Caution: Tape Measures [long]

Maker Talk
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I was getting frustrated with apparent over-cuts while doing a project recently. But I realized something else was going on when both the table saw and the CNC gave me an identical “wrong” cut.

They weren’t wrong; my tape measure was. Badly.

Because the table saw and CNC agree, I’ve been using them as the reference, via a board I cut to precisely 45 inches on the table saw. I then went around grabbing every tape measure I could find and testing them.

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The tape measures in the left pile all measure 45 inches exactly, as expected. This pile includes one of mine (the cheap Harbor Freight one), one from the tool area, one from the laser area, and the little IKEA tape that’s been floating around recently.

The middle pile all visibly measure the board to be less than 45”, most by almost but less than a sixteenth of an inch. This pile includes my decades old Craftsman, another of the cheap Harbor Freight ones (from the Asmbly cubicle), two from the tool area, one from the metal shop, and one from the 3D printing area.

The right pile were all under by more than a sixteenth of an inch at 45”. This includes one from the tool area, one from the laser area, one I found in textiles, a random one of mine, and my Woodcraft tapes. Those two were the worst, measuring more than 3/32 of an inch under. And of course, they were the ones I was relying on. :man_facepalming:t2:

Since they were all measuring under, I wondered if maybe the tabs at the end were loose. It’s supposed to move a bit, but you can imagine it pulling too far over time. While that could be a factor for some of these, my Woodcraft ones give identical mis-measurements in compression too. They’re just off.

I guess I had never noticed because, in the past, when I needed precision, I was making short measurements. (The error does seem proportional to the length you’re measuring.) Or if I was measuring something long, I didn’t need much precision. This project is probably the first where I needed precision over a 4 foot length.

So only one of mine is accurate, the cheap HF one, and it’s already wearing badly despite being quite new. And it doesn’t have auto-lock (aka self-lock), the feature that led me to use the Woodcraft ones. So I took my reference board to both Home Depot and Lowes. I only found two tape measures that I deemed accurate enough, and they didn’t have auto-lock.

I’ve been down the Google rabbit hole about accuracy classes for tape measures, but no American tapes I saw online list their class (which probably means they are unrated.)

So, 1) be careful about which measuring tapes you trust when accuracy matters, and 2) does anyone know where to get a tape measure that is accurate, inches only (not half metric), with an auto lock? Bonus if it’s subdivisions are labeled down the 1/8” level.

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Thanks for that, Ethan.
I know that I like to use the same tape for any particular build, due to possible differences. Same with metal rulers. Especially rulers / squares with thick hash marks. Terrible to measure with and get precise repeatability. It would be interesting to run a couple of laser tapes and see how they stack up. Also what multiples within the same tape. 1 foot vs 2 feet vs 4 feet, etc… That’s not a request, LOL. Just a thought.

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Thank you so much @mgmoore . This is informative, and I appreciate all the detail. I think this is an aspect of work I too often forget about. But I am very often measuring something at home and when I come into the shop to do something, I use a different ruler. So having things come out somehow differently that I designed or anticipated is making more sense now.

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FastCap is my go to for tape measures. They are the most consistently accurate I have found, have lots of measurement options, and are a good price point. That said, keep in mind that all tape measures have some level of inaccuracy when using the hook, especially as it gets stretched from pulling on it too hard. The hook part is designed with construction in mind and only needs to be accurate to 1/16" (or even an 1/8"). If you want to check for accuracy of the tape itself, check the measurement lines against a known accurate rule. Then always burn an inch for accurate measurements. Alternatively, if the piece just needs to be accurate to itself, then it doesn’t matter if an “inch” on the tape is not exactly an inch and you can just use the same tape for all of your measurements. …Or for the best accuracy, use a marking knife with the actual parts as reference to each other and don’t measure using numbers at all.

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ps when I saw your post I initially thought it was going to be about standard lengths for police caution tape

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The intended function is that it is pulled out for inside measures, and pushed in for outside measures

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Yes, I know that of course. Hence “It’s supposed to move a bit”. But with wear or damage, you can readily imagine that that tab might pull farther than it should, which would lead to an under measurement. But, as I said, my main tape measure gave the same measurement in both extension and compression, so that isn’t the problem. It’s just an inaccurately printed tape measure.