To ensure proper installation and use, we recommend you seek the advice of a professional. However, you can use the following method to obtain a rough size estimate: Remove the interior casing and measure the width, then height, of the existing door frame. Measure the door slab. Measure the thickness of the wall to determine the jamb width needed. Determine the door swing you’ll need (e.g., in-swing or out-swing, left or right). Door swing refers to the direction the door swings, based on the location of the hinges as viewed from the outside. FAQ Friday is a weekly feature where we answer Frequently Asked Questions about our products, service or industry. To see other FAQs we’ve answered, visit our FAQ page. If you have a question you don’t see an answer for, just ask in a comment or contact us.