- 1). Use the fact that one inch equals 25.4 millimeters to convert a wrench measurement in millimeters to its decimal equivalent in Imperial wrench sizes. Divide the number of millimeters by 25.4 to convert to inches. For example, if you have an 8 mm wrench, 8/25.4 = is equal to 0.315 inches. Your Imperial, or SAE, wrench should be 0.31496 inches.
- 2). Convert the decimal value of your wrench to a fractional size by multiplying the decimal value by 8, 16 and 32 to find the closest match in standard size wrenches. Following the example already started, 0.31496 multiplied by 8, 16 and 32 yields 2.519, 5.039 and 10.078, respectively. The closest fit of this decimal value is 5/16. The difference using 5/16, as measured using 16, is only 0.039, which is closer than the difference using 11/32, measured using 32, which is 0.078.
- 3). Generate the standard size based on the closest fit. Standard sizes for wrenches are in 8ths, 16ths and 32nds of inches. For the example used, 5/16, whose decimal equivalent is 0.3125, is a closer fit to the target of 0.31496 inches than 11/32, decimal equivalent is 0.34375. The difference between the decimal equivalence of an 8 mm wrench and the two standard sizes 5/16 and 11/32 are 0.00246 and 0.02879. Of the most common standard sizes in use, a 5/16-inch wrench would be the closest fit and should work for your needs without being too large.
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