How To Measure Your Door Opening

Start with the opening, not the old label

For a custom prehung door, the job is not really about what the last door was called. It is about what the opening needs now. That means width, height, jamb depth, and handing all need to be checked before the order is finalized.

What to measure

• Measure the opening width in multiple places and use the smallest reliable number.
• Measure the opening height from the finished floor to the head of the opening.
• Measure jamb depth so the prehung frame matches the wall condition.
• Confirm whether the door swings left or right and whether it is inswing or outswing.
• Check whether the project is interior or exterior before choosing slab thickness and jamb range.

Width and height mistakes to avoid

Do not assume the old slab size is automatically the right order size. Remodel work, floor changes, patched jambs, and older homes can all shift the real opening away from what the previous door suggests. If your measurement lands between common sizes, that is usually a sign to use quarter-inch custom sizing rather than forcing the project into the wrong standard size.

Jamb depth matters on prehung doors

A prehung order includes the frame, so jamb depth needs to match the wall condition closely enough that installation is clean. If you are not sure how to measure that, read the jamb depth guide before placing the order.

What to do after measuring

If the opening lands on a clean standard size, start from the closest size page in the catalog. If it lands slightly off, go into the builder and switch to custom quarter-inch sizing. For the most common edge cases, the odd-size door guide explains when that custom path becomes the safer choice.