Round holes print smaller than you drew them, reliably
A well-documented FDM quirk is that round holes, especially ones printed with their axis horizontal to the bed, tend to print measurably undersized and slightly out-of-round. The effect comes from how the extruder traces a circle in discrete line segments and how the first layer of an overhang-like hole ceiling settles slightly under gravity before it fully solidifies. A hole meant to clear a bolt or dowel can end up too tight if this is not accounted for at the design stage.
How the plan works
A default compensation allowance is applied based on orientation, larger for a horizontal-axis hole (more affected by the rounding effect) than a vertical-axis one. If you have measured your own printer's actual undersizing on a test piece, that value is used instead, since it reflects your specific machine and settings far better than a general starting figure.
Worked example
A 5 mm nominal hole, axis horizontal, no measured allowance: the tool suggests designing the hole at about 5.2 mm to compensate for typical undersizing in this orientation, and recommends confirming with a small calibration test since the exact effect varies by printer and material.
A common mistake
A frequent error is applying the same compensation value to every hole size on a part regardless of diameter. The rounding effect that causes undersizing does not scale perfectly linearly; a very small hole is often affected proportionally more than a large one. If a part has holes across a wide size range, treat the smallest ones as needing their own check rather than reusing one blanket allowance.
Limitations
This is a planning allowance based on typical FDM undersizing behavior, not a guaranteed tolerance for your specific printer, material, or hole size. For a precision fit, confirm on a test piece, or consider finishing the hole with a drill or reamer after printing rather than relying on a printed allowance alone.