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  4. How to Get a Rough Measurement for a New Door

How to Get a Rough Measurement for a New Door

Replacing your exterior door unit brings a fresh new look to your home. Proper measuring is important to ensure that the new unit will fit in the space where the old unit is removed.

The most accurate way to measure for a replacement door is with the interior casing removed. Many homeowners are uncomfortable removing the casing but with the following method, you will get a good understanding of the unit size with the casing attached.

Standing on the inside of your home, measure from the inside of the left jamb to the inside of the right jamb to find the width of the unit.

To this measurement, you will need to add 1-1/2” to account for the thickness of the jambs. Most jambs on the interior side are 3/4″ thick, and in this measurement we need to account for two jambs.

Now measure from the bottom of the sill to the inside of the head jamb to find height of the unit.

Because this is on the outside of the unit, we need to add 1-1/4” to account for the single head jamb. If we were measuring on the inside of the unit, we would add 3/4” to the measurement.


Finally, open the door and measure the jamb depth from the inside edge of the interior casing to the inside edge of the exterior Brickmould.

This method of determining the unit size will help to ensure the new unit will easily replace your current unit. The downfall to this method is that you don’t know for sure the exact size between the casing and the jamb. It is preferable to remove the interior casing to learn the distance between the door unit and the edge of the rough opening.

Updated on May 15, 2021

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