In nearly all situations, measuring trim boards for mitre cuts can be accomplished with just a tape measure. However, some understanding of how to make the cut is also necessary, along with an accurate linear dimension for the piece of trim or moulding. Forty-five degree angled cuts are the order of the day for most mitre cuts, and once you understand how this little detail works, calculating the length can be completed with one measurement.
Pull a board from the stock and set the board face down on the sawhorses. Be sure to check the board for quality and straightness.
Take your speed square and make a square line across the width of the board at one end.
Set the angle of the circular saw blade to 45 degrees and make the crosscut with the angled cut pointing towards the midpoint of the board.
Measure the distance across the wall from inside corner to corner. Do this to the nearest 1/16th of an inch.
Starting from the angled cut, extend the tape measure and make a mark at the distance that was calculated in the previous step.
Take the speed square and extend the mark across the width of the board.
With your speed square, make a perpendicular line across the width of the board. Do this within a few inches of the end of the board.
Set the angle of the circular saw blade to 45 degrees and make the crosscut with the angled cut going away from the end of the board.
Make a measurement from the inside corner to the edge of the wall, where the outside mitre will occur.
Add the thickness of the board to the overall length of your measurement. For example, if your measurement is 24 inches and the board is 3/4 of an inch thick, your final numerical value will be 24 3/4 inches.
Measure the correct length of linear dimension and make a mark. Now your mark is at the right length, but on the wrong side of the board.
Extend the mark to the edge of the board with a speed square.
Flip the board over and extend the mark across the surface of the board. You can now make your final cut.
Cutting one end before transferring the measurement to the piece of trim is a good strategy. All measurements should be made to 1/16th of an inch. If you are using an electric or non-electric mitre box, you need to turn the board at a right angle to the bottom plane of the mitre box and set the saw at 45 degrees.
Using an assistant to hold one end of the tape measure will help ensure an accurate measurement. A large framing square can be used to check the squareness of each corner. If the corner is not square, you might have to adjust the angle of the cut.