βοΈ BMI Calculator
Body Mass Index with WHO categories, imperial or metric.
Category
Estimates only, not medical advice. Talk to a clinician about what your result means for you.
How to Use This Calculator
Pick your unit system, enter your height and weight, and the calculator returns your BMI, your WHO category, and the healthy weight range for your height.
Choose Imperial (feet/inches, pounds) or Metric (centimeters, kilograms).
Enter your height. Measure in the morning without shoes, standing straight against a wall.
Enter your weight, ideally measured in the morning before eating.
Read your category and the healthy weight range for your height as a reference band.
How BMI Is Calculated
BMI is weight scaled by height squared, which keeps the number comparable between short and tall people. The WHO bands: under 18.5 underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight, 30 and above obese (with 35 and 40 marking classes II and III). The "healthy weight range" for your height is simply the weights that put your BMI between 18.5 and 24.9. For a 5'9" (69 inch) adult: lower = 18.5 x 69^2 / 703 = 125 lb, upper = 24.9 x 69^2 / 703 = 169 lb.
Worked Example
Height 5'9" (69 inches), weight 170 lb. BMI = 703 x 170 / (69 x 69) = 119,510 / 4,761 = 25.1, just inside the overweight band. Healthy range for 69 inches: 125 to 169 lb. In metric: 175 cm, 77 kg. BMI = 77 / (1.75 x 1.75) = 77 / 3.0625 = 25.1. Same person, same result, which is a good self-check that the two formulas agree.
What BMI Misses
BMI was designed in the 1830s as a population statistic, not a personal health test, and it shows. It cannot distinguish muscle from fat, so athletes read high. It ignores fat distribution, yet abdominal fat predicts risk far better than total weight. Cutoffs also fit some populations worse than others; several Asian health authorities use lower thresholds. Use BMI as a first screen: if it flags overweight or obese, follow up with waist measurement and, ideally, a clinician. A number on a screen is not a diagnosis.
Frequently Asked Questions
What is a healthy BMI?
The World Health Organization defines a healthy BMI for adults as 18.5 to 24.9. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese. These cutoffs are population-level screening bands, not personal diagnoses.
Is BMI accurate for athletes?
Not always. BMI cannot tell muscle from fat, so muscular athletes and very active people can land in the "overweight" band while having low body fat. For them, waist circumference, body-fat percentage, or a clinician's assessment is more informative than BMI alone.
Is BMI different for men and women?
The formula and the cutoffs are the same for adult men and women. On average women carry a higher body-fat percentage at the same BMI, which is one of the known limitations of the index. Age matters more than sex: the bands were validated for adults 20 and older.
How is BMI calculated?
Imperial: 703 x weight in pounds divided by height in inches squared. Metric: weight in kilograms divided by height in meters squared. A 5'9" (69 in) person at 170 lb: 703 x 170 / 69^2 = 25.1.
What are the limitations of BMI?
BMI ignores muscle mass, bone density, fat distribution, age, and ethnicity. It does not say where fat is carried, and belly fat predicts health risk better than BMI does. Treat it as a rough screening number: useful for populations and as a starting point, never the whole story.