SAE (Society of Automotive Engineers) or "English" sized bolts, nuts and wrenches were the industry standard in most English speaking countries since 1905. In 1971, the U.S. Metric Study convinced Congress the U.S. should convert to metric measurements over the course of the next 10 years. In 1974, the Education Amendments of 1974 were passed to encourage the education system to teach metric measurements and conversions to U.S. students.
To make your conversions, you will need to know there are 25.4 millimeters in an inch.
Convert the fractional English value to its decimal equivalent. For example, 1/4 = 0.25.
Multiply .25 by 25.4 to arrive at 6.35 mm.
Select a 7 mm wrench since the wrench must be larger than the nut. The 7 mm wrench will be very loose on the 1/4-inch nut. Depending on the style of nut and wrench, your wrench may not fit tight enough tighten the nut onto the bolt.
Divide the metric nut value by 25.4. For example, 8 mm ÷ 25.4 = 0.3149.
Multiply that value by 16 (0.3149 x 16 = 5.039).
The English wrench size is 5/16 inches. The 8 mm nut will fit the 5/16-inch wrench very well.
