BMI Calculator

What is BMI?

Body Mass Index (BMI) is a simple screening tool that estimates body fat based on your height and weight. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and is widely used by healthcare professionals as a quick assessment of weight status. BMI applies to adult men and women and is calculated differently for children.

How BMI is Calculated

The BMI formula divides your weight by the square of your height:

$$\text{BMI} = \frac{\text{weight (kg)}}{\text{height (m)}^2} \qquad \text{BMI} = \frac{\text{weight (lbs)} \times 703}{\text{height (in)}^2}$$

For example, a person who weighs 160 lbs and is 5'9" (69 inches) tall has a BMI of (160 × 703) / 69² ≈ 23.6, which falls in the normal weight category.

Metric Worked Example

For a person weighing 70 kg at a height of 1.75 m: $\text{BMI} = 70 / 1.75^2 = 70 / 3.0625 \approx 22.9$, also landing in the normal weight range. Because the metric formula divides directly by height in meters squared with no extra multiplier, it's slightly more intuitive to compute by hand than the imperial version, which needs the 703 conversion factor to correct for using pounds and inches instead of kilograms and meters.

BMI Categories

The World Health Organization defines four main categories: Underweight (BMI below 18.5), Normal weight (BMI 18.5 to 24.9), Overweight (BMI 25 to 29.9), and Obese (BMI 30 and above). These ranges are based on population studies linking BMI to health outcomes, and are also published in the CDC's own BMI guidance, which uses the same category cutoffs.

Limitations of BMI

BMI does not directly measure body fat and has important limitations. It may overestimate body fat in athletes and muscular individuals, and underestimate it in older adults who have lost muscle mass. BMI also does not account for fat distribution — abdominal fat carries greater health risks than fat stored elsewhere. For a fuller picture, try the Body Composition Calculator, or consult a healthcare professional.

Why a 200-Year-Old Formula Is Still in Use

Quetelet developed what he called the "Quetelet Index" as a statistical tool for studying population averages, not as a per-person health diagnostic — a distinction often lost in how BMI gets used today. It stuck around and eventually got renamed "Body Mass Index" because it's simple to calculate from just two easily measured numbers and correlates reasonably well with health risk across large populations, even though it's a much blunter instrument for evaluating any single individual.

Alternatives and Complements to BMI

Because BMI can't distinguish muscle from fat or see where fat is stored, several complementary measures are often used alongside it. Waist circumference and waist-to-hip ratio directly capture abdominal fat, which carries more health risk than fat elsewhere on the body, even at the same overall BMI. Body fat percentage — measured with methods ranging from simple skinfold calipers to bioelectrical impedance scales to DEXA scans — gives a more direct estimate of composition; try the Body Composition Calculator for an estimate. No single measurement tells the whole story, which is why healthcare providers typically weigh several factors together rather than relying on BMI alone.

Common BMI Mistakes

Treating BMI as a precise, individual health diagnosis rather than a population-level screening tool is the most consequential misunderstanding, given its origins described above. Ignoring the athlete and muscle-mass caveat is another frequent issue — a muscular individual can land in the "overweight" category by BMI alone despite having low body fat, since BMI has no way to distinguish muscle tissue from fat tissue. Comparing BMI across very different age groups without adjustment is a third: BMI categories and their associated health risks shift somewhat with age, and pediatric BMI is calculated and interpreted completely differently from the adult formula used here.

BMI Terms You Should Know

Body Composition — the proportion of fat versus lean mass (muscle, bone, water) that makes up total body weight, something BMI cannot measure directly.

Waist-to-Hip Ratio — a measurement comparing waist and hip circumference, used as an indicator of fat distribution and associated health risk.

Obesity Class — subdivisions within the "Obese" BMI category (commonly Class I, II, and III) used clinically to indicate increasing severity and associated health risk.

Percentile (Pediatric BMI) — for children, BMI is compared against age- and sex-specific population percentiles rather than the fixed adult category ranges.