an adult.

What to do after calculating your BMI

If healthy (18.5-24.9): Focus on maintenance through balanced nutrition and activity.

If underweight (<18.5): Low weight may signal nutritional issues. Discuss with a provider; focus on nutrient-dense foods and strength training.

If overweight/obese (25+): Consider this a starting point. The CDC recommends discussing results with a provider to evaluate:

Blood pressureCholesterolBlood glucoseFamily history

Sustainable change beats dramatic intervention. Modest weight loss of 5-10% produces meaningful health improvements.

When to consult a healthcare professional

BMI is a screening tool, not a diagnosis. Seek professional guidance when:

Your BMI falls outside the healthy range
You're concerned about your weight or body composition
You have family history of diabetes, heart disease, or obesity
You experience symptoms like unwanted fatigue or joint pain
You're considering significant dietary or exercise changes

BMI reference table by height and weight

Find your height, then scan across to locate your approximate BMI range.

Height	Underweight (<18.5)	Healthy (18.5-24.9)	Overweight (25-29.9)	Obese (≥30)
5'0" (152 cm)	Below 95 lbs	95-127 lbs	128-152 lbs	153+ lbs
5'4" (163 cm)	Below 108 lbs	108-144 lbs	145-173 lbs	174+ lbs
5'8" (173 cm)	Below 122 lbs	122-163 lbs	164-196 lbs	197+ lbs
6'0" (183 cm)	Below 137 lbs	137-183 lbs	184-220 lbs	221+ lbs
6'4" (193 cm)	Below 152 lbs	152-204 lbs	205-245 lbs	246+ lbs

Frequently asked questions

Disclaimer: This BMI calculator is a screening tool and is not intended to diagnose any disease or illness. BMI is one potential health indicator and should be considered with other factors when assessing overall health. This calculator does not replace professional medical advice. Consult a qualified healthcare provider with questions about your BMI or weight management.

Last updated: 2025-12-12