Fits are two problems, not one
Every failed printed fit — the bearing that wobbles, the pin that snaps the boss, the lid that needs a mallet — comes from mixing up two separate questions:
- What gap do the finished parts need? This is classic mechanical design: a press fit wants ~0.1mm of interference, a pivot wants ~0.15mm of air, a hinge that lives outdoors wants 0.35mm. Machinists have had tables for this for a century (ISO 286 and friends).
- What do I tell the printer so the finished parts actually measure that? This is FDM compensation: holes print undersize by roughly 0.1-0.25mm, external diameters print slightly oversize, materials shrink by their own percentages, and vertical-axis holes sag on top.
Most advice online smashes these together into a single "add 0.2mm" folk rule, which is why it works for some people and fails for others — they have different printers, materials, and fits in mind. The calculator keeps the two layers separate and shows you both in the breakdown.
The fit ladder
Designed gap on diameter, after printing, for rigid filaments in the 5–25mm range:
- Press fit, −0.10mm — assembled with firm hand or vise pressure; carries load; considered permanent-ish
- Snug / transition, ~0.00 to +0.05mm — slides with light friction, stays where you put it; the "detent" feel
- Close running, +0.15mm — rotates and slides freely without visible play; pivots, sliding lids, printed hinges you care about
- Free / loose, +0.35mm — obvious air; never binds even with dust, moisture swelling, or a less-than-calibrated printer
These are FDM-adapted numbers, not machinist numbers. Metal press fits use interference an order of magnitude smaller relative to size because metal barely stretches — plastic's low elastic modulus is your friend here, absorbing printer error a machinist could never tolerate.
Why holes and shafts err in opposite directions
A printed hole comes out small for three stacking reasons: the slicer approximates the circle as a polygon that sits inside the true circle, the extruder over-deposits on inside curves where it can't decelerate fast enough, and the material shrinks as it cools. The full physics is in our holes-too-small guide.
A printed shaft comes out slightly big: the same extrusion overshoot that narrows an inside curve swells an outside one, and there's no polygon undersizing on external perimeters (the polygon circumscribes differently). Net effect is typically +0.05mm on a calibrated machine — smaller than the hole error, which is why shafts are the more trustworthy half of any printed fit.
The 10-minute rule
No formula replaces one test print. Before committing to a multi-hour part, print a coupon: a 10mm slice of your hole in a small plate, plus a 10mm stub of the shaft. Test the fit, adjust by 0.05mm if needed, reprint the coupon. Two iterations maximum and you have numbers that are dead-on for your printer, your filament, this week's humidity — then print the real part once.
Fits repeat extremely well on the same machine and material. Keep a note (or the calculator's copy button output) of what worked; it's still valid months later.
Frequently asked questions
How much interference does a 3D printed press fit need?
About 0.1mm on diameter for rigid materials (PLA, PETG, ABS) in the 5-25mm range. That sounds tiny, but plastic's elasticity does the work — the hole stretches a few hundredths and grips the shaft with surprising force. Go to 0.2-0.3mm interference and you split perimeters or crack layer lines instead of gripping harder. TPU is the exception: it compresses, so double the interference.
What clearance do two 3D printed parts need to slide?
A reliable starting point is 0.15mm designed clearance on diameter for a close running fit, 0.3-0.4mm for a fit that must never bind. Remember this is the clearance AFTER print compensation — the calculator separately corrects for the fact that printed holes come out undersize and printed shafts come out slightly oversize, then adds the fit clearance on top.
Why does my printed hole + printed shaft combo bind even though the drawings show clearance?
Because both errors stack against you. The hole prints ~0.2mm small (polygon approximation + inside-curve overshoot) and the shaft prints ~0.05mm big (outer perimeter swell). A "0.1mm clearance" on paper is a 0.15mm interference in plastic. Compensate both parts — that's exactly what this calculator does — and the paper clearance becomes real.
Should the tight part be the hole or the shaft?
Put the precision on the printed shaft when you can. External dimensions print more accurately and more repeatably than internal ones — there's no bridging, no polygon undersizing, and you can measure a shaft with calipers in two seconds and sand it down if needed. You can't easily sand the inside of an 8mm hole.
Does PETG need different fits than PLA?
For press fits, no — same interference. For running fits, yes: PETG surfaces are tackier and slightly more elastic than PLA, so moving fits grab. Give PETG running fits an extra 0.05mm of clearance. Nylon is slippery (great bearing surface) but shrinks 1.5%, so nail the shrinkage compensation first.
How do I test a fit without printing the whole part?
Print a test coupon: a 10×10mm plate with the hole, and 10mm-tall stub of the shaft. Ten minutes of print time. If the fit is wrong, tweak the offending dimension by 0.05mm and reprint the coupon — not the 6-hour part. Once a fit works, write the numbers down; they're stable for your printer + material combo.
Do press fits in plastic loosen over time?
Yes — plastics creep. A PLA press fit under constant stress relaxes measurably over months, especially anywhere warm (a car dashboard, near a stepper motor). For permanent joints that carry load long-term, design a mechanical backup: a shoulder, a snap ring groove, a drop of CA glue, or switch to a screw + heat-set insert.
When should I use this vs. the Hole Tolerance Calculator?
Use the Hole Tolerance Calculator when fitting KNOWN hardware — heat-set inserts, bearings, bolts, magnets — where the part dictates the numbers. Use this calculator when you're designing the fit yourself: printed hinges, pivots, pins, boxes with sliding lids, two printed parts that must join. This one lets you choose how tight the result feels.