Every printer owner has lived it: the slicer promises 3 hours, the print takes 5. The slicer is not lying — it is modeling a printer slightly better than yours. The drift sources are known, and knowing them converts frustration into calibration.
The drift sources
- Acceleration: small models spend most of their time accelerating — a slicer assuming instant velocity overestimates speed on every short move. Marlin/Klipper acceleration tuning narrows the gap.
- Perimeter slowdown: quality profiles run outer walls at 15–30 mm/s regardless of the headline speed — detailed prints are wall-dominated, not infill-dominated.
- Layer-change overhead: Z-hops, retractions, and layer swaps add seconds per layer × hundreds of layers.
- Firmware limits: max speeds your printer physically honors cap everything the slicer asks.
Calibrating your own factor
- Record ten prints: slicer estimate vs actual time.
- Compute actual ÷ estimated — the average is YOUR printer's drift factor (typically 1.1–1.4).
- Apply it to future estimates, or feed the acceleration settings into the slicer and watch the factor fall.
- Re-run the time calculator as the independent check — mass and speed do not drift.
The planning consequence
Trust drift-corrected estimates for scheduling: "starts at 9 pm, done by 7 am" only works when the factor is known. Un-calibrated estimates fail in one direction (optimistic), which is how a 6-hour print becomes an 11-hour surprise that collides with the morning.
The cost calculator inherits the same discipline: power cost scales with true hours, not slicer hours. Calibrate once, and both numbers become trustworthy.