Nuts that are used to secure bolts come in two distinct measurements: metric (millimeters) and imperial measure (Inches). The size of the nut is determined by the diameter of the hole. You can easily convert the dimensions of a nut from metric to inches or from inches to metric. This allows you to substitute a metric nut for an English nut when a replacement nut is needed for a project.
Things You'll Need
 Metric ruler
Convert from Inches to Millimeters

Measure the diameter of a nut with the ruler. Make a note of the measurement, such as 3/8 of an inch.

Divide 3 by 8 for a result of 0.375. Multiply 0.375 by 25.4 (the number of millimeters in an inch) for a result of 9.525.

Round 9.5525 up to the next whole number which is 10 millimeters (mm). This is the size of a 3/8inch nut when it its dimension is converted to metric.
Convert from Millimeters to Inches

Measure the diameter of a nut with the metric ruler. Make a note of the measurement, such as 7 mm.

Multiply 7 mm by 25.4 for a result of 0.275. Write 0.27 as a fraction. This will look like 27/100.

Reduce the number 27 to 25 which will divide into 100. Divide 25 by itself for a result of 1. Divide 100 by 25 for a result of 4. The resulting nut size is 1/4 of an inch.
References
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