How to Measure for Interior Doors

If your old doors are no longer functioning properly or you simply need to update the look of your home, consider replacing these existing units with new interior doors. You can leave the frame as is, and simply add a new door. Another option is to remove both the door and frame and replace them with a pre-hung unit. Pre-hung units are generally easier to install, but may be more expensive than just buying a door. Before making your purchase, it's important to take careful measurements for your doors to ensure they will operate as intended.

Measuring for Doors Only

Measure the width of the existing frame. You may wish to take measurements at both the top and bottom of the frame in case the unit is not square. If you come up with different measurements, use the smaller of the two.

Take the width of the frame that you calculated in Step 1 and subtract 1/8 inch. This figure will represent the width of your new door. The 1/8 inch provides clearance so the door can swing in the opening.

Calculate the height of the opening using a tape measure. Measure from the finished floor surface to the underside of the frame.

Subtract 1/4 inch from the measurement you determined in Step 3. This will give you the height for your new door.

Determine the location of the hinges and lock strike on the existing frame, using the tape measure. Measure down from the top of the frame to the top of each hinge. To locate the strike, measure from the top of the frame to the centerline of the strike plate. Give the door manufacturer this information so he can properly prepare your door for installation.

Measuring for Pre-Hung Doors

Calculate the width of the rough opening. Measure the distance between the drywall or plaster finish on either side of the opening to determine the width of your pre-hung unit. Measure from the top of the opening to the finished floor to determine the height.

Size the unit so that it can be fit in the opening. Subtract 1 inch from the height and 2 inches from the width before making your purchase. This space will provide clearance for installing the unit, and will be concealed by trim or casing once the door is installed.

Measure the wall thickness. The frame of your pre-hung unit should be the same width as the wall on either side. For example, a 4-inch-thick wall will require a pre-hung unit with a 4-inch wide frame.

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