Getting the numbers right is the whole game. Follow these three steps and you'll order a door that drops straight into your opening — no surprises at install.
For a prehung system, measure the framed hole in the wall — width and height. Take three readings each and use the smallest. Allow about 2"–2½" over the slab size for shimming.
Replacing just the door? Measure the existing slab — most 36" steel and fiberglass slabs are about 35¾" wide and a 6'8" slab about 79" tall. Match the hinge hand and the bore positions.
Standard entry doors are 1¾" thick — confirm your jamb depth matches the wall. Note the swing direction (left or right hand) so the lock and hinges land on the correct side.
| Nominal | Actual slab | Rough opening* |
|---|---|---|
| 2'8" × 6'8" | 31¾" × 79" | ~34" × 82¼" |
| 3'0" × 6'8" | 35¾" × 79" | ~38" × 82¼" |
| 3'0" × 8'0" | 35¾" × 95" | ~38" × 98¼" |
| 3'6" × 8'0" | 41¾" × 95" | ~44" × 98¼" |
| Double 6'0" × 8'0" | 71½" × 95" | ~74" × 98¼" |
*Rough opening allows for jamb and shim space on a prehung system. Confirm against your supplier's frame.
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