What This Calculator Does

Body Mass Index is a quick screening number that relates your weight to your height, used by clinicians and public health bodies as a first-pass way to flag whether someone is likely underweight, at a healthy weight, overweight, or obese. Enter your weight and height in whichever unit system you think in — metric or imperial — and this tool returns your BMI value along with which of those four categories it falls into, using the same numeric cutoffs the WHO and CDC publish.

How to Use It

Start by choosing your Unit System from the dropdown: Metric or Imperial. In Metric mode, enter your Weight in kilograms and your Height in meters — for example, 1.75 for someone who is 175cm tall, not 175. The height field genuinely expects meters here: entering a number greater than 3 (a plausible centimeter figure, but an implausible height in meters) is rejected, since it's almost certainly a units mistake rather than a real height. In Imperial mode, enter your Weight in pounds and your Height as separate Feet and Inches fields — for example, 5 feet and 9 inches — so there's no need to convert to a single decimal-inches figure yourself. Switching the unit system resets the form to that system's fields.

The Formula

Metric: BMI = weight (kg) ÷ height (m)². Imperial: BMI = 703 × weight (lb) ÷ height (in)² — 703 is the standard CDC conversion constant that lets the same category boundaries apply to pounds and inches. Both versions round the result to 2 decimal places, and that figure is then checked against the same four category boundaries regardless of which units you used: below 18.5 is Underweight, 18.5 up to (but not including) 25 is Normal weight, 25 up to 30 is Overweight, and 30 or above is Obese.

Those cutoffs are exact, not rounded display labels — a BMI of 24.99 is still Normal weight, and 25.0 is already Overweight, even though both would round to the same figure on a chart that only shows one decimal place. If your own BMI lands close to 25 or 30, it's worth knowing you're right at a real category boundary, not just an approximate one. Because 703 is itself a rounded constant, the imperial formula can differ from the metric formula by a few thousandths of a point for the same real person — that's expected, not an error, and it won't change which category you fall into.

A Worked Example

Metric: a 70kg adult who is 1.75m tall — square the height first, 1.75 × 1.75 = 3.0625, then divide the weight by that, 70 ÷ 3.0625 = 22.857142..., which rounds to a BMI of 22.86, solidly in the Normal weight range. A 68kg adult at 1.70m works out to 68 ÷ (1.70 × 1.70) = 68 ÷ 2.89 = 23.529411..., rounding to 23.53, also Normal weight.

Imperial: a 154lb adult who is 5'9" (69 inches) tall — square the height, 69 × 69 = 4761, multiply the weight by 703, 703 × 154 = 108,262, then divide: 108,262 ÷ 4761 = 22.7355..., rounding to a BMI of 22.74 — the same Normal weight category as the metric example above, for a real-world equivalent person (70kg ≈ 154lb, 1.75m ≈ 5'9").

How to Calculate BMI Manually

Metric: it's two steps. First, multiply your height in meters by itself to get height squared (for 1.75m, that's 1.75 × 1.75 = 3.0625). Second, divide your weight in kilograms by that number (70 ÷ 3.0625 = 22.86). If you only know your height in centimeters, divide by 100 first to convert to meters — 175cm becomes 1.75m — since using centimeters directly in the formula without converting will give a wildly wrong result.

Imperial: first convert your height to total inches (multiply feet by 12 and add any remaining inches — 5'9" becomes 5 × 12 + 9 = 69in). Square that figure (69 × 69 = 4761). Multiply your weight in pounds by 703 (154 × 703 = 108,262). Finally, divide that by the squared height (108,262 ÷ 4761 = 22.74).