How to Convert a Metric Wrench to a Standard Wrench

Learn to convert between metric-size wrenches and Imperial-size wrenches.

With the exception of the United States, the majority of the nations of the world had adopted the metric system by the 1960's. Most manufacturers in the United States still use the Imperial system for measurements. Historically, Imperial units of measure became even more entrenched after their adoption by the Society of Automotive Engineers (SAE). The metric system uses millimeters for units of length that are less then approximately two centimeters, whereas the Imperial system measures in fractions of an inch. Make the conversion from metric units to Imperial units when you need to find a wrench in a specific Imperial size and all you have are metric wrenches.

Difficulty:

Moderately Easy

Instructions

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.

References

• Vaughn's Summaries: Wrench Conversion -- Size Equivalence Chart
• The Engineering Toolbox: Conversion Chart -- Inches to MM