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").

Two Bonus Metrics: BMI Prime and Ponderal Index

Alongside your BMI, this calculator also returns two related numbers computed from the same inputs. BMI Primeis simply your BMI divided by 25 — the upper boundary of the Normal weight range — so it expresses how far you are from that boundary as a ratio rather than a raw BMI number: a BMI Prime of 1.0 sits exactly at the Normal/Overweight line, below 1.0 means you're under it, above 1.0 means you're over it. For the 70kg/1.75m example above (BMI 22.86), BMI Prime is 22.86 ÷ 25 = 0.91.

Ponderal Index(also called the Corpulence Index) is an older height-weight ratio that scales weight by the cube of height instead of the square that BMI uses. That extra power makes it less distorted at height extremes — very tall or very short people tend to see BMI over- or under-state their build, and Ponderal Index corrects for that better. In metric mode it's weight(kg) ÷ height(m)³ — for a 72.57kg adult at 1.778m, that's 72.57 ÷ 1.778³ = 72.57 ÷ 5.6208 = 12.91. In imperial mode it uses a different, independently-published formula for the same concept: height (in inches) ÷ the cube root of weight (in pounds) — for a 154lb adult at 69in, that's 69 ÷ ∛154 = 69 ÷ 5.36 = 12.87. Typical adult values for both formulas fall roughly in the 11–14 range.

Health Risks Associated With BMI Extremes

Public health bodies track BMI at a population level because, on average, BMI values outside the Normal range correlate with higher rates of certain health conditions. Higher BMI (Overweight/Obese ranges) is associated with increased average risk of type 2 diabetes, high blood pressure, cardiovascular disease, certain cancers, and joint problems from added mechanical load. Lower BMI (Underweight range) is associated with increased average risk of nutrient deficiencies, reduced bone density, a weakened immune response, and — particularly in older adults — higher frailty and surgical-recovery risk. These are population-level statistical associations from public-health research, not a diagnosis or a prediction about any individual — plenty of people outside the Normal range are perfectly healthy, and plenty inside it are not.

Limitations of BMI

BMI is a fast screening number, not a diagnosis, and it has real, well-documented blind spots. It cannot distinguish muscle from fat — a muscular athlete can post a BMI in the Overweight or even Obese range while carrying very little body fat, because BMI only knows total mass and height, not composition. It doesn't account for where fat is distributed on the body, which matters for cardiovascular risk independent of total weight. It tends to run less accurate at height extremes (very tall or very short adults), which is exactly the gap the Ponderal Index above is designed to narrow. It also doesn't adjust for age or sex, even though body composition shifts with both — older adults naturally carry a higher fat percentage than younger adults at the same BMI, and typical body-fat percentage differs by sex at the same BMI too. None of this means BMI is useless — it's cheap, fast, and correlates reasonably well with health outcomes at a population level — but a single BMI number is a starting point for a conversation with a healthcare professional, not a complete picture of anyone's health.

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).