BMI Calculator
Free online BMI (body mass index) calculator with metric and imperial units. See your BMI category and the exact weight range that puts you in the healthy 18.5–24.9 zone for your height.
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How to Use This BMI Calculator
- Pick metric (kg / cm) or imperial (lb / in). The labels update instantly.
- Type your weight and height. Decimals are fine — height 5'10" is 70 inches or 177.8 cm.
- Hit Calculate. You'll get your BMI, your category, and the healthy weight range for your height.
How BMI Is Calculated
Body mass index is a simple ratio of weight to the square of height. The formula has been around since the 1830s and is the same worldwide — only the units change.
Imperial: BMI = 703 × lb ÷ (in × in)
The healthy weight range shown is just the formula run backwards — your height squared, multiplied by 18.5 to 24.9, gives the kilogram (or pound) range that lands inside the "healthy" BMI band.
What BMI can and can't tell you
BMI is a quick ratio of your weight to your height. It's useful as a rough flag — it costs nothing, needs no equipment, and gives a consistent starting point for a conversation about weight. What it can't do is tell muscle from fat, account for where fat sits on your body, or adjust for age, sex and build. A muscular person can read as "overweight" while someone in the "healthy" range carries little muscle and a lot of visceral fat.
So treat the category here as one signal among several, not a verdict. Waist measurement, body-fat percentage, blood markers and how you actually feel all add context that BMI alone misses.
This one I saw first-hand. At a couple of the companies I supported — I spent about 20 years in IT support — there were corporate "wellness" drives where everyone's BMI got measured. One colleague who trained seriously and was visibly fit got flagged "overweight" by the chart, while a few sedentary folks landed comfortably in the "normal" band. It became the running joke of the week, but it taught everyone something real: the number is a screening shortcut, not a diagnosis. I'm not a medical professional, so I'd treat BMI the way those wellness nurses eventually did — as a prompt to look closer, not a conclusion.
— Hill, 20 years in IT supportBMI is a general estimate, not medical advice or a diagnosis. For anything about your health, please talk to a doctor who can look at the full picture.
Frequently Asked Questions
What does BMI actually measure?
BMI is a ratio of weight to height squared. It is a population-level screening tool — useful for tracking trends, but it does not directly measure body fat and ignores muscle mass, bone density, and body composition.
What are the BMI categories?
Underweight: below 18.5. Healthy weight: 18.5 to 24.9. Overweight: 25 to 29.9. Obesity: 30 and above. The WHO uses these thresholds for adults.
Is BMI accurate for athletes?
No — muscle is denser than fat, so very muscular people often score in the "overweight" or "obese" range despite having low body fat. For athletes, body fat percent or waist-to-height ratio is more meaningful.
What should I do if my BMI is outside the healthy range?
Use it as a prompt, not a panic button — a single number doesn't capture your full health. If you're concerned, a doctor can look at waist measurement, body composition, blood work and your activity level together, which is far more meaningful than BMI on its own.