How to Use This Tool

BMI calculator

Enter your height and weight to calculate your Body Mass Index. BMI is a screening tool that estimates body fat based on your height-to-weight ratio. It works for adults 20 and older. This page explains the formula, the category ranges, and what BMI does and does not tell you about health.

BMI categories

These categories come from the World Health Organization and the US National Institutes of Health. They apply to adults of all ages and both sexes, though the relationship between BMI and body fat percentage varies by age, sex, and ethnicity.

BMI formulas

Metric formula

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

Example: A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 / (1.75 x 1.75) = 70 / 3.0625 = 22.9.

Imperial formula

BMI = (weight (lb) x 703) / height (in) squared

Example: A person who weighs 160 lb and is 5 feet 8 inches (68 inches) tall has a BMI of (160 x 703) / (68 x 68) = 112,480 / 4,624 = 24.3.

Why 703?

The factor 703 converts from pounds and inches to the same result as the metric formula. It accounts for the unit differences: 1 kg = 2.20462 lb, and 1 inch = 0.0254 m. The math works out to approximately 703.069, rounded to 703.

BMI reference by height and weight

This table shows BMI values for common height and weight combinations using imperial units.

What BMI does and does not measure

What it does

BMI is a quick screening tool that correlates with body fat at the population level. It requires only a scale and a tape measure. Research has found that higher BMIs are associated with increased risk of type 2 diabetes, cardiovascular disease, and certain cancers across large groups. Public health agencies use it because it is simple, inexpensive, and consistent.

What it misses

BMI does not distinguish between muscle and fat. An athlete with a lot of muscle mass can have a BMI in the “overweight” range while having low body fat. BMI also does not account for where fat is stored. Abdominal fat carries more health risk than fat stored in the hips and thighs, but BMI treats all weight the same regardless of distribution.

BMI categories were developed primarily from studies of white European populations. Research has shown that health risks can occur at different BMI thresholds for people of different ethnic backgrounds. Some Asian populations face elevated metabolic risk at BMIs below 25, while some Pacific Islander populations may have lower risk at higher BMIs.

Tips for using BMI

• BMI is a starting point, not a diagnosis. A doctor considers BMI alongside other measurements like waist circumference, blood pressure, cholesterol levels, and blood sugar to assess health risks.
• For children and teenagers, BMI is calculated the same way but interpreted differently. Pediatric BMI uses age- and sex-specific percentile charts because body composition changes as children grow.
• If you measure your height in feet and inches, convert everything to inches first. 5 feet 6 inches = 66 inches. Then square that number (66 x 66 = 4,356) and use the imperial formula.
• Weigh yourself at the same time of day for consistency. Body weight fluctuates by 2 to 4 pounds throughout the day due to food, water, and other factors.

Other body composition measurements

Waist circumference is a simple alternative that specifically targets abdominal fat. The WHO considers a waist circumference above 40 inches (102 cm) for men or 35 inches (88 cm) for women to indicate elevated health risk, regardless of BMI.

Waist-to-hip ratio divides waist circumference by hip circumference. The WHO considers a ratio above 0.90 for men or 0.85 for women to indicate elevated risk. This measurement captures fat distribution better than BMI.

Body fat percentage can be measured through methods like skinfold calipers, bioelectrical impedance, DEXA scans, or hydrostatic weighing. These provide more detail than BMI but require equipment and trained operators.

Frequently asked questions

What is a healthy BMI?

The “normal weight” range is 18.5 to 24.9. But a healthy BMI depends on individual factors including age, sex, muscle mass, and overall health. Talk to a healthcare provider for personalized guidance.

How do I calculate BMI in metric?

Divide your weight in kilograms by your height in meters squared. A 75 kg person at 1.80 m tall has a BMI of 75 / (1.80 x 1.80) = 23.1.

Why is my BMI high even though I exercise?

Muscle weighs more than fat by volume. If you have significant muscle mass from strength training, your BMI may fall in the “overweight” category even with low body fat. This is a well-known limitation of BMI.

Does BMI apply to older adults?

The same formula applies, but some research suggests that slightly higher BMIs (25 to 27) may be associated with lower mortality risk in adults over 65. Body composition changes with age as muscle mass naturally decreases.

Is BMI different for men and women?

The formula and categories are the same, but women generally have more body fat than men at the same BMI. A BMI of 25 corresponds to roughly 30% body fat in women and 20% in men.

What BMI is considered obese?

A BMI of 30 or above is classified as obese by the WHO. Class 1 is 30 to 34.9, Class 2 is 35 to 39.9, and Class 3 is 40 and above.

Related tools

• BMR Calculator
• Weight Converter
• Pounds to Kilograms Converter
• Feet to Meters Converter

Measurement guides

• How Big Is X Inches? A Visual Size Guide From 1 to 16 Inches
• Inches Explained: Conversion Chart, Tools, and Real-World Examples

Formula & Explanation

The BMI formula was developed by Adolphe Quetelet in the 1830s. The WHO established the current category thresholds in 1997 based on epidemiological data linking BMI to health outcomes.