A beginner woodworker would (rationally) assume that a 2×4 board is 2 inches by 4 inches, and base resulting plans or measurements on that. As much sense as it might make, unfortunately that’s not the case. The dimensions (called Nominal Dimensions) are actually the size of the board before it is dried and cleaned up. What’s left after that is smaller in both directions. Here’s a handy little cheat sheet!

Nominal | Actual | Actual – Metric |
---|---|---|

1″ x 2″ | 3/4″ x 1-1/2″ | 19 x 38 mm |

1″ x 3″ | 3/4″ x 2-1/2″ | 19 x 64 mm |

1″ x 4″ | 3/4″ x 3-1/2″ | 19 x 89 mm |

1″ x 5″ | 3/4″ x 4-1/2″ | 19 x 114 mm |

1″ x 6″ | 3/4″ x 5-1/2″ | 19 x 140 mm |

1″ x 7″ | 3/4″ x 6-1/4″ | 19 x 159 mm |

1″ x 8″ | 3/4″ x 7-1/4″ | 19 x 184 mm |

1″ x 10″ | 3/4″ x 9-1/4″ | 19 x 235 mm |

1″ x 12″ | 3/4″ x 11-1/4″ | 19 x 286 mm |

1-1/4″ x 4″ | 1″ x 3-1/2″ | 25 x 89 mm |

1-1/4″ x 6″ | 1″ x 5-1/2″ | 25 x 140 mm |

1-1/4″ x 8″ | 1″ x 7-1/4″ | 25 x 184 mm |

1-1/4″ x 10″ | 1″ x 9-1/4″ | 25 x 235 mm |

1-1/4″ x 12″ | 1″ x 11-1/4″ | 25 x 286 mm |

1-1/2″ x 4″ | 1-1/4″ x 3-1/2″ | 32 x 89 mm |

1-1/2″ x 6″ | 1-1/4″ x 5-1/2″ | 32 x 140 mm |

1-1/2″ x 8″ | 1-1/4″ x 7-1/4″ | 32 x 184 mm |

1-1/2″ x 10″ | 1-1/4″ x 9-1/4″ | 32 x 235 mm |

1-1/2″ x 12″ | 1-1/4″ x 11-1/4″ | 32 x 286 mm |

2″ x 2″ | 1-1/2″ x 1-1/2″ | 38 x 38 mm |

2″ x 4″ | 1-1/2″ x 3-1/2″ | 38 x 89 mm |

2″ x 6″ | 1-1/2″ x 5-1/2″ | 38 x 140 mm |

2″ x 8″ | 1-1/2″ x 7-1/4″ | 38 x 184 mm |

2″ x 10″ | 1-1/2″ x 9-1/4″ | 38 x 235 mm |

2″ x 12″ | 1-1/2″ x 11-1/4″ | 38 x 286 mm |

3″ x 6″ | 2-1/2″ x 5-1/2″ | 64 x 140 mm |

4″ x 4″ | 3-1/2″ x 3-1/2″ | 89 x 89 mm |

4″ x 6″ | 3-1/2″ x 5-1/2″ | 89 x 140 mm |