A dimension often appears in a maintenance manual or on a blueprint as a decimal instead of a fraction. To use the dimension, it may need to be converted to a fraction. An aviation mechanic frequently uses a steel rule that is calibrated in units of 1⁄64 of an inch. To change a decimal to the nearest equivalent common fraction, multiply the decimal by 64. The product of the decimal and 64 is the numerator of the fraction and 64 is the denominator. Reduce the fraction, if needed.

Example: The width of a hex head bolt is 0.3123 inches. Convert the decimal 0.3123 to a common fraction to decide which socket would be the best fit for the bolt head. First, multiply the 0.3123 decimal by 64:

Next, round the product to the nearest whole number:

Use this whole number (20) as the numerator and 64 as the denominator: 20⁄64.

Now, reduce 20⁄64 to 5⁄16 as 4 is common to both the numerator and denominator. Therefore, the correct socket would be the 5⁄16 inch socket (20⁄64 reduced).

Example: When accurate holes of uniform diameter are required for aircraft structures, they are first drilled approximately 1⁄64 inch undersized and then reamed to the final desired diameter. What size drill bit should be selected for the undersized hole if the final hole is reamed to a diameter of 0.763 inches? First, multiply the 0.763 decimal by 64.

Next, round the product to the nearest whole number:

Use this number (49) as the numerator and 64 as the denominator: 49⁄64 is the closest fraction to the final reaming diameter of 0.763 inches. To determine the drill size for the initial undersized hole, subtract 1⁄64 inch from the finished hole size.

Therefore, a 3⁄4-inch drill bit should be used for the initial undersized holes.

To convert any fraction to a decimal, simply divide the top number (numerator) by the bottom number (denominator). Every fraction has an approximate decimal equivalent.

Example:

0.375