# Easy SAE to Metric socket size conversion

There are approximately 25.6 millimeters per inch.

What’s that? You say there are only *25.4* millimeters per inch?

You’d be right in saying that 25.4 is a *closer* approximation, but 25.6
is more *useful*, because it leads to this rule of thumb:

There are 256 millimeters in 10 inches.

Since 256 is a power of 2, this makes it really easy to convert between metric socket sizes (which are in millimeters) to SAE socket sizes (which are in power-of-2 fractions of an inch).

Consider a 1⁄2” battery terminal. If there are 256mm/10in, then there are 128mm/in, or 12.8mm per inch. So a 13mm socket will be ~.2mm larger than the bolt head.

I find going the other way a little trickier, but not too bad. For the 13mm socket, I remember that 1mm is 10*1⁄256”, so 13mm is 130⁄256”. The closest power of two to 130 is 128, and 128⁄256” is 1⁄2”. So a 13mm socket is just 2⁄256” bigger than a 1⁄2”.

Why is this useful? Sometimes it useful to use a socket size that’s close rather than an exact match. A battery terminal is usually a 1⁄2” socket, but what if you’re on the side of the road and you only have metric sockets on hand? Grab a 13mm and it will be a close enough fit. Or what if your 13mm bolt head is rusted? Chances are it’s not a 13mm head any more, and a 1⁄2” socket will be a tighter fit.

Here’s a full conversion table for common socket types:

SAE | Metric | Metric | SAE | |
---|---|---|---|---|

1⁄8 | 3.2mm | 8mm | 5⁄16 | |

3⁄16 | 4.8mm | 9mm | 11⁄32 - 3⁄8 | |

1⁄4 | 6.4mm | 10mm | 3⁄8 - 13⁄32 | |

5⁄16 | 8mm | 11mm | 13⁄32 - 7⁄16 | |

3⁄8 | 9.6mm | 12mm | 15⁄32 | |

7⁄16 | 11.2mm | 13mm | 1⁄2 - 17⁄32 | |

1⁄2 | 12.8mm | 14mm | 17⁄32 - 9⁄16 | |

9⁄16 | 14.4mm | 15mm | 9⁄16 - 19⁄32 | |

5⁄8 | 16mm | 16mm | 5⁄8 | |

11⁄16 | 17.6mm | 17mm | 21⁄32 - 11⁄16 | |

3⁄4 | 19.2mm | 18mm | 11⁄16 - 23⁄32 | |

13⁄16 | 20.8mm | 19mm | 23⁄32 - 3⁄4 | |

7⁄8 | 22.4mm | 21mm | 13⁄16 - 27⁄32 | |

15⁄16 | 24mm | 23mm | 7⁄8 - 15⁄16 | |

1 | 25.6mm | 25mm | 15⁄16 - 1 |

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