When your math doesn’t match…

When your math does match your measurements, check your the accuracy of your ruler or tape measure :man_shrugging:t3::man_facepalming:t3::man_facepalming:t3::man_facepalming:t3::man_facepalming:t3:

I spent a hour this morning chasing my tail trying to figure out why my bottom panel measurements didn’t fit the math. I redid my math over and over and over until I knew every fraction to decimal to mm conversion by heart. Then a lightbulb went…could the tape measure I’ve been using for a long time gone bad? Yep!

It worked perfectly for outside measurements, but for internal measurements, it would not repeat the same measurements by a 1/16 to 1/8 inch, and perhaps another 1/32”.

I’m not sure when this happened, but it must be recent as I check the accuracy all my measuring instruments every 3-5 months.

So I just wanted to put this out there as a reminder to y’all to make sure whatever your using, to check its accuracy- this goes for squares too.

I hope y’all have a brilliant weekend!


This is great advice, and hopefully saves someone similar troubles.
I’ve also learned to use the measuring instruments throughout a priject, as different tools tend to have some variances in them.

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Using the same measuring instrument for an entire project (as James mentions) was something I only encountered relatively recently, and it explained so many prior projects that had gone just slightly awry.

The greatest (and at times most frustrating) thing about making stuff is that you will genuinely never have all the knowledge, so learning opportunities are endless

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You’re absolutely correct @jamesfreeman! I use a steel rule or my combination/double square for all my layout joinery. I’ve been using the same tape measure for rough measurements for several years, but this project is the first time I’ve used it for this level of layout detail.

@stepho you’re certainly right, we never stop learning or growing if we’re willing to take risks and go outside our comfort zone.

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Ok, I hoped someone else would ask, but…how can a tape measure change over time? Or are you saying it’s always been inaccurate? Let the chuckling begin!

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I believe he’s probably experiencing a problem with the slipping on the end of the tape measure. The “clip” on the end of a tape measure is supposed to move when you measure either inside or outside, to account for the thickness of the “clip.” I believe this amount can move over time and cause error. It’s why some people measure from the tape only, i.e. starting at the 1" line.

See number 5:
Tape measure features


P.s. @tomthm Never feel silly for asking a question here.

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Yes, James is correct, it’s the slipping hook on the tape measure that gone bad. As I mentioned above, the internal measurements are intermittent—I can get up 1/8” of variance between reading. After I realized what was wrong, I tested it against my steel rule. However, measuring length remains accurate. I have of course disposed of the tape measure in favor of my 6’ fastcap which I really like.

And @tomthm, it’s always fair to question something that seems sketchy, odd or just somehow improbable.

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Letting a tape measure slam back into the housing repeatedly is a root cause of tape measurement drift. Once it twice probably won’t change much, but over months or years, yep.

When retracting a tape measure, catch the end and don’t let it slam closed.

Ethan did an interesting study a couple of years back, where he accurately measured several different tape measures. That supported the argument to always use the same one on a project.

I’ll often triple check measurements, if I’m using inside and outside of the tab. Or check the tape. You get paranoid on joinery, and rightfully so.

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Makes sense. In fact I remember that discussion earlier this year, but had forgotten.i usually burn the first inch, so wasn’t visualizing well. Appreciate the review!