I want to measure a tube's diameter with a precision of about 0.1 mm. I have a caliper but sadly the tube is wider than my tool.

I would like to know what methods exist to precisely measure a diameter. Ideally, without buying any (expensive) tools.

A few ideas I have are:

  • Tape measure the circumference. Then divide by 3.1416.
  • Roll the tube once and measure the distance traveled. Divide by 3.1416.

I'm not sure if I would be able to achieve the desired accuracy with one of those methods. Any ideas will be appreciated.

Edit: Why I can't use my caliper: enter image description here

  • 2
    $\begingroup$ Buy a larger caliper... $\endgroup$
    – Solar Mike
    Commented Jul 18, 2018 at 6:30
  • $\begingroup$ If you know the material it's made of then you know its density. Weigh the tube on some accurate scales. Then, measure its length and its wall thickness accurately then you can calculate it's OD and ID. Then to verify your calculations, do as @SolarMike suggested, buy a bigger caliper :) $\endgroup$
    – user6335
    Commented Jul 18, 2018 at 7:18
  • $\begingroup$ Your restriction on buying tools limits the accuracy and precision you can attain. 0.1 mm is less than four thousandths of an inch for the non-metric readers, very small! A fabric or paper tape measure wrapped around the tube will have at least that much lack of precision. $\endgroup$
    – fred_dot_u
    Commented Jul 18, 2018 at 9:28
  • $\begingroup$ Both of your proposed methods have a lot of room for compounding errors when compared to a direct measurement. $\endgroup$
    – JMac
    Commented Jul 18, 2018 at 9:42
  • 2
    $\begingroup$ Can you measure at the end of the tube? Regarding your drawing, rotate the calipers about the centerline you drew until they're just less than in the center plane. The slight angle will allow you to make sure you're hitting the widest point. $\endgroup$
    – ericksonla
    Commented Jul 18, 2018 at 22:23

4 Answers 4


Accurate circumference measurements by string of tape ted to be difficult.

I would suggest buying or making some simple external cailpers enter image description here

And use them to transfer the measurement you your vernier calipers using the external jaws on the back

You will lose a bit of accuracy in the transfer but not that much.

Some skill is required to use jaw calipers as you need to feel for the actual diameter by rocking them and adjusting untill you just feel very light touch.

The other option is to measure the end of the tube ie rotate the vernier 90 degrees, again you need to feel for the max measurement.

Having aid that 0.1mm accuracy is perhaps a bit ambitious without the right tools and ERW tube can easily vary by more than that depending on where you measure it and there are only a finite number of nominal diameters standard tube can possibly be.


The time-honored method is to attach a string, or wire, or straightedge, to one point on the perimeter. Hold the string taut and sweep it across the tube. Take note of where the greatest length of string is needed to reach the perimeter. That's a diameter.

  • $\begingroup$ This only works if you have access to the end of the tube, of course... Not so helpful if it's installed such that both ends are fixed $\endgroup$ Commented Jul 18, 2018 at 20:50
  • 1
    $\begingroup$ Of course you could wrap a wire around the tube tightly, score a mark across the wire where it crosses - that gives a more than reasonable perimeter length : divide by pi and bingo! $\endgroup$
    – Solar Mike
    Commented Jul 18, 2018 at 20:53
  • $\begingroup$ @SolarMike OK, so long as you still slide things around to minimize the length used (which will occur at a true circumference. $\endgroup$ Commented Jul 19, 2018 at 17:22
  • $\begingroup$ @JonathanRSwift since the OP suggested rolling his tube, I figured it wasn't installed anywhere just yet. $\endgroup$ Commented Jul 19, 2018 at 17:22

This should work in theory, but the accuracy would decrease with increasing diameter:

Your image shows measuring a chord of the circle at an unknown distance from the center of the circle.

If you place a gauge block (of known thickness) between the beam of the caliper and the circumference of the tube, you can measure the length of a second chord of the circle at a known radial displacement from the first chord.

The diameter of the circle should be obtainable from the lengths of the two chords and the known radial distance between them.

The illustration at the top of this page is applicable: http://jwilson.coe.uga.edu/emt725/ParallelChords/ParallelChords.html

The result would be affected by the roundness of the tube, surface roughness on the circumference, the perpendicularity of the caliper to the axis of the tube, and the accuracy with which the thickness of the gauge block is known.

I haven't done this, worked out the formula, or determined how sensitive the result would be to the variables listed above, but it might be an adequate and inexpensive approach in some cases.


One other possible and simple solution would be to tape or twist-wire a couple of pieces of flat aluminum to the jaws of the caliper. The diameter of the pipe would be the measurement minus twice the thickness of the flat aluminum. Pretty straightforward.

There's also this item on Amazon, if you're willing to spend fewer than 10 bucks.


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