Cast iron pipe cutting is something you will have to deal with on many renovation projects on older homes, especially those built before the 1960s, and knowing how to cut them is essential. Often people will try to cut cast iron pipe with a hack saw but because of the extensive time and effort involved with this method (you can go through multiple hack saw blades just trying to get through one pipe) it is not the most efficient way.
Step 1 - Decide Which Method of Cutting Best Suits Your Needs
If the pipe you're cutting is not connected to the wall and you have access to all sides of it the easiest way will be with a hammer and chisel. With chalk draw a line around the pipe where you want to cut it, put the edge of the chisel on the line and hammer the end of it. Rotate the pipe, hammering around the circumference until the pipe breaks. Luckily cast iron pipes are fairly brittle so it should break easily. This method has the advantage of being done with tools you most likely already have, but if the pipe is connected to a wall it won't be an option so you'll have to use a snap cutter.
Note that if the pipe is horizontal, it's a good idea to support it using strap hangers to help reduce vibration or movement during cutting which could lead to a broken pipe. If the pipe is vertically oriented, employ riser clamps to stabilize it.
Step 2 - Get Your Snap Cutter
A snap cutter (also known as a chain pipe cutter) is a fairly expensive piece of equipment, especially if you're only going to be using it a few times on your own home improvement projects. Snap cutters can be rented from most home improvement or hardware stores for a reasonable fee.
Step 3 - Mark Your Cut
Just as you would if you were using the hammer and chisel method, mark where you intend to cut around the diameter of the pipe with chalk. Chalk is not necessary, but a good option because it wipes clean so if you make a mistake you won't be confused by previous (and permanent) marks and there won't be any visible evidence of your home improvement project when it's all done.
Step 4 - Wrap The Chain
A snap cutter has a steel roller chain attached to the end of a ratchet, wrap the chain around your line and thread it through the other side of the jaw so it's wrapped completely around the diameter of the pipe.
Step 5 - Tighten the Chain
Open and close the handles of the ratchet which will put the chain tighter and tighter around the pipe. The rims of each chain link will press into the pipe until it lives up to it's name and "snaps" the pipe in half. If there are any sharp or uneven areas left on the edge of the pipe you can grab them with a wrench and hit it with a hammer (hailing back to the alternative hammer and chisel method) which should remove it.
Now stand back and take a look at your work, but not for too long, you've got the rest of your plumbing project to finish!