How to Calculate BMI: Formula, Chart, and What Your Number Means

What Is BMI?

Body Mass Index (BMI) is a simple mathematical formula that relates your weight to your height to produce a single number. It was originally designed as a population-level statistical tool to identify weight trends across groups, not as a diagnostic tool for individual health. However, because of its simplicity — requiring only a scale and a tape measure — it has become the standard screening tool used by doctors, insurance companies, and public health agencies worldwide.

BMI does not directly measure body fat. It cannot tell you whether your weight comes from muscle, fat, bone, or water. Despite this limitation, BMI correlates reasonably well with more direct measures of body fat for the majority of the population, which is why it remains in widespread use as a first-pass screening metric.

You can quickly find your BMI using our BMI calculator, or follow the manual calculations below to understand exactly how the formula works.

The BMI Formula: Metric and Imperial

BMI can be calculated using either metric or imperial (U.S. customary) measurements. Both formulas produce the same result.

Metric Formula

BMI = weight (kg) / height (m)2

In the metric formula, weight is measured in kilograms and height in meters. You divide your weight by the square of your height.

Example: A person weighing 75 kg who is 1.75 m tall:

• Height squared: 1.75 x 1.75 = 3.0625
• BMI = 75 / 3.0625 = 24.5

Imperial Formula

BMI = (weight (lbs) / height (in)2) x 703

In the imperial formula, weight is measured in pounds and height in inches. You divide weight by height squared, then multiply by 703 (a conversion factor that adjusts for the unit difference between the metric and imperial systems).

Example: A person weighing 165 lbs who is 5'9" (69 inches) tall:

• Height squared: 69 x 69 = 4,761
• Weight / height squared: 165 / 4,761 = 0.034657
• BMI = 0.034657 x 703 = 24.4

Step-by-Step: Calculating Your BMI Manually

Follow these steps using the imperial formula (the most common in the United States):

1. Measure your height in inches. If you are 5 feet 6 inches, that is (5 x 12) + 6 = 66 inches. If you are 6 feet 1 inch, that is (6 x 12) + 1 = 73 inches.
2. Square your height. Multiply the height in inches by itself. For 66 inches: 66 x 66 = 4,356.
3. Divide your weight by the squared height. If you weigh 155 pounds: 155 / 4,356 = 0.03558.
4. Multiply by 703. 0.03558 x 703 = 25.0.
5. Interpret your result using the BMI categories chart below.

For a faster result, simply plug your numbers into our BMI calculator and get your result instantly with a detailed interpretation.

BMI Categories Chart

The World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC) use the following BMI ranges to classify weight status in adults (ages 20 and older):

• Underweight: BMI below 18.5
• Normal weight: BMI 18.5 to 24.9
• Overweight: BMI 25.0 to 29.9
• Obesity Class I: BMI 30.0 to 34.9
• Obesity Class II: BMI 35.0 to 39.9
• Obesity Class III (severe): BMI 40.0 and above

What Each Category Means for Health

Underweight (below 18.5): Being underweight can indicate malnutrition, eating disorders, or underlying health conditions. It is associated with weakened immune function, osteoporosis, fertility issues, and increased surgical risk. If your BMI falls in this range and you are not intentionally maintaining a low weight under medical supervision, consult a healthcare provider.

Normal weight (18.5-24.9): This range is associated with the lowest statistical risk of weight-related health problems. However, a "normal" BMI does not guarantee good health — diet quality, physical activity, sleep, stress, and genetics all play crucial roles.

Overweight (25.0-29.9): This range indicates slightly elevated risk for conditions like type 2 diabetes, hypertension, and cardiovascular disease. However, many people in this range — particularly those who exercise regularly and have high muscle mass — may be in excellent health. Context matters enormously in this category.

Obese (30.0 and above): Higher BMI values are associated with increased risk for numerous health conditions including type 2 diabetes, heart disease, stroke, certain cancers, sleep apnea, and joint problems. The risk increases with higher BMI categories. However, factors like fitness level, metabolic health, and fat distribution are also important. Some individuals with BMIs over 30 have normal blood pressure, blood sugar, and cholesterol levels.

Quick Reference: BMI for Common Heights and Weights

Here are BMI values for several common height/weight combinations:

• 5'4" (64"), 130 lbs: BMI = 22.3 (normal)
• 5'6" (66"), 155 lbs: BMI = 25.0 (overweight threshold)
• 5'8" (68"), 165 lbs: BMI = 25.1 (overweight)
• 5'10" (70"), 175 lbs: BMI = 25.1 (overweight)
• 6'0" (72"), 185 lbs: BMI = 25.1 (overweight)
• 6'0" (72"), 200 lbs: BMI = 27.1 (overweight)
• 6'2" (74"), 195 lbs: BMI = 25.0 (overweight threshold)

Browse our complete BMI reference tables for a comprehensive lookup covering a wide range of heights and weights.

BMI for Children and Teens

BMI is calculated the same way for children and teenagers, but the interpretation is different. Because body composition changes significantly during growth and development, and differs between boys and girls at the same age, children's BMI is expressed as a percentile relative to other children of the same age and sex.

• Underweight: Below the 5th percentile
• Healthy weight: 5th percentile to below the 85th percentile
• Overweight: 85th percentile to below the 95th percentile
• Obese: 95th percentile and above

A BMI of 23 might be perfectly normal for a 15-year-old boy but at the 85th percentile for a 10-year-old girl. This is why age- and sex-specific growth charts (maintained by the CDC) are used for anyone under 20. Pediatricians routinely track BMI percentiles at annual checkups.

The Limitations of BMI

While BMI is useful as a quick screening tool, it has well-documented limitations that every health-conscious person should understand. Relying on BMI alone can lead to misleading conclusions about your health.

1. BMI Does Not Distinguish Muscle from Fat

This is the most frequently cited limitation. Muscle tissue is denser than fat tissue, so muscular individuals often have BMIs in the "overweight" or even "obese" range despite having low body fat percentages. Professional athletes provide extreme examples — many football players, bodybuilders, and rugby players have BMIs well above 30 while being in peak physical condition. Even regular gym-goers with moderate muscle mass can be miscategorized.

2. BMI Does Not Account for Fat Distribution

Where you carry fat matters as much as how much you carry. Visceral fat — fat stored around the internal organs in the abdominal cavity — is significantly more dangerous than subcutaneous fat (fat under the skin on your arms, legs, and hips). Two people with the same BMI of 28 could have very different health risk profiles depending on whether their excess weight is concentrated in the midsection or distributed more evenly.

3. BMI Does Not Account for Age, Sex, or Ethnicity

As people age, they naturally lose muscle mass and gain fat, even if their weight stays the same. A BMI of 24 at age 25 represents a very different body composition than a BMI of 24 at age 65. Women naturally carry more body fat than men at the same BMI. Additionally, research has shown that health risks associated with specific BMI values differ across ethnic groups. Some Asian populations experience increased health risks at lower BMI values (23-24), while some Black populations may tolerate slightly higher BMIs with fewer metabolic consequences.

4. BMI Was Designed for Populations, Not Individuals

Adolphe Quetelet created the BMI formula in the 1830s as a tool for studying population-level trends, not for assessing individual health. It was designed to be simple and require minimal measurements. While it works well for identifying weight trends in large groups, applying it rigidly to any single person can be misleading.

5. BMI Does Not Measure Fitness or Metabolic Health

A sedentary person with a "normal" BMI of 23 but poor cardiovascular fitness, high blood sugar, and unhealthy cholesterol levels may be at greater health risk than an active person with a BMI of 28 who exercises regularly and has excellent metabolic markers. BMI tells you nothing about your blood pressure, blood glucose, cholesterol, fitness level, or overall metabolic health.

Better Alternatives and Supplements to BMI

Given BMI's limitations, health professionals increasingly recommend using additional metrics alongside or instead of BMI for a more complete picture of body composition and health risk.

Waist Circumference

Measuring your waist at the navel provides a direct indicator of abdominal (visceral) fat, which is the type most closely linked to heart disease, diabetes, and metabolic syndrome. Guidelines suggest that health risk increases significantly when waist circumference exceeds:

• Men: 40 inches (102 cm)
• Women: 35 inches (88 cm)

Waist-to-Hip Ratio

Divide your waist circumference by your hip circumference. The WHO considers health risk elevated when the ratio exceeds 0.90 for men or 0.85 for women. This ratio captures body fat distribution better than BMI.

Waist-to-Height Ratio

A simple and increasingly validated metric: your waist circumference should ideally be less than half your height. If you are 70 inches tall, your waist should be under 35 inches. Research suggests this ratio may be a better predictor of cardiovascular risk than BMI.

Body Fat Percentage

Directly measuring body fat percentage provides the most meaningful assessment of body composition. Healthy ranges vary by age and sex:

• Men (age 20-39): 8-20% body fat
• Men (age 40-59): 11-22% body fat
• Women (age 20-39): 21-33% body fat
• Women (age 40-59): 23-34% body fat

Body fat can be measured through DEXA scans (the gold standard), bioelectrical impedance scales, skinfold calipers, hydrostatic weighing, or bod pod air displacement. Use our body fat calculator to estimate your body fat percentage.

The Best Approach: Use Multiple Metrics

No single number tells the complete story of your health. The most informative approach is to track BMI alongside waist circumference and body fat percentage, while also monitoring metabolic health markers (blood pressure, blood sugar, cholesterol) through regular checkups. If your BMI puts you in the "overweight" category but your waist circumference is healthy, your body fat percentage is normal, and your metabolic markers are good, you likely have nothing to worry about.

How to Use BMI Effectively

Despite its limitations, BMI remains a useful tool when used appropriately. Here is how to get the most value from it:

1. Use it as a starting point, not a final verdict. BMI is a screening tool that identifies potential weight-related health risks. It is not a diagnosis.
2. Track changes over time. Even if your absolute BMI number is less meaningful for you (due to high muscle mass, for example), tracking changes in your BMI over months and years can reveal important trends.
3. Combine it with other measurements. Add waist circumference and body fat percentage to create a much more complete health profile.
4. Consider your full health picture. Factor in physical activity level, diet quality, blood work results, family history, sleep quality, and mental health. A number on a chart is only one data point among many.
5. Discuss with your doctor. If your BMI falls outside the normal range, have a conversation with your healthcare provider about what it means in your specific context, especially if you are muscular, an athlete, or have other factors that affect the interpretation.

Calculate your BMI now with our BMI calculator and explore your BMI reference charts for a detailed breakdown of where your numbers fall.