GD&T Calculator

Bonus Tolerance

Enter feature size limits and the stated geometric tolerance at MMC or LMC. Returns bonus tolerance, virtual condition, resultant condition, and a breakdown across the full size range.

Feature Type
Modifier
Units
Size Input
in
in
in
The tolerance value in the feature control frame
For labeling — math is the same for all

Enter feature size limits and geometric tolerance

How Bonus Tolerance Works

The one rule
Bonus = Departure from MMC (or LMC)

When a geometric tolerance is modified with Ⓜ (MMC) or Ⓛ (LMC), the tolerance zone grows as the feature departs from the specified material condition. The additional tolerance equals the departure — no formula beyond subtraction.

MMC and LMC by feature type
Internal feature (hole) MMC = smallest hole (lower limit). LMC = largest hole (upper limit). A hole at MMC has the most material remaining in the part.
External feature (shaft) MMC = largest shaft (upper limit). LMC = smallest shaft (lower limit). A shaft at MMC has the most material on the part.
Virtual and resultant conditions
Virtual Condition (VC) The constant boundary generated by the collective effect of a feature's specified material condition and its geometric tolerance. For MMC, this is the tightest boundary (smallest hole or largest shaft). For LMC, it's the opposite (largest hole or smallest shaft). Functional gauging checks this boundary.
Resultant Condition (RC) The variable boundary at the opposite extreme — the worst-case loosest fit (MMC modifier) or tightest fit (LMC modifier) when the feature is at maximum departure with full bonus tolerance applied.
VC (hole at MMC) = MMC − geo tol For an internal feature modified at MMC, the virtual condition is the smallest size minus the stated tolerance. This is the gauge pin diameter.
VC (shaft at MMC) = MMC + geo tol For an external feature modified at MMC, the virtual condition is the largest size plus the stated tolerance. This is the gauge hole diameter.
Why it matters

Bonus tolerance is free — it doesn't cost anything in manufacturing and it doesn't compromise function. A bolt pattern with Ø.500 +.010/-.000 holes and .010 positional tolerance at MMC gets .010 total positional tolerance at the smallest hole, but .020 total at the largest hole. The assembly still works because the virtual condition (the functional gauge) stays fixed.

Inspectors who ignore the Ⓜ modifier reject good parts. Every feature produced away from MMC earns bonus tolerance that the print already allows. This calculator makes that visible.

Per ASME Y14.5-2018. Bonus tolerance applies to position, perpendicularity, and parallelism when modified at MMC or LMC. Does not apply at RFS (no modifier), which is the default condition. Concentricity and symmetry were removed in the 2018 revision but still appear on legacy drawings per Y14.5-2009.

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