I recently posted on cutting batt insulation; although you use pretty much the same tools, cutting backer board is a whole different animal. Backer board is generally used as a substrate for tile, mortared rock, or cultured stone, or as a primary surface, suitably painted or stained, in high-moisture areas like bathrooms.

It is substantially more difficult to cut than insulation or sheet rock, but with the proper tools, you can get through the task. First off -- safety: since you're going to be whacking away at the board with (probably) a razor knife, use sharp blades, wear gloves, have a stable work surface, and wear a dust mask. Do not stand directly in the cutting path of the razor knife; femoral artery red is generally not considered a good color for a bathroom floor.

After the jump, a bunch of pictures to help you along the way, and the rest of the process.

Gallery: Cutting concrete backer board

Materials: some backer board. When you measure your project, add about 10% for waste. The board I am most familiar with is 1/2" thick and 3' x 5' in size; it's clumsy to handle, but on the plus side, it's heavy (heh). If your project will consume a bunch of board, you might consider having some help to man-handle the stuff.

Tools:

• a razor or utility knife. If you have a large project, you can use a circular saw, with the appropriate blade, but you'll generate a lot of dust, so you'll need some sort of dust collection system. Most DIY projects can be done with a knife (not a ginsu). I change my blades after I cut 5 or 6 pieces (you are cutting concrete, after all); dull blades make the work harder and more dangerous, and make you more irritable.
• a straight edge -- I use a 4' level.
• tape measure
• carpenters pencil
• small pliers
• suitable, stable work surface

Time: after some practice, it will take you 3-4 minutes to cut a full board in half; smaller pieces take less since they are easier to handle.

The process:

• Take your measurements, keeping in mind that most projects requiring backer board usually have confined or irregular spaces to work in, so you only want to cut this stuff once -- be accurate; you'll eventually tile (or put some sort of edge molding) over this board, so it doesn't require finish cabinet accuracy -- I generally subtract 1/8" from the overall dimensions before I cut. Remember, also, that your shower stall, walls, or back splash may not be perfectly rectangular (say it ain't so); be prepared to accommodate the irregularities. Measure it three times, cut it once.
• Backer board generally has a smooth side, for painting or staining, and a rough side, for mastic, thin set, or mortar. To make it easy, lay out your measurements on the smooth side.
• Put your straight edge on your marks, move your legs out of the path of the knife and, with moderate pressure, scribe a guide line 5-6 times along the edge.
• Set the straight edge aside. Now, make 8-10 cuts, staying in the guide line, using heavy pressure. Remember, it's concrete. You don't have to cut all the way through the board, but the deeper you cut, the cleaner the break will be.
• Put the knife safely aside; it's always a good idea to retract the blade until the next cut. Lay the board so the cut line is just over the edge of your work surface and push down hard. If your cuts were done properly, it will snap, making you happy. If it doesn't snap, you'll have to make more cuts, making you unhappy.
• At this point, set the board up on edge, and from the back side, cut the netting; you'll see what the netting is, on your first cut. The two pieces will separate and, since the material is (previously mentioned) clumsy and heavy, hang on to both chunks of board.
• Using your utility knife, carefully clean up the edge of your board and it's ready to go.
• At this juncture in your reading, you may ask "what about the pliers"? At some point in the project, you'll have to make a cut within an inch or two of the edge of the board. This piece is not easily snapped by exerting pressure with your hand or fingers. In this event, use the pliers to "nibble" at the edge, moving along the board and breaking away 2-3" pieces at a time. it's not much fun and really slows you down. If you do a lot of this stuff, you can buy pliers whose very wide jaws look like the business end of a hammerhead shark and that allows you to take a wider nibble, but with small pieces, nibble you must, no getting around it. Sorry ...

So, now you know how to cut backer board. The first time, it will take you a half dozen pieces to get the hang of the process, but after that the good news is that it's just hard work! The end result is that you'll have a sturdy substrate for the really cool tile your significant other wants installed (by you).

Have fun.