Calculate GD&T bonus tolerance, feature size at MMC, or actual feature size for internal and external features in mm, cm, or inches.
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Bonus Tolerance Formula
Bonus tolerance is the extra geometric tolerance gained when an actual feature size departs from maximum material condition, or MMC. The formula depends on whether the feature is internal or external.
Internal feature formula
Use this for holes, slots, and other internal features. For an internal feature, MMC is the smallest allowed size.
External feature formula
Use this for pins, shafts, tabs, and other external features. For an external feature, MMC is the largest allowed size.
- BT = bonus tolerance
- AV = actual feature size
- MMC = feature size at maximum material condition
The calculator uses the selected feature type to decide which subtraction order is valid. For an internal feature, the actual size must be greater than or equal to the MMC size. For an external feature, the actual size must be less than or equal to the MMC size.
- To calculate bonus tolerance: enter the MMC size and actual feature size, then leave bonus tolerance blank.
- To calculate feature size at MMC: enter the actual feature size and bonus tolerance, then leave feature size at MMC blank.
- To calculate actual feature size: enter the MMC size and bonus tolerance, then leave actual feature size blank.
Internal and External Feature Reference
| Feature type | Common examples | MMC condition | Bonus tolerance occurs when |
|---|---|---|---|
| Internal feature | Hole, slot, pocket width | Smallest size | Actual size is larger than MMC |
| External feature | Pin, shaft, tab width | Largest size | Actual size is smaller than MMC |
Unit Conversion Reference
| Unit | Equivalent in millimeters | Use note |
|---|---|---|
| 1 mm | 1 mm | Base metric unit |
| 1 cm | 10 mm | Useful for larger metric dimensions |
| 1 in | 25.4 mm | Use when the drawing is dimensioned in inches |
Example Problems
Example 1: Internal feature
A hole has an MMC size of 10.000 mm. The actual measured hole size is 10.120 mm.
The bonus tolerance is 0.120 mm.
Example 2: External feature
A pin has an MMC size of 0.500 in. The actual measured pin size is 0.497 in.
The bonus tolerance is 0.003 in.
FAQ
What does bonus tolerance mean in GD&T?
Bonus tolerance is extra geometric tolerance allowed when a feature moves away from MMC. For example, a hole at its smallest allowed size has no bonus tolerance. As the hole gets larger, it gains bonus tolerance equal to the amount it departs from MMC.
Is bonus tolerance the same as total tolerance?
No. Bonus tolerance is only the added tolerance from MMC departure. If a feature control frame already gives a geometric tolerance at MMC, the total available tolerance is usually the stated geometric tolerance plus the bonus tolerance.
Why can bonus tolerance not be negative?
A negative result means the actual size is on the wrong side of MMC for the selected feature type. For an internal feature, the actual size cannot be smaller than the MMC size and still create bonus tolerance. For an external feature, the actual size cannot be larger than the MMC size and still create bonus tolerance.
