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FIT-CALC · ISO 286 · FDM PROCESS · MM · REV 2026.09

3D Print Clearance & Fit Calculator

Enter the size and the job the joint has to do. This returns the dimensions to model in CAD, compensated for extrusion width and material shrinkage. Clearance scales with diameter the way ISO 286 does, because a 5 mm pin and a 50 mm bore do not want the same gap.

The joint

The size the feature is supposed to be. For a purchased 8 mm rod, enter 8.

Pick “shaft is nominal” whenever the peg is a bought part you cannot change.

This matters more than people expect. Printing both lets shrinkage cancel; printing one does not.

The process

Blank means auto: width = 1.125 × nozzle, shrinkage from the material table.

Machine-specific. Defaults suit a tuned bed-slinger with a 0.4 mm nozzle. Use the calibration block below to replace them with your own numbers.

Model these dimensions

Where the numbers come from

Step by step breakdown of the compensation
Calibrate this to your machine (20 minutes, once per material)

Every number above is a starting point until you measure your own machine. Print one coupon and the model stops guessing.

  1. Model a plate with a Ø20.00 mm through hole and, beside it, a Ø20.00 mm peg about 10 mm tall. Do not apply any compensation.
  2. Slice with the profile you actually use and print it in the material you actually use.
  3. Measure the peg with calipers. Measure the bore with pin gauges or a bore gauge if you have them — caliper knife edges read a printed hole low, which will poison the calibration.
  4. Enter both measurements. The constants update.

Why “just leave 0.2 mm” keeps failing

The folk rule is not wrong so much as it is a single data point. Run the numbers above at 20 mm with a sliding fit and you get roughly 0.2 mm — that is exactly where the rule comes from. The problem is what happens either side of 20 mm.

1. Clearance has to scale with size

ISO 286 defines a tolerance unit i = 0.45·D^⅓ + 0.001·D in microns, and every fit grade is a multiple of it. The cube root matters: clearance grows with diameter, but much more slowly than diameter itself. A flat 0.2 mm is a slop-fest on a 4 mm pin and a jam on a 60 mm bore. This calculator expresses each fit class as a multiple of i, so the gap tracks size the way it does in machine design.

2. The error is driven by extrusion width, not by your model

A printed hole comes out undersize and a printed peg comes out oversize, both for the same reason: the extruded bead is a finite width being placed on a curve, and the slicer approximates that curve with straight segments that sit inside it. Fix the geometry, change the nozzle, and the error changes anyway — because it never belonged to the geometry. That is why the process allowance here is a function of extrusion width, with a smaller layer-height term for vertical stepping, and a curvature factor that grows as the bore gets small relative to the bead.

Why a printed hole comes out undersize A bore seen from above. The slicer replaces the circle with straight segments that fall inside it, and the extruded bead has a finite width. Both take material inward, so the finished bore is smaller than the one modelled. Ø CAD Ø as printed 1 2
A bore seen from above, with the bead width exaggerated so the two effects are visible at all. At true scale a 0.4 mm bead on a 20 mm bore is a hairline.
  1. The slicer approximates the circle with straight segments. Every chord falls inside the arc, so the path is already smaller than what you modelled.
  2. The bead has a finite width and its inner edge sits inside that path, taking a further bite out of the bore.

Neither term belongs to your geometry, which is why editing the model does not fix it and changing the nozzle does. The allowance is computed from extrusion width for exactly this reason.

3. Shrinkage only cancels when both parts are printed

This is the one that catches people who had fits working fine for months. Print both members in the same material and shrinkage largely cancels — both parts come out proportionally small, so the relationship survives. Print one member against a purchased shaft, bearing or steel rod and there is nothing to cancel against. Shrinkage becomes a full error term: 0.5% on PETG at 20 mm is 0.10 mm, and on Nylon it is 0.24 mm, which is larger than most fits. Switch the “which parts are you printing” selector and watch the numbers move.

4. Tolerance is not the same as repeatability

You can ask for a 0.05 mm clearance. A tuned consumer FDM machine holds roughly ±0.08 mm on a 0.2 mm layer, part to part, day to day. Asking for a fit tighter than twice that band means some parts press, some slide, some rattle, and none of it is your CAD. The calculator flags this instead of pretending the number is achievable, and reports the nearest ISO IT grade for both what you asked for and what the process can actually hold — usually IT12 to IT14 territory, where a milling machine sits around IT7.

5. Press fits are a stress problem, not a clearance problem

Interference loads the boss in hoop tension. Thin walls split, and brittle materials split sooner because they cannot creep to relieve the stress. Keep the wall around the bore at roughly 0.25 × diameter, chamfer the lead-in so the peg self-aligns rather than shaving the bore, and prefer PETG or ABS over PLA when the diameter gets large. TPU should not be used in interference at all — it relaxes and loses grip over days.

The fit classes

Fit classes and their clearances

Clearances shown for the diameter currently entered. Values are diametral. Negative means interference.

Limits worth knowing

Below about 4 mm the bore is only a few extrusion widths across, bead placement dominates the geometry, and no formula will save you — drill or ream instead. Above about 150 mm, thermal distortion and bed flatness swamp the compensation. Hygroscopic materials (Nylon, PP, PETG-CF) move after printing, so calibrate with dried filament and expect service drift. And every default constant here is a starting point: the calibration coupon is what turns this from a reasonable estimate into your machine's actual behaviour.

Millimetre / inch converter

The calculator above works in millimetres, on purpose: one unit on the page means you can never misread a number. This block is here for the parts you buy in inches — rod, bushings, bearings, fasteners. Convert, then type the millimetre figure above.

Common inch sizes

Common inch sizes

These are nominal sizes. A rod sold as 1/4 in still needs measuring — that is what the calibration block above is for.