When ordering pre-hung doors, measuring your door jamb correctly is essential. If you are keeping your existing frames, then door slabs only should be ordered. If you are looking to replace your door frames with new door frames, it is essential to understand how door frames are measured. Follow the tips below to help you understand the various parts of the frame, wall, and how to measure correctly. This may seem like a daunting task, but it is a very simple and fast process.

How is the door jamb measured? It is the measurement of the wall thickness that your new door will be filling in. This includes any sheetrock (drywall) and siding that is on the inside or outside as well. In simple terms, look at your doorway. See the frame around the opening where the door closes? Where does the door latch lock in place by the door handle? That is the door jamb. Make sure not to include any decorative door casing our any other door molding in the measurements.
When selecting a door replacement, this is one of the most important things to do, if you purchase a door with a jamb too small than there will be an opening where the frame doesn’t cover the gaps completely. If it is too large, then the door frame will need to be trimmed to fit in the framing of your home. Not having a jamb of the correct size will lead to the door being unable to close or have a tight seal and can cause the elements to more easily enter your home and create drafts or openings for insects to enter.

Step one: Measure the Jamb

You will need a tape measure and know how to read it. The picture below will help you if you are unsure of what line correlate with which measurement. You must know how to read the tape measure to get eh correct size properly. In measurements (“) signifies inches while (‘) signifies feet.
Where the red arrow points are 1 foot and 3/8 inches, or 12 and 3/8 inches, the black arrow is 13 and ¾ inches.

measure door jam

Now measure the frame, not including any trim on the wall.

measure door jam

Step two: Measure your casing

Measure from the door frame to where the drywall starts, this is your casing, the measurement of the size trim you will put around the door. You will need to measure this on the outside and the inside, and they may be different measurements. Generally, two to three-inch molding is all you need as sometimes there is no gap to cover, or the difference is very small. The molding includes the door frame on the sides. In the image below, notice the gap indicated by the red box with a yellow border. This is the gap you are covering with casing.

measure jam casing

Now you have the proper measurements, and you can order the correct jambs for your door. If you are still not confident with measurements, consider hiring a door professional to assist with the measurements. The door installer is best suited to help and provide you with these measurements.