Convert from Inches to Millimeters

• 1). Measure the diameter of a nut with the ruler. Make a note of the measurement, such as 3/8 of an inch.
  
  
• 2). 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.
  
  
• 3). Round 9.5525 up to the next whole number which is 10 millimeters (mm). This is the size of a 3/8-inch nut when it its dimension is converted to metric.
  
  

Convert from Millimeters to Inches

• 1). Measure the diameter of a nut with the metric ruler. Make a note of the measurement, such as 7 mm.
  
  
• 2). Multiply 7 mm by 25.4 for a result of 0.275. Write 0.27 as a fraction. This will look like 27/100.
  
  
• 3). 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.