Before you choose your new pre-hung door, it's important you know the size of the opening. Getting the right size door for the opening helps to make your installation smooth and will eliminate having problems down the road.
Pre-Hung Door Measurements
Measure the opening to your door frame. Be sure to allow for about a 1½ inch larger opening than the standard door sizes. Typically a pre-hung door is measured in 30-inch and 36-inch sizes with additional sizes available in non-standard widths.
The doors are also a standard 80 inches tall. Presuming the door is a 36-inch wide door, the dimensions of your opening should be 37½ inch wide by 81½ inch tall.
Reasons for Door Opening Sizing
When a pre-hung door is being installed into an opening there have to be considerations for leveling the door, and ensuring that it has an obstruction free swing. If the pre-hung door is butted up into an opening, the swing will be hindered by the door. It may scrape the floor or it may get stuck as it is closing.
An opening that is the same size as the frame on the pre-hung door can alter the shape of the frame and make the installation more difficult.
Taking the guesswork out of choosing the correct size pre-hung door makes things much easier. Now that you have the know-how, you're all set to head to the hardware store.