How to Measure an Exterior Door in 4 Steps
When beginning to measure for your new exterior door, start by simply measuring the width and height of your old, existing door. Round these measurements up to full inches in order to more easily find the replacement door size.
Next, record your jamb measurements. Make sure when you are measuring these dimensions that you start to measure from the backside of the interior trim to the back side of the exterior trim. You'll need this jamb width when ordering your new pre-hung door. Why? This will ensure that the interior trim will fit nice and flush with the wall without the need to add extra jamb extensions.
It's important to know how much space you're working with, so measuring the rough opening is important. Make sure you remove the interior trim (gently) to get the most accurate measurements. Start with the opening width between the framing and then proceed by measuring from the top of the opening to the bottom of the sill. You'll cross-check these measurements with the rough opening requirements of your new door to ensure a proper fit.
Additional steps for sidelights: Measuring your rough opening also applies to measuring for a door with decorative sidelights. If you choose to leave existing sidelights in place, you will not need to include them in your rough opening measurements. However, if you do want to replace your sidelights, follow the same steps above, taking into account the length and width of your sidelights slated for replacement.
(Pro tip: Most standard, pre-hung options include two sidelights, measuring 10 inches each in width and varying to the length of the door or a shorter or longer custom length.)
The last step in measuring for your new entry door is to measure to the outer exterior casings and jot down the dimensions from the top of the trim to the bottom of the sill. Cross check these dimensions with those of a standard pre-hung door with 2-inch-wide molding trim.