Five steps to trim the bottom of a wood exterior door in order to get it to the right length.  With the door resting on sawhorses:

1. Accurately measure up from the bottom of the door with a metal ruler, preferably a carpenters’ square, not a measuring tape. Tape ends move. Do it again. Measure twice at least. This measurement will identify where you need to do step 2.

2. Fully mask the area of the cut line with blue tape. Include the area that your saw base will travel over (or tape the cut line area, and separately tape the saw base to prevent it from marring the door). The tape on the cut line is to help prevent splintering, particularly at the door stile sides where you will crosscut, not rip. While most saw blade blowout will occur at the exit side of the cut, I like to tape both sides of the door for good measure.

Note: Depending on the type of threshold, it may be necessary to bevel the cut. Usually a slight bevel, with the outside of the door lower than the inside is helpful to keeping the weather out and makes for a tight fit against the threshold seal strip. 3-5 degree bevel is sufficient.

3. Repeat step 1 – marking the masked tape. Using a metal or otherwise crisp straightedge and a utility knife, score the cutline once it is marked out on the taped door. Using a utility knife to mark the cut line will assist with minimizing the splintering by relieving the surface tension of wood fibers.

4. Clamp a straightedge to use as a guide for the saw at the appropriate offset from the saw blade cut line. It is usually about 5.5”, but check against your circular saw. Make certain you have allowed for the saw kerf width to the correct side of the cut line. I like to use 60 tooth blade for door cuts, but depending on the door finish, 40 tooth or 80 tooth may be fine/needed. Rockler has a saw blade 101 to review blade types.

5. Once setup, pucker up, goggle up, and make the cut……. Sand all edges to finish and breathe…..

Good luck! Hope all goes well……