3D Printing Tolerances: How Much Clearance Each Fit Needs (in mm)
For FDM prints in PLA with a 0.4 mm nozzle, design 0.05-0.1 mm of clearance per side for a press fit, 0.2-0.3 mm for a sliding fit and 0.4-0.5 mm for a loose fit. Then make holes another 0.2 mm larger, because printed holes come out small.
Most fit problems come from two things: mixing up per-side and total clearance, and modeling holes at their exact size. Both are covered below.
Clearance for each fit
All numbers are per side: the gap between one face of the inner part and the facing wall of the outer part.
| Fit | Per side | Total (on a diameter) | Use it for |
|---|---|---|---|
| Press fit | 0.05-0.1 mm | 0.1-0.2 mm | Magnets, dowel pins, bearings pressed in |
| Snug fit | 0.1-0.15 mm | 0.2-0.3 mm | Parts that stay put but come apart by hand |
| Sliding fit | 0.2-0.3 mm | 0.4-0.6 mm | Lids, drawers, telescoping tubes |
| Loose fit | 0.4-0.5 mm | 0.8-1.0 mm | Hinge pins, hooks, chain links |
| Print-in-place | 0.4-0.6 mm | 0.8-1.2 mm | Hinges and joints printed already assembled |
For a sliding fit longer than 50 mm, add 0.05 mm per side: small errors add up along the length and the part binds halfway in.
Per side vs. total
If someone says "use 0.2 mm", ask which. 0.2 mm total is a snug fit; 0.2 mm per side is a sliding fit. Twice the difference.
Worked example: a lid that slides over a box 80 x 60 mm outside.
- Pick sliding fit: 0.25 mm per side.
- Add twice that to each outside dimension: 80 + 0.5 = 80.5 mm, 60 + 0.5 = 60.5 mm.
- Model the inside of the lid at 80.5 x 60.5 mm.
For a 6 mm pin in a hole with a loose fit (0.4 mm per side), the hole is 6 + 0.8 = 6.8 mm, then plus 0.2 mm for hole shrink: 7.0 mm.
Why holes print small
A printed hole is traced by the nozzle going around its edge. The plastic squishes out slightly in every direction, including into the hole. Curves are also made of short straight segments, which cut the corners of the circle. Together, holes come out 0.1-0.3 mm smaller than modeled.
Fixes, in order of preference:
- Model holes 0.2-0.3 mm larger than the pin (on top of the fit clearance).
- For exact holes, model them 0.5 mm small and drill to size after printing.
- Holes printed on their side (axis horizontal) sag at the top. Use a teardrop shape or a flat top so they print round without supports.
Outside dimensions do the opposite and come out slightly large. That's why both halves of a printed pair need clearance, not just one.
Scaling won't fix a fit
If a lid is too tight, scaling the whole part up 1% doesn't help: on an 80 mm box that's 0.8 mm more width but only 0.4 mm on the 40 mm side, and the walls get thicker too. Change the clearance in the model instead. If you don't have the source file, our measuring guide covers taking dimensions off the real part.
Snap fits, threads and magnets
- Snap fits need their own math: the hook bends into place, so clearance at rest is small (0.1-0.2 mm) and the arm length decides how hard it snaps. See designing snap-fit joints.
- Threads under M6 rarely print well. Use 0.2-0.3 mm per side on printed threads above M10, or use heat-set inserts. See adding threads to 3D printed parts.
- Magnets press in with 0.05-0.1 mm per side. Leave a 0.2 mm skin on top so you can print over them, or add glue if they're loose.
Print a test before the big part
Every printer and filament is a little different. Before a 4-hour enclosure, print a 10-minute test: a 20 mm peg and a row of holes at 0.1, 0.2, 0.3 and 0.4 mm per side. Try the peg in each and use the gap that feels right. A test piece like that weighs a few grams and costs about the same as our small bracket example, $9.42 delivered.
How accurate our prints are
Mandarin 3D prints in PLA at 0.2 mm layers. Our stated tolerance is about ±0.3 mm or ±0.5%, whichever is greater (terms). Sliding and loose fits designed with the numbers above work as printed. For press fits, which sit inside that tolerance, either:
- print one test piece first, or
- add the file check ($5.00) on the quote page: we review printability and fit before printing and email you if something looks off.
Get it printed
Drop your STL, 3MF or STEP in the box below for an instant price (what affects it). No file yet? Describe the part with the fit you need, like "a box 80 x 60 x 40 mm with a sliding lid, 0.25 mm per side", and the AI designer builds a model you can check and order. New to this? Read how to get something 3D printed.
Questions
What tolerance should I use for 3D printing?
Per side, on a 0.4 mm nozzle in PLA: 0.05-0.1 mm for a press fit, 0.1-0.15 mm for a snug fit, 0.2-0.3 mm for a sliding fit and 0.4-0.5 mm for a loose or print-in-place fit.
Is clearance per side or total?
Both are used, which is why forum numbers conflict. This guide gives per side. For a pin in a hole, the hole diameter is the pin diameter plus twice the per-side gap.
Why do 3D printed holes come out too small?
Holes print 0.1-0.3 mm undersize because the plastic squishes inward and round holes are made of short straight segments. Model holes 0.2-0.3 mm larger, or drill them to size.
How accurate are Mandarin 3D prints?
Our dimensional tolerance is about ±0.3 mm or ±0.5%, whichever is greater. For press fits tighter than that, add the $5 file check or print a small test piece first.