BMI Calculator
Calculate your Body Mass Index in seconds using imperial or metric units. Your BMI is a quick screening tool that places your weight relative to your height into one of four categories — underweight, normal, overweight, or obese.
Your BMI
What your BMI score means
BMI is calculated by dividing your weight in kilograms by your height in meters squared. A BMI under 18.5 is considered underweight, 18.5-24.9 is the normal range, 25-29.9 is overweight, and 30 or above falls into the obese category. Athletes and very muscular individuals may register as "overweight" despite low body fat, since BMI doesn't distinguish muscle from fat.
How This Calculator Works
This tool uses the standard Body Mass Index formula adopted by the World Health Organization: BMI = weight (kg) ÷ height (m)². If you enter imperial units, the calculator first converts your height to meters and your weight to kilograms, then applies the same equation, so metric and imperial inputs return an identical result. The output is a single number with no units that gets matched against four fixed categories — underweight (below 18.5), normal (18.5–24.9), overweight (25–29.9) and obese (30 and above). The color marker on the scale is positioned by mapping your BMI across a 15-to-40 range, which is why extreme values sit pinned at either end.
A Worked Example
Take someone who is 5′9″ tall and weighs 165 pounds — the default values in the calculator above. First convert the height: 5 feet 9 inches is 69 inches, which is 175.3 cm, or 1.753 m. Square that height: 1.753 × 1.753 = 3.07 m². Next convert the weight: 165 lb × 0.4536 = 74.8 kg. Finally divide: 74.8 ÷ 3.07 = 24.4. A BMI of 24.4 falls in the upper end of the normal range, just shy of the 25 overweight threshold. Add roughly 7 pounds and this same person would tip into the overweight category — a good illustration of how narrow these bands are.
What Affects Your Result
- Muscle mass: Muscle weighs more than the same volume of fat, so athletes and lifters often score higher than their leanness suggests.
- Frame and bone density: A naturally broad, heavy-boned build adds weight that BMI reads as excess mass.
- Age: Older adults tend to lose muscle and gain fat, so the same BMI can mean more body fat at 60 than at 25.
- Sex: Women carry more essential body fat than men at any given BMI, yet the categories do not adjust for this.
- Ethnicity: Health risks appear at lower BMIs in some Asian populations and higher BMIs in some others, so the universal cut-offs fit imperfectly.
- Measurement accuracy: Weighing yourself clothed, after meals, or at different times of day can swing the figure by a point or more.
Frequently Asked Questions
Is a BMI of 25 actually unhealthy?
Not necessarily. A BMI of 25 sits right at the line between the normal and overweight ranges, and the number alone says nothing about your body composition. A muscular person can carry a BMI of 27 with low body fat, while a sedentary person at 24 may carry excess fat around the waist. Use BMI as a starting screen, then look at waist circumference and body fat percentage for a fuller picture.
What is a healthy BMI range for adults?
For most adults aged 20 and over, a BMI between 18.5 and 24.9 is classified as the normal weight range by the World Health Organization and CDC. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese. These cut-offs apply to both men and women but were derived mainly from populations of European descent, so they fit some ethnic groups better than others.
Does BMI work for children and teenagers?
No, not in the same way. For anyone under 20, BMI must be compared against age- and sex-specific percentile charts rather than the fixed adult categories, because healthy body fat changes dramatically through growth and puberty. This calculator is built for adults; for children, ask a pediatrician to plot BMI-for-age percentiles.
Why does my BMI say overweight when I lift weights?
Because BMI uses only height and weight, it cannot tell muscle from fat. Muscle is denser than fat, so a lean, well-trained person often weighs more than the BMI scale expects and lands in the overweight band despite being in excellent shape. If you train regularly, trust a body fat measurement and how your clothes fit over the BMI label.
These figures are estimates for general education and screening only. BMI is not a diagnosis and does not measure body composition or health. It is not medical advice — talk to a qualified healthcare professional before acting on any result.