Assembly-clearance calculator from a calibration coupon you measured yourself

What the calculator does and what it leaves alone

The tool takes four inputs: the shared nominal diameter of the hole and pin on your coupon, the hole diameter you measured after printing, the pin diameter you measured after printing, and the diametral clearance you want. It returns two outputs: the signed adjustment in millimeters and the new hole diameter to enter in CAD. Everything is diametral, never radial, and nothing is divided by two. Both coupon features must share the same nominal diameter, or the comparison is meaningless.

What it does not do matters just as much. It does not predict a printer's tolerance, it does not rank machines, and it does not stand in for a slicer's XY compensation. It does not handle ovality, warped parts, or a fit that depends on several dimensions at once. It is a linear one-step approximation defined by this site, not a formula taken from the Prusa documentation. The math assumes you keep the same printer, filament, profile, orientation, and geometry between the coupon and the final part.

Designing a coupon that yields usable measurements

Model a small plate with a through hole and, next to it or on a second piece, a cylindrical pin at the same nominal diameter. The Prusa documentation warns that your modeling software may display a smooth curve while the exported mesh only has a handful of faces: check how many sides your cylinders really have before exporting, or you will be measuring a polygon rather than a circle. The guide on holes and fits covers that CAD preparation.

Print the coupon using the profile and orientation intended for the final part. Prusa lists orientation and model size among the factors that affect fit, so a coupon that differs greatly from the target gives less direct guidance. Measure the hole and pin consistently, at several positions if possible. If readings vary notably by angle, that may indicate a non-round feature or a measurement issue; do not collapse the differences into one diameter before examining them. See dimensional accuracy troubleshooting if needed.

The formula, step by step, on a hypothetical case

Measured clearance is measured hole minus measured pin. Adjustment is desired clearance minus measured clearance, added to the shared nominal. Hypothetical example, with no real print: both features have a 10 mm nominal, the hole measures 9.8 mm, the pin measures 10 mm and desired clearance is 0.3 mm. Measured clearance is −0.2 mm; adjustment is 0.3 − (−0.2) = 0.5 mm. The tool proposes a 10.5 mm nominal hole and a 0.5 mm adjustment. The CAD pin remains unchanged; print another trial to inspect the actual resulting clearance.

The reading rule is simple. A negative or undersized measured clearance yields a positive adjustment, so the hole grows. A measured clearance already larger than your target yields a negative adjustment, so the hole shrinks. The tool displays results with at most three decimals. Inputs must be finite numbers, positive for the nominal and both measurements, and the desired clearance may be zero but not negative. A new hole diameter at or below zero is rejected as invalid. The form limits each input to one million millimeters. Those input bounds do not define a validated physical fit range.

Why no universal tolerance can replace your own measurement

Prusa's page on modeling with 3D printing in mind states plainly that there is no single universal tolerance value. It lists the most common factors: size of the model, horizontal or vertical orientation, the geometry of the interlocking parts, calibration, settings, and material properties. It adds that different materials can warp or shrink during printing, which changes the outcome even when the CAD file has not changed.

The same source says an Original Prusa will be accurate to at least 0.2 mm and that an initial good measurement for movable parts is at least 0.3 mm. Those are manufacturer figures for its own machines, not a rule of this site and not a value you can assume for every printer. Prusa also separates two different needs: parts that should move, like a hinge, and parts that should lock or snap together with a friction fit. The calculator only accepts zero or positive desired clearance; it does not size negative-interference fits or their insertion force. Picking that clearance is your call, and the result says nothing about press-fit force or the strength of the assembled joint.

Isolate first-layer and elephant-foot effects

PrusaSlicer documentation says the first layer, squished against the bed, is usually wider than expected. An opening that meets this layer may therefore be affected by its geometry; measure in a defined, comparable area, away from the first layer if possible. The calculator does not correct for this effect separately. See the page on elephant foot if the lower edge appears flared. If the fit specifically uses the base, test that functional area as well; a measurement higher up does not qualify it.

PrusaSlicer's Elephant foot compensation setting, documented for version 2.9 Legacy, shrinks the first layer only. It lives under Print settings, Advanced, and is hidden in Simple mode; you need Advanced or Expert mode to reach it. Prusa says values around 0.2 mm usually work well for the default 0.4 mm nozzle, that official profiles ship with it enabled, and that thin first-layer lines are detected so they are not over-shrunk, a behavior present since version 2.2. That number is about first-layer squish, not hole clearance, so do not confuse it with the calculator's output. Prusa also notes that a brim that fails to connect to the part is a sign the compensation is set too high.

Verify with a second coupon before touching the final part

Prusa states that you will have to make test prints before you can consider a design final, and that you should expect to tweak tolerances and geometry along the way. The calculator fits that workflow: it proposes a dimension for the next trial without removing the need to verify it. Enter the new hole diameter in CAD, reprint the same coupon with the same profile, measure again, and recalculate if the resulting clearance is still off target. A parametric approach, which Prusa recommends, lets you change that dimension directly because the hole diameter becomes a single parameter to change.

If two successive trials do not reach the intended clearance, do not assume the CAD diameter is the only cause. Check measurement repeatability, geometry, orientation and profile before making another change. The print cost tool can complement the coupon budget using your inputs without predicting actual material mass. Keep file versions so you can roll back if a correction worsens the fit.

Sources and limits

• Modeling with 3D printing in mind | Prusa
• Elephant foot compensation | Prusa

We organize manufacturer and community documentation; we have not measured these products ourselves. The manual for your exact model takes precedence over general guidance.