In terms of both style and size, no one standard applies to all doors. While hollow-core flat-surface doors are commonly used inside the home, exterior doors often feature panels and windows. Door widths vary from 28 to 36 inches, and the two most common heights are 78 inches -- usually expressed as 6-foot-6 -- and 80 inches, which is 6-foot-8. The latter height is the more common size, and you'll have to cut the door to fit into a 6-foot-6 opening. To preserve the symmetry of the door, it's best to cut equal amounts from both ends.
Things You'll Need
3-inch masking tape
2 door hinges
Take a measurement of the distance from the top jamb to the floor with a tape measure. It might not be exactly 78 inches, especially if there is carpet on the floor. Measure the exact length of the door and subtract from it the measurement of the height of the opening.
Add an inch to this difference to allow for a 1/2-inch gap at the top and bottom of the door, then divide the number in two. This is the amount you need to cut from each end of the door.
Set the door on a pair of sawhorses and lay 3-inch masking tape along the top and bottom edges on both sides of the door -- that's four pieces of tape. The tape will prevent the wood from chipping when you cut the door with a saw.
Measure the amount you need to cut from the top of the door and draw a straight line along the tape on one side of the door with a straightedge and pencil. Make the same measurement from the bottom of the door and draw a similar line on the tape at the bottom on the same side of the door.
Cut along each line with a circular saw. When you've finished the cuts, remove the tape from both sides of the door and sand the edges with 150-grit sandpaper to smooth them and remove splinters.
Chisel mortises for the hinges at equal distances from the top and bottom of the door, using a hammer and chisel. Chisel similar mortises in the jamb, remembering to account for the gaps at the top and bottom of the door when measuring their distances from the top jamb and the floor.
Separate two door hinges by pulling the pins.Screw one half of each hinge to the door and half to the jamb. Hang the door by fitting the hinges together and tapping in the pins with a hammer.
To further minimize chipping when cutting the door, it's best to use a blade with a large number of teeth -- from 60 to 80. This is especially true if you're cutting a hollow-core door covered with plywood paneling.
Wear safety glasses when cutting with a circular saw.