Even a well-tuned printer makes holes undersized. The nozzle traces a polygon inside a circle, so the hole ends up smaller than commissioned — this is geometry, not a fault.
Phase 5 — Verificationabout 30 min
How to do it
Print the tolerance gauge.
Measure X and Y outer dimensions with calipers. Measure each one three times in different places and use the average — a single reading catches a high spot and sends you tuning something that is not wrong.
Work out the error: measured minus commanded. A 20 mm cube reading 20.2 mm is +0.2 mm, and the compensation you enter is HALF that per side, because the error is shared between two walls.
Find the setting. OrcaSlicer and FlashStudio call it XY SIZE COMPENSATION under Quality; PrusaSlicer and SuperSlicer call it XY size compensation under Advanced; Cura calls it Horizontal Expansion under Shell. A part that reads too big takes a NEGATIVE number.
Measure the test holes separately. They will read undersized, and that is geometry rather than a fault — the nozzle traces a many-sided shape inside the circle.
For holes, use hole compensation if your slicer has it (OrcaSlicer: "X-Y hole compensation"), or simply oversize holes in CAD by the error you measured. Do NOT fix holes with XY size compensation — that moves the outer walls too, and you will break the dimensions you just corrected.
Check the bottom edge for elephant's foot, the slight bulge on the first layer or two. If present, revisit Z-offset rather than compensating here — compensation would shrink the whole part to hide a first-layer problem.
How to read the result
Outer dimensions land within about 0.2 mm. Holes read undersized; that is expected and gets compensated, not tuned away.