Measuring a part so the print fits

What to measure with

A digital caliper is the best tool for this, and a cheap one is accurate enough. It measures outside sizes, inside sizes and depths to about 0.1 mm.

A ruler or tape measure is fine for sizes above about 100 mm, where half a millimeter matters less. For a snap fit or a press fit, use a caliper.

How to measure

• Measure in three places and use the largest value for something the print has to go around, and the smallest for something the print has to go into.
• Round things are rarely perfectly round. Turn the part a quarter and measure again.
• Close the caliper gently. Pressing hard on a soft part, such as a cable or a rubber hose, gives a size that is too small.
• Measure the part itself, not the number on its packaging. A "25 mm" pipe is often 25.4 or 26.9 mm on the outside.

Enter the real size, not the size plus a bit

Tools on this site that fit around something have a clearance setting. Enter the size you measured and let the clearance add the room. If you also add room yourself, the part ends up loose.

For the right clearance value on your printer, see the guide on fit and tolerances.

Shapes that are hard to measure

• Rounded corners: lay coins or drill bits in the corner until one matches, then measure that. Its radius is half its diameter.
• Angles: fold a piece of paper to match the angle and measure it with a protractor, or take a photo straight from the side.
• Hole spacing: measure from the left edge of one hole to the left edge of the next. That equals the distance between the centers.
• Thread: measure the outside diameter and count the threads over 10 mm. Ten threads in 10 mm is a 1 mm pitch.

Test before you print the whole thing

For a large part, print only the bit that has to fit. Most slicers can cut a model, so you can print a 5 mm high slice of a holder in a few minutes and try it on the real part. Change the size in the tool, then print the full model.