11

Usually I use a tape measure to do carpentry, but a problem arises when I need to take a precise inside measurement.

Because the tape is longer than the area inside the cabinet (or whatever it is) I can only bend the tape and estimate the distance, but this estimate is only good to about a 1/8" at best. How can I make a more precise measurement of an inside dimension?

Improve this question
1
  • 3
    As a side note / tip: measure the width at both the bottom and the top (same for vertical measures, do both sides). Sometimes these things are not perfectly perpendicular everywhere, so this will help prevent annoying surprises like gaps or excess material.
    – MiG
    Feb 2, 2022 at 19:20

6 Answers 6

Reset to default
33

Most quality tape measures have the length of the tape measure marked on the body of the tape.

For example, here's the marking on one of the tape measures I own:

tape measure with dimensions called out
Image courtesy of lowes.com. Click to embiggen

  • Put the tab against one inside edge
  • Put the other side of the body of the tape against the other edge.
  • Read the dimension where the tape goes into the body.
  • Add the size of the body to the measured dimension.
  • That is your final measurement.

If your tape measure body does not have this marking, use your other tape measure (doesn't everybody have 3 or 4 tape measures?) to measure the size of this one and use a Sharpie™ or other permanent marker or make some sort of label to write it on the measure so you don't have to figure it out again in the future.

Improve this answer
4
  • 27
    Perhaps interesting to note that many tape measures are specifically designed to take inside measurements. You'll notice the tab on the end of some tape measures feels loose - this lets the tab itself move by one tab-width relative to the tape, so that the tab width isn't counted in the measurement when measuring from the outside of an object, but is counted in the measurement when measuring from the inside of an object.
    – Nuclear Hoagie
    Feb 2, 2022 at 20:24
  • 3
    The main reasons I don't like this technique for precision are: 1) Many tapes are not nicely flat on the back side. This can result in error unless careful attention is paid to position. 2) The opening for the blade tends to not provide a sharp index at which to read the measurement, and the shape of the tape body tends to interfere with aligned sighting (it bulges out the front). 3) It's often quite dark when inside things. Shadow from the tape body and hands is a problem.
    – isherwood
    Feb 3, 2022 at 17:11
  • TBH, I don't particularly like it either, @isherwood, for exactly the same reasons and I do what you described in your answer, though I still learned some good tips from yours.
    – FreeMan
    Feb 3, 2022 at 17:15
  • 1
    It's certainly useful, so I wouldn't downvote, but it's not quick or precise in my experience.
    – isherwood
    Feb 3, 2022 at 17:17
31

You can make an inside measuring gauge using two sticks. When you get the sticks positioned, clamp them, remove them and measure the distance.

enter image description here

Improve this answer
10
  • 18
    You'll want to make them a perpendicular to the opposing surfaces as possible to avoid error. The picture has a little too much "angle" to the sticks. The resulting measurement could be off significantly as is.
    – gnicko
    Feb 2, 2022 at 19:20
  • 4
    @gnicko Good call. That was the best image I could find.
    – Steve Wellens
    Feb 3, 2022 at 5:31
  • 4
    Instead of trying to extract the sticks together, simply draw a line across both with a pencil. You can then take them out individually, and then line up the pencil marks to measure them outside the box. Or just measure the distances from ends to pencil marks and add them.
    – Graham
    Feb 3, 2022 at 23:22
  • 1
    @isherwood If you line up the pencil lines on the sticks, the error cannot be any higher than half the width of a pencil line. That's more precise than it's possible to read a tape measure (even with your method of guesstimating sub-divisions by eye), and that's before we consider any play in the tape measure hook or in the tape itself. If you want better precision than that, score a line with a scalpel instead of marking with a pencil; at which point the error is the width of a scalpel blade cut.
    – Graham
    Feb 4, 2022 at 14:51
  • 1
    @isherwood I'd certainly agree that trying to clamp them and remove them in situ, as per the picture, is never going to work. :) Lining up two planed battens isn't too hard though, and with the sides flush then you can normally get a decent line across them. (Of course pick your battens so they're straight!) And yes, you'd need to make sure they didn't move whilst you took your measurement after removal. It's certainly a lot less quick than your method, but the skills required (just lining up marks) are probably easier for someone less familiar. Anyway, it's an option.
    – Graham
    Feb 4, 2022 at 14:59
26

I disagree with your primary thesis. It's quite easy* to measure to 1/32" or better accuracy with a standard tape measure, even of the heavy 25-30' variety. As a professional finish and framing carpenter I've done it many thousands of times. It's just a matter of technique.

  • Press the bend into the corner firmly. Even steel tapes are plenty flexible.
  • Tilt the blade to one side. Doing so concentrates the force on the edge, allowing a very sharp bend.
  • Read the measurement at an angle perpendicular to the blade at the center of the bend. This reduces parallax error, that arising from gaps between the blade and the surface when viewed from an angle.
  • When in doubt, cut long, then cut again. Modern miter saws allow for razor-thin adjustments, assuming a quality blade in sharp condition. For a snug fit, intentionally cut just a bit long and flex the piece into place by bowing out the middle.

No need for goofy gadgets or in-your-head addition of complex fractions using the tape body size**. Both introduce error, either in mathematical rounding or in mechanical slop, and they slow you down. This technique works quickly and without a doubt even for hair-fine fitment.

enter image description here

enter image description here

I would read the above measurement as twenty-nine and a shy three eighths, or 11/32 if you prefer.

Fun fact: Most tape measures have a bit of play in their hook attachment. This is to account for the thickness of the hook when used in either a hooked or butted manner.

* Really. It's easy. Once you're aware of this technique, it takes very little practice or effort to use it.

** Assuming you're not lucky enough to have a round-number size. Rightly or wrongly, you'd probably get laughed off a jobsite for backing your tape into a corner to get a measurement anyway. It's half a step from lifting the big end to the top of a wall to get its height.

Improve this answer
9
  • 1
    Careful to keep it square. If you're off by 5 degrees over 29" then that's almost an 1/8" too long.
    – Mazura
    Feb 2, 2022 at 23:50
  • 3
    Perhaps it is a good idea to reconsider "It's quite easy [...]. As a professional finish and framing carpenter, [sic] I've done it many thousands of times" (emphasis mine). Is OP a professional finish and framing carpenter like you? Are all future readers? If not, this line is not that useful, and worse, may come off as elitist/gate-keeping. I would recommend just sticking to the facts, e.g. "you can do this; here's how".
    – TylerH
    Feb 3, 2022 at 16:57
  • 2
    @isherwood Something cannot be objectively easy. That's not what the term "objective" means. What if the reader is missing an arm? An eye? I don't troll (well, sometimes on Meta...); this is a genuine criticism of a detrimental section of your otherwise good answer. Keep in mind that, by the rules of the network, you should assume good faith. It's your right to disagree that the language is problematic, but not to dismiss someone with a valid critique as trolling just because you happen to disagree.
    – TylerH
    Feb 3, 2022 at 17:07
  • Well, obviously I disagree, and I assert that it's easy partly to distinguish this technique from several others that look easy to the layperson but actually aren't. This is much easier than trying to hold two sticks in position, then move them, then measure them, for example. To me it's important to make that clear.
    – isherwood
    Feb 3, 2022 at 17:20
  • 1
    Yeah, normally you'd measure along the top or bottom panel, or some other perpendicular reference. It's rare that you're out in space, but if so the trick is to swing the tape until it reads the smallest value.
    – isherwood
    Feb 3, 2022 at 17:32
13

I find this type of measure, called a folding measure or folding rule, to be very helpful:

enter image description here

Note that it has a metal extension that is used when making inside measurements.

Improve this answer
1
  • 2
    I've always heard them referred to as a "carpenter's ruler". Terms may vary by region.
    – FreeMan
    Feb 2, 2022 at 17:05
11

There are 2 basic ways to achieve this:

  1. Measure from one inside corner to a spot you can comfortably reach with the tape without having to bend it, and best to make it a round number. For example, if the total inside length of the cabinet is 20 1/4", you could measure to 15". Make a mark at this spot. Now measure the other direction, from the opposite corner, to your mark, and add up your measurements. If you don't want to mark directly on the material you're measuring, you can place a piece of masking tape and put your mark on that.
  2. Many tape measure have stamped or written a measurement on the back. This indicates how much you need to add to measurements from the front of the tape measure body to the back of it. So in this case, you would simply place the tape inside the cabinet, with the tip in one corner, and the back of the tape body in the other corner. Now read the measurement where the tape enters the body of the measure, and add the number written on the back of the tape measure to this. There are some good photos of this process on this website.
Improve this answer
1
  • As a 20+ year cabinet maker, PhilippNagel's method #1 is the method I have used and trust the most. I do my best to mark to a multiple of 10 for the first step, if possible, to make the final math easier.
    – Triplefault
    Feb 3, 2022 at 22:05
5

As others have said, you can definitely achieve this with a standard tape or rule.

However, if you're looking for an technical alternative, consider a small, hand-held laser rangefinder. I have a basic one one from a local hardware outlet that cost less than $100 and achieves millimeter-level accuracy for any distance from a few inches up to about 80 feet. It's a fantastic time saver.

laser rangefinder example

Improve this answer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.