BMI Calculator

Body mass index compares a person’s weight with the square of their height. It offers a consistent way to describe weight relative to height, using only two measurements. It does not directly measure body fat, fitness, nutrition or overall health.

For adults aged 20 and older, this calculator uses the CDC’s adult screening categories. The same arithmetic formula works for different ages, but interpretation changes during growth. An adult BMI chart must not be applied to a child or teenager.

1. Select centimeters, meters, feet & inches, or total inches for height.
2. Enter your height, then select kilograms or pounds and enter weight.
3. Enter age in whole years. The supported input range is 2–120; adult references appear only from 20.
4. Read the BMI number and, when appropriate, the adult category. Review the formula and limitations alongside the result.

Results update as you type. Unit changes convert the current physical measurement, so switching from kilograms to pounds does not replace 70 kg with 70 lb. Copy, print and CSV actions include the measurements and interpretation rules. Reset restores the labeled starting example of 70 kg, 175 cm and age 30.

Convert height to meters and weight to kilograms first. Square the height, then divide weight by that squared value. Keep the converted values at full precision until the final result.

Example: 175 cm = 1.75 m; 1.75² = 3.0625 m²; 70 ÷ 3.0625 = 22.857142857… kg/m². The displayed BMI is 22.86. For an adult, the unrounded value belongs to Healthy Weight (Adult).

Age does not appear in the BMI equation. Here it determines whether adult categories, BMI Prime and the mathematical weight interval are shown. No sex input is needed to calculate an adult BMI.

BMI = weight in kg ÷ (height in meters × height in meters).

With centimeters, the equivalent formula is BMI = 10,000 × kg ÷ cm². The factor 10,000 comes from squaring the 100 centimeters in one meter. Dividing kg by centimeters squared without this conversion gives a result that is 10,000 times too small.

The calculator accepts height from 30 to 300 cm and weight from 1 to 1,000 kg, including unit equivalents. These broad input limits prevent zero, negative or extreme arithmetic; they are not assertions about what measurements are typical or clinically appropriate.

First combine feet and inches: total inches = 12 × feet + inches. The inches field accepts values from zero to below 12. For example, 5 ft 9 in is 69 total inches.

BMI = (pounds × 0.45359237) ÷ (total inches × 0.0254)². One avoirdupois pound is exactly 0.45359237 kg; one inch is exactly 0.0254 m. These definitions avoid rounding the familiar imperial multiplier to 703.

For 154 lb and 5 ft 9 in, the converted weight is 69.85322498 kg and height is 1.7526 m. BMI is approximately 22.74. A small difference from a manually rounded conversion is expected; the calculator does not round height before squaring it.

The following categories apply at age 20 or older. A boundary belongs to the category that starts at it: exactly 18.5 is Healthy Weight, exactly 25 is Overweight, and exactly 30 is Obesity. Classification uses the stored, unrounded BMI.

The live horizontal scale places the unrounded BMI against the adult boundaries. Its visual range is 10–40, with markers at 18.5, 25 and 30. A result outside that drawing range keeps its actual number and category; the marker stays at the relevant edge with an explanation.

The text labels and the category table provide the same interpretation without relying on color. Segment lengths reflect the numeric distance between boundaries. For someone under 20, this adult scale is hidden because BMI-for-age assessment is required instead.

The adult category named Healthy Weight covers BMI values from 18.5 inclusive to 25 exclusive. It is a population screening reference, not a guarantee of health or an individual treatment target.

At a particular height, the corresponding mathematical weight interval is 18.5 × height in meters² through below 25 × height in meters². At 1.75 m this is 56.65625 kg to below 76.5625 kg, displayed as 56.66 to below 76.56 kg. Those displayed bounds are rounded; the full-precision bounds define the interval.

BMI Prime simply divides BMI by 25. For a BMI of 22.857142857…, it is approximately 0.9143. It is another mathematical expression of the same inputs and does not add a separate medical assessment.

At a fixed height, BMI changes in direct proportion to weight. At a fixed weight, it changes with the inverse square of height. A height entry error therefore affects the calculation twice through the squared term.

Measure height consistently and check that the chosen units match your measurements. When entering feet and inches, 5 ft 9 in is not 5.9 feet: the latter is 5 ft 10.8 in. Ordinary measurement variation can move a result near a boundary; displaying more decimals does not make the measurements more precise.

BMI uses total weight, including muscle, fat, bone and other tissues. Two people of the same height and weight have the same BMI even when their body compositions differ substantially.

Body fat percentage addresses a different quantity: the fraction of total mass estimated to be fat. It requires additional measurements or assessment methods, each with its own limitations. A BMI value cannot by itself establish a body fat percentage.

High muscle mass can raise BMI without BMI distinguishing muscle from fat. Pregnancy changes the meaning of current body weight, while age-related changes, fluid retention and other medical circumstances can make a simple height-and-weight screen less informative.

Children and teens need age- and sex-specific growth assessment. Adult categories are not pediatric percentiles. This tool deliberately provides no invented child percentile or adult weight interval for an entry under 20.

Use the result as information to discuss in context. A professional assessment can incorporate medical history, physical findings and other relevant measures; BMI alone cannot diagnose a condition or decide whether a weight change is appropriate.

Method: kg ÷ m² with exact defined imperial conversions; two decimal places for displayed BMI, full precision for classification and interval calculations. Entries are calculated in browser memory and are not sent to a measurement service.

• CDC: adult BMI categories and screening limitations
• CDC: adult calculator and ages 20+ scope
• NIST: defined measurement conversions