How to install an interior door: rough opening, jambs, shims and casing

Should you buy a prehung door or a slab?

Buy a prehung door unless the old jamb is straight, square and fit the old door well. The $54 extra buys the hinge mortises and a frame already square to the door.

A prehung door is the slab already hung on hinges in its own frame (the jambs), with the stop attached. You set the whole frame into the rough opening, which is the framed hole in the wall. A slab is the door alone, and you hang it in a jamb that is already there.

At Menards on September 19, 2026, a Mastercraft 30 by 80 inch primed flush slab was $65.00 and the same style prehung was $119.00. The $54 difference buys three hinge mortises cut at the right heights, a bored lockset hole, a frame that is already square to the door, and the stop set to the slab. Buy the slab only when the old jamb is straight and square and the old door fit it well; then you copy the old hinge positions onto the new door. For everything else, including "how to install door jambs," the answer is a prehung unit, because the jambs come with the door.

What size rough opening does an interior door need?

The rough opening should be 1/2 inch wider and taller than the door frame; for a 30-inch door, Masonite's chart calls for 32 by 82 inches. Measure before you buy. Take the casing off one side of the old door, pull the old frame or look at the framing with a light, and take four measurements.

1. Width between the studs, at the top, middle and bottom. Use the smallest. Masonite's chart calls for 32 inches of rough opening for a unit 30 to 31-1/2 inches wide. JELD-WEN states the rule behind it: the frame should be 1/2 inch smaller than the opening in width and in height. Check: the smallest width is at least the frame width plus 1/2 inch.
2. Height from the finished floor to the underside of the header, on both sides. Masonite's chart uses 82 inches for an 81-1/2 inch unit. Check: the lower of the two readings still gives 1/2 inch over the frame.
3. Both diagonals, corner to corner. JELD-WEN allows 1/4 inch of difference at most. Check: the two diagonals are within 1/4 inch.
4. Wall thickness, from drywall face to drywall face. A 2x4 wall with 1/2-inch drywall on both sides is 3-1/2 + 1/2 + 1/2 = 4-1/2 inches. The jamb must match the wall, or the casing will not sit flat on both edges. Check: the jamb width printed on the door's label matches your wall, or you have a plan for a jamb extension or a plane.

Last, the handing. Masonite's definition: when the door opens toward you and the knob is on the left, it is a left-hand door; knob on the right, right-hand. Stand on the side the door swings toward and check before you buy. A wrong-handed prehung unit cannot be flipped.

Interior door materials, tools and what they cost

The declared case: one 30 by 80 inch primed flush hollow core prehung door, replacing an old door in an existing 32 by 82 inch rough opening in a 2x4 interior wall, cased on both sides.

The casing count: four legs, one per side of the opening on each face of the wall, each under 7 feet for an 80-inch door, plus one 7-foot stick cut into the two head pieces. Two packs of shims give 24, enough for the seven shim points on the jambs with pairs at each point and a few to spare. Wood filler, caulk, 4d nails for the casing's outer edge and paint are left out of the total because most people already have them.

Tools you cannot skip: a 4-foot level (JELD-WEN asks for at least 4 feet; a 2-foot level misses a bow in the jamb), a tape, a hammer and nail set, a utility knife, a combination square and a miter saw or miter box for the casing. Worth renting or borrowing: a finish nailer, which makes the casing faster and splits less MDF. Nice to have: a pry bar for the old trim and an oscillating saw to trim shims flush.

How to install a prehung interior door, step by step

1. Let the door acclimate. JELD-WEN asks for at least 48 hours in the room at normal temperature before hanging. Check: the door has been indoors, standing up, for two days.
2. Unpack and inspect. Pull every shipping clip, block and any screw that runs through the head into the slab. Measure the unit's diagonals; JELD-WEN rejects anything more than 1/4 inch out. Check: door swings freely in its frame on the floor, correct handing.
3. Remove the old door, casing and jamb. Cut the paint line along the casing with a utility knife first so the drywall paper does not tear. Check: bare studs, header and floor, no nails sticking out.
4. Check the floor under the jambs. Prehung jambs run long below the slab so they can be cut to your floor. If the floor is out of level across the opening, trim the higher side's jamb leg so the head ends up level. Leave the air gap under the door that your ventilation needs, as JELD-WEN notes. Check: with the unit standing in the opening, the head jamb reads level.
5. Set the unit and shim the hinge side first. Center it in the opening, face flush with the wall on both sides. JELD-WEN puts the first shims about 2 inches from the top and bottom of the hinge jamb; Masonite nails 6 inches from the top, 6 from the bottom and in the middle. Use shims in opposing pairs so the jamb face stays flat, and put one behind each hinge. Nail through the jamb and shims with 8d finish nails, heads left proud for now. Check: the 4-foot level reads plumb on the face and the edge of the hinge jamb, and a straightedge shows no bow.
6. Close the door and shim the latch side. Masonite places shims 6 inches from the top, 6 inches from the bottom and at the strike. Adjust until the gap between the door and the jamb is even from top to bottom. Check: the gap along the latch side is the same at the top, the middle and the bottom, and the door touches the stop evenly.
7. Shim and nail the head, then the rest. Shim the head jamb at its center. JELD-WEN then nails through the stop, jamb and shims every 12 to 16 inches. Check: the door opens and closes without rubbing, and stays where you leave it at half open.
8. Anchor the hinges. On solid core and heavier doors, JELD-WEN has you replace the center screw in the top two hinges with the long #9 x 2-1/2 inch screws it supplies, which reach the stud. Check: the gap at the top of the latch side did not change when you drove them.
9. Set nails, trim shims. Once the door is right, set the nail heads and score the shims flush with the jamb edge. Check: nothing proud of the jamb face, where the casing will land.
10. Case the door. Mark the reveal, the small setback between the jamb edge and the casing, with a combination square at a few points on each jamb. The exact number matters less than keeping it the same all the way around, and Masonite notes the casing sits back so the hinge pins can be pulled. Cut the head first, then the legs. Nail to the jamb edge and into the framing. Check: miters closed at both top corners, reveal even by eye.
11. Install the knob and fill. Follow the lockset instructions, fill the nail holes, caulk the casing to the wall and paint. JELD-WEN asks for all six sides of the slab to be finished, including the top and bottom edges. Check: the latch catches the strike with a light push, and the door does not rattle.

Door installation mistakes that cost money

Is it worth installing an interior door yourself?

For most people, yes: the gap between $173.01 in materials and $529 to $804 hired is $355.99 to $630.99 per door. Carpenters earned a median $29.12 an hour in May 2025, according to BLS. Wages are 69.5% of total compensation in private construction, so a loaded carpenter hour costs $41.90. Homewyse assumes a 2-hour minimum for one door, which is $83.80 of labor to the contractor. The rest of the $529 to $804 is the door (a mid-range six-panel in Homewyse's case, more expensive than the flush unit priced here), supplies, disposal, travel, overhead and profit.

Part of that gap is the cheaper door, so the fair comparison is somewhat smaller. It is still a good rate for an afternoon, and it gets better from the second door on, when you already own the level and the saw and you know the sequence. A house with six doors to replace is where the DIY math is strongest.

Hire when the opening is badly out of square, the wall is bearing and needs a new header, the door is the garage entry covered by R302.5.1, or the trim is stain-grade wood where a bad miter shows forever. For those, get two quotes and ask each to say how many hours they price per door. The contractor pricing guide explains how to read the markup in the answer.