how do you cut an inside corner of crown molding on a 45 degree angle instead of a 90 degree angle?

^what the...?

Heres an online calculator that will tell you how to set your saw angles, just type in the wall angle, hit inside/outside or whatever and it will give you a precise setting:

http://www.issi1.com/corwin/crown.html

None of the previous answers are correct. If two walls come together at a 45 degree angle it would be impossible to install crown molding. Standard compound miter saws can only cut 45 degrees to the left and 45 degrees to the right, some higher end models extend to 50 degrees. Take for example a wall that comes together at a 60 degree angle, if your crown molding is standard (sits on the ceiling at a 52 degree angle and on the wall at a 38 degree angle, known as the “spring angle”) the miter would be 46.8 degrees and the bevel (tilt of the saw blade) would be set at 43 degrees. Anything sharper than a 59 or 60 degree angle would require a saw that would spin 360 degrees. Basically a 45 degree wall angle is beyond the range of a saw. The answers other people gave would be correct if your question were about baseboard or another type of trim that sits flat on the wall.

I think he meant 'mitre' saw, just in case you couldn't find a 'mider' saw!

You can get a mitre block, a wooden jig if you like, with slots already cut at specific angles, one of which being 45 degrees. You put the molding on the flat edge and run the saw through the slot, an easy way to cut 45 degrees with spending out on a whole new tool.

assuming u have found out how to cut a 90.....just set the saw to 22.5 degrees....thats half of 45

several ways actually. You can use a table saw tilting the blade at a 45 or a mider saw

22.5 is correct Though it may be a compound.If so set your molding against your miter stops exactly as it will set against the wall,with a right triangle space behind your molding.