The new door must be the same size as the old. It can even be slightly larger, but not smaller. Most standard entry doors are 3-0 by 6-8.

You'll want to match door thickness, and if you're getting one already bored, make sure the hinge cutouts, as well as the knob holes are the proper size and backset.

Same for the deadbolt hole, if there is one.

Most doors are standardized, so you shouldn't have any problems unless your old door is quite old or custom.

On your existing door, when you close the door and look at the hinges, each hinge has a hinge pin. Usually the hinge pins can be popped out, from bottom to top. If there is a decorative finial on the pin remove that, then use a hammer and a nailset os something similar to pop the pin out of the hinge knuckle, from bottom to top.

Pop the pins almost all the way out, but not completely. You still want the pin engaging the two knuckles. Open the door, then while supporting the door at the knob end, remove the pins completely.

Remove the door.

Place the new door in the opening, shim under the bottom to "lift" it off the threshold, and size it to fit for an even reveal all around. Match the reveal on the existing door.

If the door is predrilled for knobs/deadbolt, any triming to the width should come off the hinge side or you'll alter the predrilled knob backset. If you trim off the hinge side, you'll need to chisel the hinge mortises to replicate the depth.

The height should be okay, most doors are 6'8" tall. Some doors, even though listed at 3-0 in width, are slightly larger or smaller.

Once the door is sized, do any priming/painting/staining/etc to the door, don;t forget the bottom edge.

Remove the hinge knuckles from the old door. Screw the hinge knuckles to the door.

Place the door in the opening, engaging the knuckles, and slide the hinge pins in about halfway.

Close the door to match up the knob hole/and striker plate.

Set the hing pins fully, open the door, install the knob and deadbolt, and alter the height of the striker on the jamb if required.

Viola!

That's one way, off the top of my head. These days doors are pretty much standardized, so it should be fairly easy.

Best, Mongo

Get a pro! No disrespect, but hanging a door isn't a diy. Removing your existing jamb,trim and replacing with a pre-hung door would be easier, but more costly,and you have to re-trim, many times not matching the existing. I have replaced many doors to the existing jamb, and always consider it a challenge,as there are many factors to consider and things to check before placing the new door.Get a door hanger,(finish carpenter), and have them just do the hanging. You can do the rest as far as finishing, weatherstripping, lockset installs,etc.

Well, there's a diversity of opinion!

Could be easy, or not so. How old is the door, anyhow? What type/brand?

Since you have "no clue", you should get a pro. They could do either of the above, depending...

ok...so I have one vote for DIY and one not. I say I have no clue, but I do have a FIL who knows his stuff, and I'm sure he could help us out. The house was built in 1978 and I assume It's the original door. IT seems a pretty standard size to me. I assume the pre-drilled holes for the knob would line up with the existing plate?
What' s the difference with a "pre-hung" door?
If we did go with a pro, how much $$ are we talking about? And what type of pro should we look for?

You might be better off replacing the door and frame all at once. That would be what a prehung door is. You will get a better door with better weather stripping,sealing it against the weather. Also your present frame may have hidden problems such as rot or be out of plumb. There is a wide selection of prehung door units on the market from real wood to fiberglass to a metal door. If your FIL is handy with tools, It is a not too difficult job to do if you read up on before hand. I have replaced a total of 3 exterior doors on 2 houses and quite a few interior doors. Just educate yourself before hand before tackling the project.

We had our front door replaced and I grimaced at the estimate but went ahead. The guy who does this for a living worked three 10 hour days and I never regretted for one minute writing the check. I second the motion for a professional.

There is no way to see the existing door/frame assembly---so no way to come up with the correct advice. Is the door causing a problem, or is the frame causing a problem. If you replace the door, and the frame is bad, you still have a problem.

If there are no problems and you just want a new door, then that is relatively simple. You will need some tools for the job. Home improvement stores sell door holesaw kits that will drill the hardware holes for about $20. The mortices for the hinges---those are the cutouts in the door where the hinges set in----have to be cut. A very sharp chisel and hammer can cut those, or there are jigs sold at the HI stores and other places for using a router with a straight bit for making the mortices. Drill bits for predrilling the screw holes are a good idea---VIX bits center and drill screw holes. You may also need a hand/power plane or a saw to trim if the new door is larger.

Measure the old door---width and height. Get the new exterior door you like that is the closest but larger size. If either the width or height is less, the door will not fit properly.

Set up two saw horses. lay the new door on the saw horses. If it has an inside/outside, make sure the inside faces up.

Remove the old door, by removing the hinge pin, and hardware and lay it on the new door, aligning the lockset side and the top. Mark the holes for the lock set and also mark any part of the door on the hinge side or bottom that needs trimmed---using the old door as a pattern.

Trim off any of the new door that needs trimming.

Lay the new door back on the saw horses and the old door on top again. Align the top/sides/marked hardware hole and make sure the new door is now an exact copy.

Now mark the hinge mortices. That means top and bottom as well as how far the hinge is inset in the width of the old door. Make sure they are exactly the same as the old door.

Cut the mortices and install the hinge halves---predrilling the screw holes.

Hang the new door and double check the location of the marks for the hardware holes. Cut the hole for the latch---in the edge of tew door first, using the alignment marks and the plastic template in the door holesaw kit. Then drill the knob hole, stopping just short of drilling all the way through the door. Finish the hole fron the opposite side to prevent chipping.

Install the old/new hardware and you are done.

If you do not think you can do these steps exactly---1/16" off will result in the door not fitting ----then getting a pro willm be less expensive than ruining a door and then getting a pro.

nadine-
When you install a new door, you want everything to align absolutely correctly, and I guarantee that if you go buy just a door of the right size without the matching frame something is going to be off...the hinges, the height of the hole for the lock, the weatherstripping, etc. That's why it might be easier in the long run to pull the existing frame as well, and that's not a job for you. You need the knowhow and the right tools. If you have a friend or relative who is truly competent, that's fine, but if it's just someone who is merely handy, this might turn into a frustrating mess.

One thing to consider:

We just replaced our door with a new pre-hung and NONE of the trim fit properly. The new door jams seem to be narrower than older ones. We had to custom make each piece (dadoes, routering, chisel, sandpaper etc.) and it took a long time with many tools that the average homeowner might not have.
It looks great now, and seals well but it was a HUGE job.

nadine-
Are you seeing a general theme emerging here...?

We had our front door replaced about a year or so ago. It seemed to be quite a job for the installer who did it. (standard front door with little half moon shaped glass thingy at the top) Nothing fancy, but nicer than the original 17 yr old plain door. The door itself (even in the home store) was about $200 and change, and the installation was probably about the same, but it seemed worth it for a job done the right way.

But I am quite sure there are some really handy dudes out there that could definitely do it themselves, but for me and my husband (very NOT HANDY), the $400 was money well spent at the time.

jmho

Shari :)