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I have an OLD framing square. Like many of my “I can’t part with it” tools, it was my grandfather’s. It doesn’t matter how far out of square it gets, I will keep it until it’s time to pass it on to one of my sons. Just like those old tools, though, I want to still be able to use it. So how do I check to make sure it’s actually square, and what do I do about it if it isn’t?
Checking for Square
There are two methods for this. The easy method is pretty darn accurate, and I’d venture to say the slightly harder method is VERY accurate.
Easy method
Lay the framing square on a table with a fence (I use my radial arm saw) with one leg butted up to the fence and pointing left, and draw a line on the table along the edge of the square. Flip the square so the same leg is butted up to the fence, but pointing right. Draw another line on the table along the leg pointing toward you. If the lines are parallel, you’re good. If they aren’t, however, you’ll need to adjust the square. If the “right” line deviates to the right, your square is less than 90 degrees; if it deviates left, the square is more than 90 degrees. Back to making the adjustment in a little bit.
Harder method
The harder method requires some algebra and accurate measuring. It depends on the pythagorean theorem (say it with me, “A squared plus B squared equals C squared.”). Connecting any two points on the outside edge of the legs of your framing square are supposed to make a right triangle. Remembering high school algebra again, the quickest right triangle to remember is a 3-4-5 right triangle. So, if you measure 3 units up one leg from the point on your square and 4 units up the other leg from the point, a ruler laid across those two points should measure 5 units. Notice I said “units;” I didn’t say inches. If you take a standard 24-inch by 16-inch framing square and measure 3 inches up one side, 4 inches up the other, expecting the diagonal to be 5 inches, it probably will be. 
At that distance from the point, any difference positive or negative, is going to be miniscule. It’s like a boat being 1 degree off on its heading. For the first mile, the difference between where the boat is and where it’s supposed to be will be pretty small. Go 15 miles on that same incorrect heading and suddenly you’re a mile to one side or the other of where you want to be. That’s why I use a longer unit when checking my framing square. The closest I can get to both ends of the square and keep my right triangle is 15 3/4 inches (5 1/4 units) from the point on the short leg. That gives me 21 inches on the long leg, and they should be 26 1/4 inches apart. To check this accurately, I mark 2 inches from the end of a jointed edge on a scrap piece of wood at least 30 inches long. I then mark 28 1/4 inches from the same end. I scribe both lines and fill in with a pen to make sure the marks are accurate. I do it this way because the first two inches of my tape measure get beat up a lot. By shifting everything two inches, I can ensure accurate measurements. Now all I have to do is line up one mark with the OUTSIDE 15 3/4 inch mark on the short leg of the framing square and see if the other mark will line up with the 21-inch mark on the OUTSIDE of the long leg. If it does, super! My framing square is square. If not, it needs to be adjusted.
Adjusting the Square
This is actually the easy part. No measuring, no nothing. Take a nail set or punch and a hammer (ball peen is my preference). If the square is less than 90 degrees (easy method: if the second line deviates right; harder method: if the legs are less than 26 1/4 inches apart), place the punch near the inside corner of the square and give it a gentle whack with the hammer. If it’s over, place the punch near the outside corner of the square and give it a whack. Go back and check again for square. Keep checking and adjusting until your lines are parallel (easy method) or until the difference between the measuring points is 5 units (harder method).
That’s it! Now grandpa’s framing square is square again! I can use it now to square up my radial arm saw or anything else where I need a square corner. The cool thing is this 3-4-5 squaring idea can be used for all kinds of things. A friend and I used it when helping his dad lay the first boards on a new deck. 3 feet along the wall, 4 feet out from the same point along the edge of the board. When those two points were 5 feet apart, we nailed it down. The rest was just a matter of maintaining board spacing. And you thought you’d never use algebra again after high school.