Parts don’t fit together, holes are too small for bolts, or a part measured 20.0mm in the model comes out 19.6mm or 20.4mm. The error is consistent across similar dimensions on the same axis, or different in X vs. Y.
Measure First, Guess Second
Print a 20x20x20mm calibration cube and measure with digital calipers — not a ruler, not an estimate. Measure the outside dimensions at mid-height (not at the base), across multiple locations, and write down the numbers. This tells you whether the error is:
- Axis-specific: X measures 20.4mm, Y measures 20.1mm, Z measures 20.0mm — motion calibration problem on X
- All axes equally off: Flow rate or e-steps problem — more or less material everywhere
- Only at the base: Elephant foot (see that guide separately)
- Only on internal features (holes): Expansion compensation needed, not calibration
When It’s Axis Motion (Steps/mm)
If one axis consistently reads wrong, the steps/mm needs adjustment. Most modern printers have this calibrated from factory but it can drift. The formula: new_steps = old_steps × (commanded_distance / actual_distance). If you commanded 100mm and got 99.2mm: new_steps = old_steps × (100 / 99.2). Make this change in firmware or EEPROM, then verify with another cube before declaring it fixed.
Belts affect accuracy too. A loose X belt causes the print head to lag behind commanded positions on fast direction changes. Tighten belts until they twang at a consistent pitch, and retension equally on both sides of dual-belt axes.
When It’s Extrusion (Flow Rate)
If all walls measure thicker than expected, you’re over-extruding. If they measure thinner, under-extruding. Print a single-wall test (0.4mm nozzle, 0.4mm extrusion width, 1 perimeter) and measure the wall thickness with calipers. Target is your extrusion width. Adjust flow rate until it hits that target.
Don’t set e-steps using this test — use a direct 100mm extrusion test instead. Steps/mm and flow rate are separate adjustments for separate problems.
When It’s Thermal Shrinkage
ABS and ASA shrink 0.5–0.8% as they cool. A 100mm ABS part will measure 99.2–99.5mm. This isn’t a calibration error — it’s material physics. Either use horizontal expansion compensation in your slicer (+0.3 to +0.5mm for ABS on typical part sizes) or scale the model up before slicing.
PLA shrinks 0.2–0.3%. Usually insignificant. Nylon: 1–2% depending on grade and whether it’s dry — nylon’s moisture content affects dimensional stability significantly.
Holes Are a Special Case
Round holes always print smaller than the model diameter, regardless of how well-calibrated the printer is. This happens because the toolpath follows the inside of the circle, so extrusion width effectively reduces the hole diameter by approximately one extrusion width. The fix is to use the “hole horizontal expansion” setting in your slicer (typically +0.1 to +0.2mm) or design the hole slightly oversized in CAD.
What Not to Adjust
Don’t touch belt tension, steps/mm, and flow rate simultaneously — you won’t be able to tell which change fixed or broke what. Change one variable, print a verification cube, measure, then decide on the next adjustment.