How square should a square be?

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    Matt Wilcox

    Obviously “perfectly square”, but what are the tolerances?

    For example; I have a combination square I got from B&Q a few weeks ago, and I’ve just tested it by drawing a line, flipping it over and comparing the line. At a distance of 240mm it’s off by 3.5mm. Is that a lot or is that within expected tolerance?

    If it’s off by a lot and going to have an impact on my work, is it a case of go buy a new square, or is there a way to fix a square that’s off? There doesn’t seem to be an obvious way to adjust a combination square’s angle.

    Thanks for any help and advice 🙂


    If I can use imperial measures you are talking something that is close to an 1/8 of inch out of square in a 9 inch distance?  To me thats quite significant, that is of course if my rough conversion is correct.

    I have no idea how you would go about tuning a metal combination square, sorry.


    Michael Petre

    It should be square along the entire length of the ruler, otherwise it isn’t square.

    First, check if the ruler is rocking in the head with the screw tightened. If it is, return it as defective and test another one until you find a good one.

    From this point, you must decide if you want to attempt fixing it or to simply return it. Below are the various things I would try if I decided against returning it.

    Check the face of the head for imperfections. Lap the face until they are gone and test the square again.

    If there is no imperfection on the face, test the ruler at various extensions. If it is only inaccurate from a certain extension, check the groove of the ruler around the area where the nut is for that extension for any imperfection.

    Lightly wax the ruler blade, oil the screw and nut. That should make the ruler move more easily, which can have some impact on the accuracy in my experience.

    Matt Wilcox

    According to Google that’s 9.44″ and off by 0.138″ (which would be a little over 1/8th)

    Hmm, guess the B&Q square is considerably out then. Thanks.


    Matt, some combination squares have a small nib inside the housing where the bar slides.  This nib could be filed so the angle of the bar to the housing is adjusted.      Boyce

    Matt Wilcox

    Lapping is a no-go; I’m just starting out in Woodwork and have almost no experience, very limited tools, and nowhere to get access to better tools. I’ve made a note though for future use, thank you.

    Alas, returning it is also likely to be out of the question as it’s a good few weeks ago now.

    I’ll order one of the ones recommended by Paul in his Tools series and (now I know I should) I’ll check it for square when it arrives.

    Thanks for the help 🙂

    Steve Follis


    Here is a pretty interesting read on this subject by Chris Schwarz.

    Memphis, Tennessee

    Rob Young

    Return that combination square to the store if you can. If you can’t, then do something to dispose of it.

    You can find used Starrett, Brown & Sharpe, Muitoyio (spelling?) and several other machinist grade combination squares on eBay in the USA and UK. A missing scriber or damaged bubble level is of little consequence to their usefulness in woodworking but will generally lower the cost. Likewise, a little corrosion on the blade isn’t a big deal if the marks are incised. A quick scrub and you are back in business.

    Baring finding a quality used square, one brand to look for is PEC but I’m not sure if it is distributed in the UK. I can get factory second PEC combination squares, 12″ for $20US and they are of good quality. Just have the occasional ding or dent that will not affect their use in woodworking. Here is the mail-order side of the store where I’ve purchased PEC squares:

    Good quality squares have a rib running through the slot in the head that can be filed to fine-tune the angle of the beam and thus the squareness. A quality 12″ combination square should be less than 0.01″ out over its full extension length (11″) for woodworking, and less than 0.005″ is the “B” grade machinist square.

    One additional note on vintage combination squares, there are cast iron heads that can be rather brittle, cracking or breaking if dropped. Starrett still offers one model with a cast head that (I believe) is not ductile cast-iron. So as always with used tools, caveat emptor.

    Gary Hodgin

    I’d definitely take it back if out’s that much. Make sure you have the ruler locked in correctly and you’re measuring square from a known straight edge.

    I gave up on using a combination square for everyday bench work a few years ago. Just couldn’t trust them. The mechanism that locks the ruler in seemed to always give me a problem. Finally, got a good Starrett but only use it unless I need a square with a ruler.

    I use a set of engineer squares 2″, 4″, 6″, and 12″. I found these are accurate and stay that way unless abused. Disadvantage is that only one or two doesn’t give me the flexibility of the 12″ combination square. I almost never use the 12″ and seldom use the 2″, but the 2″ comes in handy for certain tasks.



           I have a combination square Stanley model 122, you get tested and passes the prueva ie matching the line on both sides, that you have opinions on this model, someone has a Stanley model, however the model using Paul Sellers is if I’m not mistaken Rabone Chesterman besides old and you do not beat about the tolerances discussed here and as you see Paul does not give much importance. Do you think about it?

    Jim Burcicki

    One item that was not mentioned is that the edge you are registering against, is it flat? If it has a bump in it and you just quickly place the square, you will get inaccurate readings both ways which could account for the error. But, like everyone else, if the edge is flat, I would trash it unless you can lap it and correct it in order to have confidence in it.

    I purchased a Starrett combination square. I found the 12″ combination square to be cumbersome especially when marking a knife line/wall on the edge of a board. I would always be putting the wood in a vise. I decided to purchase a 2″ and a 6″ PEC engineer square on eBay. I find myself grabbing those to make knife walls. I would recommend the 6″ along with the combination square for the projects presented so far, but, I decided to spend the $14 for the 2″ which is nice for marking the edges and checking for square. I think the 4″ might be a bit redundant.

    Interestingly enough, at the Somerset, NJ wood show, Paul was using his well-used Craftsman combination square.

    I have come to think that Paul recommends tools so that the average person can have access to tools in order to be able to work wood. You can spend a lot for a tool, as in the case of Starrett, or spend not as much like Craftsman. The question is “What is good enough?” Maybe, down the road, if my skillset develops to the point where I would be able to recognize the difference in the quality of my work, I would then purchase a Starrett. But, for now, I don’t think it is all that necessary. That is why I purchased the PEC engineer squares. Less expensive, does what I need them to do and I will get use out of them until I “need” Starrett quality.


    Paul goes into some detail on the subject here


    André Gaudet

    hello Matt,

    I am not sure if this will help you or not but i most certainly hope you may take something from it. like you i am quite new to woodworking and as a university student i find myself on a tight budget.

    i read this post in hopes of solving my own square problems. i knew my own was not at all perfect, far from it. but on a 12.99$ budget allowance i picked up a square at my local home hardware. it was the best i could muster.

    seeing the post by Boyce about the “nibs” i quickly ran to dig out my own combination square. i threw of the rule and looked inside. sure enough i saw the nibs. i determined which direction i was off on the square and proceeded by deducing which nib was high. using the corner of the rule i scratched the nib (very crude but functional). its relatively soft and wears nice. i reassembled the square and gave it a test. still off i reworked the nib and tested again. i did this a few times working slowly not to over do it. little by little i got my square to line up with a very satisfactory degree.

    if you haven’t already solved your square issues i would recommend giving this a go. though i am limited on my resources, i take great pride in things that work well and making things work well. i was frustrated with the state of my square (i even tested all the squares at the store to get the best one) and gave up on the fact that it was all i could get with my position. i am now confident my tool is trustworthy for all practical purposes.

    i really hope this helps =)



    Squares – Choosing and Using Them in the Workshop


    A person can also check their squares using the above.  Just knife mark on a piece of wood to the lengths and your square should be right on, if not then its out of square.



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