Tolerances for 3D Printed Parts
The single biggest reason a 3D printed part does not fit is tolerance: the printer lays down a little more plastic than the model says. Here are the clearance values that make parts fit the first time, so you stop reprinting.
Why printed parts come out tight
An FDM printer pushes molten plastic through a nozzle, and that bead spreads slightly as it cools. The result: holes end up a touch smaller than designed, and outside dimensions a touch larger. A 5 mm hole modeled exactly to 5 mm often measures 4.8 mm once printed — which is why a 5 mm pin will not go in. The fix is to design in a deliberate gap, called clearance.
Clearance values that work
These are good starting points for a typical FDM printer with a 0.4 mm nozzle. Tune from here based on your machine — but start here and most parts fit on the first try.
| Fit type | Clearance (per side) | Use it for |
|---|---|---|
| Press / interference | 0.0–0.1 mm | Parts you want to stay put (pins, dowels) |
| Snug / sliding | 0.2 mm | Lids, covers, parts that fit firmly but come apart |
| Loose / free | 0.3–0.5 mm | Moving parts, hinges, easy assembly |
| Bolt clearance hole | +0.2–0.4 mm on diameter | Bolts that pass through (see the bolt hole chart) |
Apply clearance to the right side
For a hole, make it larger by the clearance. For a peg or boss going into a hole, make it smaller. Adding clearance to both at once doubles the gap.
Designing tolerances with AI CAD
With PartWork.ai you bake the clearance straight into the dimension you ask for. Designing a lid that should slide onto a 50 mm box with a snug fit? Ask for the opening at 50.4 mm (0.2 mm per side):
Create a lid with a 50.4 mm square inner opening, 2 mm walls, 8 mm deepPrinted it and it is still tight? Just say "make the inner opening 0.3 mm bigger on each side" and re-export — no remodeling. More on editing dimensions in Modifying Parts, and on exporting in Exporting.
Print a test piece first
Every printer is a little different. For a critical fit, print a small test coupon with the hole or slot before committing to the full part — it saves a long reprint.
Design a part that fits
Start with 2 free AI generations — no card required
Describe the part and the fit you need in plain English, get editable 3D geometry, export an STL. Open the studio. Need more after your free generations? 100 credits for $4.99 (about 5¢ each).