US Units of Length: Videos & Practice Problems

To convert feet to inches, you use the fact that 1 foot equals 12 inches. The conversion is done by multiplying the number of feet by the unit fraction $\frac{12}{1}$ inches per foot. For example, to convert 14 feet to inches, multiply 14 by 12: $14\times 12=168$. So, 14 feet equals 168 inches. This method uses unit fractions or conversion factors to cancel out the original unit and replace it with the new unit.

When adding mixed units such as feet and inches, first add the like units separately. For example, if you add 3 feet 5 inches and 9 feet 4 inches, add the inches: 5 + 4 = 9 inches, and add the feet: 3 + 9 = 12 feet. Since 9 inches is less than 12 inches (1 foot), no further conversion is needed. The final answer is 12 feet 9 inches. Always add or subtract like units first, then convert if necessary.

When subtracting mixed units such as yards and feet, and the smaller unit in the minuend is less than in the subtrahend, you need to borrow from the larger unit. For example, subtracting 2 yards 8 feet from 10 yards 5 feet requires borrowing 1 yard (which equals 3 feet) from the 10 yards, reducing it to 9 yards. Add the borrowed 3 feet to the 5 feet, making 8 feet. Now subtract: 8 feet - 8 feet = 0 feet, and 9 yards - 2 yards = 7 yards. The result is 7 yards. Borrowing involves converting the larger unit to the smaller unit before subtracting.

To multiply a mixed length measurement by a constant, multiply each unit separately by the constant. For example, multiplying 7 feet 6 inches by 4 means multiplying 7 feet by 4 and 6 inches by 4. This gives 28 feet and 24 inches. Since 24 inches is more than 12 inches, convert 24 inches to feet: $\frac{24}{12}=2$ feet. Add these 2 feet to 28 feet, resulting in 30 feet total. This method ensures the final answer is expressed in standard US units.

The US system of length uses these equivalent measures: 12 inches = 1 foot, 3 feet = 1 yard, 1760 yards = 1 mile, and 5280 feet = 1 mile. These equivalences allow conversions between units by using unit fractions or conversion factors. For example, to convert yards to feet, multiply by 3, or to convert miles to feet, multiply by 5280. Understanding these relationships is essential for converting and working with US units of length.