Why Do Holes Print Smaller Than They Should?

You designed a 10 mm hole in CAD, but after printing, it measures only 9.7 mm. If you've ever wondered where those missing fractions of a millimeter went, you're not alone.

Undersized holes are one of the most common dimensional issues in FDM 3D printing.

The Nozzle Has a Real Width

A 3D printer doesn't create a hole with a perfectly thin line. Instead, the nozzle deposits a bead of molten filament along a calculated toolpath.

When printing an internal wall, that bead takes up a small amount of space inside the hole. At the same time, the material on the outside of the part can extend slightly outward.

The result is simple: outside dimensions may become slightly larger, while internal holes become slightly smaller.

This effect is usually more noticeable with smaller holes, where even a small amount of dimensional error can make a significant difference.

Flow Makes a Difference

Extrusion flow is another important factor.

If the printer is pushing slightly more material than expected, the excess filament has to go somewhere. Around an internal wall, that can mean more material encroaching into the hole.

Nozzle temperature, flow rate, filament diameter, and extrusion calibration can all influence how much material is actually deposited.

That's why a printer with slightly incorrect extrusion settings may produce parts that look fine at first glance but have tight holes and difficult-to-fit components.

Different Filaments Can Behave Differently

Even when two filaments are made from the same basic material, they may not print exactly the same way.

Different formulations, pigments, additives, and cooling behavior can affect how the filament flows and contracts during printing. Switching from one PLA to another, for example, may produce slightly different hole dimensions even when the same slicer profile is used.

For functional parts, it's worth checking the results rather than assuming every spool will behave identically.

Small Holes Are the Hardest

A large 20 mm hole may print close to its designed size, while a 3 mm hole can be noticeably undersized.

Small circular features leave less room for dimensional errors, and the printer has fewer extrusion paths available to reproduce the shape accurately.

Print orientation can also matter. Horizontal holes may require bridging or support and can become less circular, while holes close to the build plate can be affected by first-layer expansion.

How Can You Fix It?

Start by checking your extrusion and flow calibration. If the printer is consistently producing undersized holes, you can then compensate by slightly increasing the hole diameter in your CAD model or adjusting the slicer's XY/hole compensation settings.

There is no universal compensation value. The ideal adjustment depends on your printer, nozzle, material, and hole size.

A simple calibration model with several different hole diameters can help you find a value that works reliably for your setup.

So if your 10 mm hole comes out closer to 9.7 mm, it doesn't necessarily mean something is wrong with your model.

In FDM printing, creating an accurate hole is a little more complicated than drawing one.