I have two filleted corners that are not at 90deg on an aluminium part. I want to measure the radius of the fillet, but I don't have fancy radius measuring tools, only calipers.

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Is there any way I can measure the fillet radius of those corners using only calipers or maybe other common tools ?

3 Answers 3


I use low tech things for that kind of stuff. Bottle caps, jar lids, rolls of tape, 5 gallon buckets, whatever may fit the curve I am trying to measure. Once I find one that fits the curve, I measure the diameter and divide for the radius. There is a formula for finding diameter if you can set a straightedge or at least create a straight line between 2 points of an arc.

I will need to find it and add it later. It is very simple, and I used to use it a lot...


The proper way to make this measurement accurately is with a set of radius gauges.

They are a set of know concave and convex radii essentially punched out of steel that you push up against your piece to check if the radius matches the tool. You can purchase these fairly inexpensively (~$30 for a cheap set on Amazon), or use a printer to make a paper set of your own out of paper or cardboard.


Jack's idea is probably the best practical solution. The problem is that measurement precision becomes increasingly more critical as the radius being measured decreases. Obviously, when dealing with a fillet, we're talking about very small dimensions. A measurement error of 1/16" results in a substantial calculation error.

Calculators such as this one do the math for you, but you need accurate measurements from the tangent points and from where they cross in space.

  • The link you have is the very same formula I use. I has served me very well over the years.
    – Jack
    Apr 25, 2016 at 19:24
  • "The resource you are looking for has been removed, had its name changed, or is temporarily unavailable." (link is down)
    – Nicu Surdu
    Oct 5, 2018 at 15:25

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