Tolerance & Clearance Calculator
Model a hole against a known part size — a bearing, a dowel, a screw shaft — and get the diameter to actually print for the fit you want.
The known size of the part the hole needs to fit — a bearing's outer diameter, a dowel, a bolt shaft.
Why the hole gets sized up, not the part
Print holes come out slightly undersized and printed pegs/pins come out slightly oversized on most FDM printers, a side effect of how the nozzle traces and slightly over-extrudes at each corner. Rather than fighting that by trying to compensate a shaft's diameter, it's more reliable to model the hole a bit larger than nominal and leave the mating part at its real, known measurement — that's what these offsets are calibrated for.
These numbers are a starting point, not a law
Every printer's dimensional accuracy is a little different, and calibration (extrusion multiplier, actual nozzle wear, belt tension) shifts these numbers directly. If a press fit is consistently too tight or too loose on your specific printer, that's worth adjusting for permanently rather than re-guessing it each time — print a quick test set of holes at a few offsets once, measure which one actually presses in the way you want, and use that offset going forward.