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Most modern, standard doors are 1 ⅜-inches thick, so you should aim for this thickness unless you're building a door to replace an antique door or fit into a non-standard wall opening. Thickness varies when installing a pocket door or sliding barn door, so a good rule of thumb is to measure your existing door, if possible. For standard doors, you can also measure from the inside of the door to the door stop to gauge the proper thickness.

A prehung door is a door that comes ready to install, already mounted in its frame with hinges and stop molding included. Because everything is factory-aligned, installation is quicker, more precise, and convenient, as you don't need to drill or align the hinges yourself. This makes it a smart option if your current frame is damaged, uneven, or if you're working on a new build. Installation involves fitting the entire unit into the rough opening, shimming and leveling it, and then securing it. If you are replacing an existing prehung door, the process is even simpler, often just requiring you to swap the old unit for the new one and secure it with a few screws. Prehung doors typically cost only slightly more (around $20) than a door slab alone, making them an efficient choice.

Most modern door knobs will all fit in standard, modern doors, so if you know your door was made in the last 50 years or so, you should be fine buying a standard replacement door knob. The only exception would be a thumb latch handleset for an exterior door, as this type of door knob may not fit your door without drilling a mounting hole for the bottom of the handle. Antique or custom doors might also have different sizes and knob mounting holes, so call in a professional if you don’t believe your door is a standard one.

You put shims between the three-sided frame to which the door hinges are attached and the framing of the rough wall opening. Using shims like this will center your door in the wall opening and allows you to make small adjustments to get your door plumb and level for smooth operation. Your shims and the gap they fill will be covered when you install door trim, so don’t worry too much about the appearance.