Feet to Inches

The US version of the body mass index uses pounds and total inches: BMI equals 703 times weight in pounds divided by height in inches squared. That means a height like 5 ft 9 in has to become 69 in before it goes into the formula. For a 160 lb person, 703 × 160 ÷ 69² gives a BMI of 23.6, the same as the metric result for 72.6 kg and 1.753 m.

Doctors, growth charts and pediatric dosing tables in the US also record height in total inches, so 4 ft 2 in becomes 50 in on the chart.

Carriers write size limits in inches even when people think of the item in feet. A typical US carry-on limit of 22 × 14 × 9 in is roughly 1.8 × 1.2 × 0.75 ft, and a 3-foot box is 36 in long. UPS caps packages at 108 in long, which is 9 ft, and 165 in for length plus girth combined, so a 6-foot box with a girth of 8 ft (72 + 96 = 168 in) is over the limit even though each number sounds modest.

Home projects follow the same pattern. A tape measure is marked in inches with foot markers every 12, so a 7 ft door opening reads as 84 in, and a 10 ft wall is 120 in for anyone laying out tiles or studs at 16-inch spacing.

A measurement such as 5.5 ft means five and a half feet, which is 66 in, not 55 in or 5 ft 5 in. Multiply the whole value by 12 and the decimal takes care of itself: 5.5 × 12 = 66. Since the foot is exactly 30.48 cm and the inch 2.54 cm under the 1959 agreement, the factor 12 is exact.

To convert back, see inches to feet, or switch to centimeters in the unit converter.