How this calculator works
SAE sizes are inch-based wrench and fastener sizes, named as fractions of an inch and associated with the Society of Automotive Engineers. Metric sizes are given in millimetres. The exact conversion uses 1 in = 25.4 mm, so a 13 mm wrench is 0.511811 in, close to but not the same as a 1/2 in wrench at 12.7 mm. The nearest standard size is informational: a slightly mismatched wrench can round a soft fastener or slip under load, so use the size the manufacturer specifies.
Formula
inches = millimetres / 25.4. millimetres = inches x 25.4. The nearest SAE size is the fraction from 1/8 in to 1 in whose millimetre equivalent is closest to the entered metric size. The nearest metric size is chosen from the common 5 mm to 25 mm series.
Worked example
13 mm = 0.511811 in, nearest to 1/2 in at 12.7 mm, 0.3 mm smaller. 10 mm = 0.393701 in, nearest to 13/32 in at 10.31875 mm, about 0.32 mm larger. 19 mm = 0.748031 in, nearest to 3/4 in at 19.05 mm. A 1/2 in wrench is 12.7 mm.
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Frequently asked questions
Can I use a 13 mm wrench on a 1/2 inch bolt?
They are close, but not identical: 13 mm is 0.511811 in against 0.5 in, a difference of 0.3 mm. On a tight or soft fastener the extra clearance can cause slipping or rounding. Use the specified size when you can.
What does SAE mean for tool sizes?
SAE refers to the Society of Automotive Engineers, whose standards popularised inch-based fastener sizes in North America. In tool catalogues SAE simply means the inch series, as opposed to metric.
Why are SAE sizes written as fractions?
The inch system divides the inch into halves, quarters, eighths, sixteenths and thirty-seconds. A size such as 9/16 in is exactly 0.5625 in, which converts to 14.2875 mm.
Is there a metric size exactly equal to an inch size?
Only where the fraction lands on a whole or simple decimal millimetre value. For example, 1 in is exactly 25.4 mm, but 1/2 in is 12.7 mm, which is not a standard metric wrench size. Most pairs are close rather than equal.