Hacking measurements

On several occasions I’ve found myself needing to measure something and not having my trusty digital calipers with me. What’s a boy to do? Well, I’ve learned that as long as you have pocket change, you can measure just about anything!

You see, a lot of governments have coins minted with incredibly narrow tolerances in order to thwart counterfeiting. This is good news for you and me because that means that any coin in your pocket, provided it’s not been run over by a train, has a known, specific size that you can use as a basis for other measurements.

Measurement purists and old timers may have a problem with this method, but for folks who don’t mind “winging it”, here’s what you do:

  • Get a coin (I use US quarters) and place it up against to or next to the item you need to measure
  • Take a picture with your other hand making sure that:
    • you are far enough away to avoid any lens distortion,
    • the coin and desired object to measure are both in sharp focus,
    • you hold the camera as close to parallel to the coin as you can
  • Open the picture in SketchUp or other 3D modelling software package that supports showing a picture as an object or texture
  • Draw a circle or hollow cylinder the same size as the quarter in your photo and overlay it in the exact same spot
  • Place measurement labels on the circle if your software supports it
  • Resize the image and the circle until the size is as close as your software supports to the proper size of the coin you used
    • Wikipedia has all the details on most common coin sizes (US quarters are 24.26mm in diameter)

Now you can measure other items in the picture with pretty good accuracy.

Close only counts in horseshoes and hand-grenades

I only say “pretty good” here because really you will never get an exact measurement using this method; there are many things that will affect the precision of your measurements based on the coin image and you shouldn’t, therefore, rely on this method for precision.

For instance, one of those things that can mess it all up is that SketchUp only shows decimals to the tenth place — that is, the closest I can get the coin circle diameter is 24.2mm or 24.3mm (I normally go with the lower one). Another thing, lens distortion–no matter how slight–will make any measurements except those immediately near the coin just plain wrong. Also, it can be hard to hold smaller coins in place without your finger blocking any two points on the perimeter that you can measure from.

But, if you don’t need exacting precision, this method works wonders. I’ve used this in many different scenarios, each time with good results; I’m typically within 1mm with my first test print (at which point I can kind-of fudge it by adding or subtracting “shells” before regenerating the gcode).


You may think that such measuring methods are just hacks and should never be used for anything “real”. Well, one of the first places I remember seeing this method was on a the OYO Glasses website which (if they ever launch) will let you design custom eyeglass frames online by–yup, you guessed it–snapping a picture of your face with a quarter pressed against it! If that’s not “real”, I don’t know what is. 😉

Happy measuring!

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