If you own a power miter saw, especially one with a laser guide, and you are having trouble getting accurate cuts, I have a dirty little secret to tell you. YOUR LASER GUIDE LINE IS LYING TO YOU.


Although that guide line is there to tell you where your cut line will be, if you line up your cut exactly along the laser guide there is a good chance that the final cut piece will not be the exact size you want, at least 50% of the time.  It will be slightly (and in some cases, quite a bit) off, depending on which side of the board is the piece you are keeping and which side is the ‘leftover’ piece.

Case in point: On my saw, if I draw a line down the EXACT center of a board, and line the laser level directly on top of the line and then cut, when I take the two pieces and lay them on top of each other this is what I get:
One is significantly shorter than the other.  That is NOT GOOD! NOT GOOD AT ALL!
When making my cuts you can only trust the laser line HALF of the time.

What??  Blasphemy you say! Why would tool companies add those handy little laser beams if they weren’t accurate?

Well.. It isn’t that they aren’t accurate, it is that they CAN’T be, at least not all the time.   And the reason that is happening is because of something called KERF.

What is a KERF? 

I know it sounds like a muppet or perhaps a lifestyle blogger, but alas, no.  It is not. A KERF is actually the notch in a piece of lumber left behind by the saw blade. 

You see, when you cut a piece of wood you aren’t really splitting it in half.  You are actually removing a tiny area at the cut site, which is *just* slightly wider than the width of your saw blade.  
On my blade the kerf is nearly A QUARTER INCH!And if you are trying to do any sort of project that requires hyper-accurate cuts, that difference can be a project breaker.   When I try to cut a board in half, one side ends up 1/4 inch shorter every.single.time.  Multiplied over the course of a project can mean I have to buy wood filler by the truck load.

How do you take into account the width of the saw blade when making cuts? 

Well, the short answer is:  "ALWAYS CUT ON THE “WASTE” SIDE OF THE LINE"
And if you are a pro-woodworker you probably know exactly how to do that.  But if you are a regular person who is just getting used to their saw or have been fudging it for a while now.. let me explain to you what that means.

When you make a mark on your board, you always want the blade to fall right NEXT to the line, so that the kerf is removed from the piece of wood you are not keeping.  That also means the board you are keeping comes out at exactly the correct length.

Which side of the blade is the “waste” side?

This is going to be different for every saw. But there is an easy way to figure it out.

The first thing you need to do is determine exactly where you blade cuts, and if you are using the laser guide, you want to know where it cuts relative to that line.

Here is a quick test:

Take two pieces of scrap wood and draw a cut line on them.  On each board, mark one end as the “Keep” side.  You are going to cut one board with the keep side on the right of the blade and one board with the keep side on the left of the blade.  And if you have a laser guide on your saw, you are going to use that to line up the cut.

So here is what my test set up looked like (you can barely make out the red laser line on top of my pen marks):
If your saw doesn’t have a laser guide, you can just pull the saw blade down (WITHOUT IT SPINNING) until it touches the wood. Your goal is to line up the blade next to line on the "waste" side of the board.

Next,  make the cuts and look closely at your cut lines.

When my board is placed with the “keep” side on the left and my “waste” side on the right, this is what the cut looks like. The cut is perfect:
But when my board is placed with the “keep” side on the right and my “waste” side on the left, this is what the cut looks like:
In this case, you can see that my cut mark is actually left behind on my waste board and the 1/4 inch kerf has been removed from the board I wanted to keep and use.  NO BUENO!!

The difference is super clear when you line them up side-by-side:
The board on the left is significantly longer than the board on the right. Even the tip of my arrow is missing.

So in MY CASE, my saw blade comes down ON THE RIGHT of my laser line.  That means, every time I use my saw, I want my “waste” end of the board to be on the right.  I always need to make sure I want to keep the piece on the left.  

For many years now I have had this reminded myself of this with a note on my saw blade itself (this was a tip from my dad).  
However, I recently made it way more obvious for myself, so now I every time I put a board down on the saw I know the correct orientation of the cut:
That being said, on some saws you can adjust the laser line.  In those cases you could set it up to make either side the waste side.  That would be good if your workbench is set up to accommodate longer boards only on one side.   My miter saw doesn’t have that option.

If your cuts end up looking exactly the same (the kerf ends up directly on top of your mark), that means your laser is actually centered.  This isn't necessarily ideal since you would be losing a little bit of length on both sides of the cut (in my case I'd lose 1/8 inch from both the waste and keep board).

If your laser guide is centered, and you can't adjust it,  I would make sure to push my board over just SLIGHTLY away from the “keep” side when making the cut. Line your laser guide up NEXT to your cut mark on the waste side of the line.

Now let's revisit my sample board, the one I wanted to cut perfectly in half.  To accomplish that, on my saw, I have to slide my board so the laser line is 1/8 inch to the left of my center mark. This would mean the kerf would be split evenly between my ‘keep’ piece and my ‘waste' piece.  That means they are the same length, but keep in mind now BOTH boards are 1/8 inch shorter than the half the original width.  The 1/4 inch loss to the kerf has to be accounted for somewhere.

Which brings me to my final tip about accurate cutting on a miter saw:

You should not mark multiple measurements on a board without re-measuring after every cut.

If you take a 4-foot board and mark it into four 1-foot sections and go down the line cutting, those boards will not be the same size. The first piece may be 12 inches (if your cut was on the waste side of the board) but that means the second piece will be 1/4 inch too short.  To be accurate you either need to measure each time, or ‘cheat’ 1/8 inch into the ‘keep’ side each time so that all the boards are 11 7/8” inches wide.  (However if your plan calls for four accurate 1 foot lengths this could screw you up!)

When buying lumber make sure you always take into account a little waste from the kerf in every single cut!  

I hope this explanation alleviates some frustration you may have been having when your cuts weren’t coming out correctly.  On large building projects, it probably doesn’t matter as much, but the more accurate the cuts, the more tightly fitting your pieces will be.  And especially in cases of miters or other tricky joints, you really want your measurements to be dead on!   So don’t ignore the KERF!!