Imperial BMI Formula: Pounds and Inches Explained for US Readers

United States customary units used in BMI calculation

In the United States, height is commonly recorded in feet and inches and weight in pounds, so a BMI calculation has to work from those units rather than from meters and kilograms. The U.S. customary version of the formula takes weight in pounds and height in total inches, then applies a conversion factor so the result lands on the same scale as the metric BMI.

The Centers for Disease Control and Prevention states the U.S. customary formula as weight in pounds divided by height in inches squared, multiplied by 703. The American Cancer Society gives the same relationship in a slightly different order: weight in pounds times 703, divided by height in inches squared. Both produce the same number.

Only two measurements are needed: a weight in pounds and a height converted to total inches. No separate metric step is required.

• Weight: pounds (lb); ounces are converted at 16 ounces to 1 pound
• Height: total inches, not feet alone and not feet plus a separate inches figure
• Conversion factor: 703, which aligns pound-and-inch inputs with the metric BMI scale

Sources: Centers for Disease Control and Prevention, American Cancer Society

Converting feet and inches into total inches

The most common imperial mistake is entering a height in feet without converting the remaining inches. The formula expects one number for height, expressed in total inches. To get there, multiply the feet by 12 and add any additional inches.

For example, 5 feet 3 inches becomes 5 times 12 plus 3, which is 63 inches. A height of 6 feet becomes 6 times 12, which is 72 inches. The American Cancer Society uses exactly this conversion in its worked example, and CalculatorSoup describes the same feet-times-12-plus-inches step.

Once the height is in total inches, the formula uses that single figure twice: height times height gives the squared value that appears in the denominator.

1. Write the height in feet and inches, for example 5 ft 10 in.
2. Multiply the feet by 12 to convert them to inches: 5 × 12 = 60.
3. Add the remaining inches: 60 + 10 = 70 inches total.
4. Use 70 inches as the height value in the imperial BMI formula.

Sources: American Cancer Society, CalculatorSoup, RegisteredNurseRN

Applying the 703 factor

The 703 factor is a unit conversion constant. It adjusts a result calculated from pounds and inches so that it matches the metric BMI scale, where BMI is weight in kilograms divided by height in meters squared. Without the factor, dividing pounds by inches squared would give a number that does not correspond to the familiar BMI categories.

In practice, the factor can be applied in either of two equivalent ways: multiply the weight by 703 before dividing, or divide weight by inches squared and then multiply by 703. The order does not change the answer.

The imperial formula is an adaptation of the metric formula. It is not a different measurement of body composition; it is the same index expressed for U.S. customary inputs.

• Metric base formula: weight in kilograms divided by height in meters squared
• Imperial adaptation: weight in pounds times 703, divided by height in inches squared
• The factor exists because pounds and inches are not the metric units used to define the BMI scale

Sources: Centers for Disease Control and Prevention, American Cancer Society, The Calculator Site, Imperial Calculator, CalculateMyBMI

Worked imperial examples at three heights

The examples below use the imperial formula with pounds and total inches. Each one shows the intermediate arithmetic so the result can be checked step by step. The first comes from the American Cancer Society, the second follows the steps in an educational nursing video, and the third uses an example from a secondary calculator resource.

1. Example 1: 120 lb at 5 ft 3 in. Convert height: 5 × 12 + 3 = 63 inches. Square the height: 63 × 63 = 3,969. Multiply weight by 703: 120 × 703 = 84,360. Divide: 84,360 ÷ 3,969 = 21.3. This falls in the CDC healthy weight range.
2. Example 2: 167 lb at 6 ft 0 in. Convert height: 6 × 12 = 72 inches. Square the height: 72 × 72 = 5,184. Multiply weight by 703: 167 × 703 = 117,401. Divide: 117,401 ÷ 5,184 ≈ 22.6. This also falls in the CDC healthy weight range.
3. Example 3: 190 lb at 5 ft 10 in. Convert height: 5 × 12 + 10 = 70 inches. Square the height: 70 × 70 = 4,900. Multiply weight by 703: 190 × 703 = 133,570. Divide: 133,570 ÷ 4,900 ≈ 27.3. This falls in the CDC overweight range.

Sources: American Cancer Society, RegisteredNurseRN, Calculatio, Centers for Disease Control and Prevention

Common imperial conversion mistakes

Most errors in imperial BMI calculations come from unit handling rather than from arithmetic. A few simple checks catch nearly all of them before the formula is applied.

The CDC adult calculator accepts heights from 3 feet (36 inches) to 9 feet (108 inches), with the inches field between 0 and 11, and weights from 55 pounds to 1,000 pounds. Heights and weights outside those ranges should be treated as a sign that a value was entered incorrectly.

• Entering feet instead of total inches: 5 ft 10 in must become 70 inches, not 5.10 or 5.83.
• Forgetting to square the total inches: the denominator is height × height, not height alone.
• Mixing units: pounds must pair with inches, and the 703 factor only applies to that pair.
• Rounding too early: keep the full intermediate numbers and round only the final BMI if desired.
• Using an inches value of 12 or more: remaining inches should be between 0 and 11.

Sources: Centers for Disease Control and Prevention, CalculatorSoup

Cross-checking an imperial result against a category range

Once a BMI number is calculated, it can be checked against the CDC adult category ranges. For adults age 20 and older, the CDC classifies BMI below 18.5 as underweight, 18.5 to 24.9 as healthy weight, 25.0 to 29.9 as overweight, and 30.0 or greater as obesity. Obesity is further divided into Class 1 (30.0 to 34.9), Class 2 (35.0 to 39.9), and Class 3 or severe obesity (40.0 or greater).

A second style of cross-check works backward from the category to a weight range, which can be useful when thinking about a specific height. The American Cancer Society gives two adult examples: at 5 ft 10 in, overweight corresponds roughly to 174 to 202 pounds and obesity to 209 pounds or more; at 5 ft 4 in, overweight corresponds roughly to 145 to 169 pounds and obesity to 174 pounds or more.

BMI is a screening measure and is not intended to diagnose disease or illness. It can be less accurate for people with greater muscle mass, such as athletes, and for older people and others who have lost muscle mass.

Sources: Centers for Disease Control and Prevention, American Cancer Society

Adults, children and the limits of the imperial result

The same imperial formula is used for adults, children and teens, but the interpretation differs by age. For children and teens, BMI is read against age- and sex-specific percentile charts rather than the adult category ranges shown above. The CDC notes that its calculators use the same formulas, while interpretation for younger people relies on growth chart percentiles.

For adults, the result is a screening number. The CDC describes BMI as a screening measure, not a diagnosis, and the American Cancer Society notes that it can be inaccurate for people with greater muscle mass and for older adults or others who have lost muscle mass.

This page explains the formula and the arithmetic. It does not offer personalized medical advice, and a BMI result should not be treated as a diagnosis. Anyone with concerns about their weight or health should discuss them with a qualified health professional.

Sources: Centers for Disease Control and Prevention, American Cancer Society