GD&T Approximation Tool
GD&T plus-minus tolerance approximation converter
Convert common GD&T tolerance zones into rough plus-minus values for explanation, review discussions, and intuition.
Reference only. This converter is not exactly correct, and it is not a substitute for GD&T standards, drawing authority, metrology planning, or tolerance analysis. It is only an approximation to make GD&T tolerances simpler to understand.
Live converter
Approximate GD&T zones as simpler plus-minus values
Choose a control type, enter the tolerance value, and use the result as a plain-language reference. Position controls include simplified MMC and LMC bonus tolerance examples for holes and pins.
Reference approximation only
This tool is intentionally not exact. It does not replace ASME Y14.5, ISO GPS requirements, datum simulation, inspection planning, tolerance stack analysis, or a released drawing interpretation. Use it only as a rough way to make GD&T tolerance zones easier to understand.
Position with bonus
Adds a simplified MMC bonus tolerance before converting the resulting position diameter into plus/minus approximations.
Generated feature control frame
⌖ | ⌀0.20 Ⓜ | A | B | CUses the same symbol set as the GD&T PNG builder. Actual feature size affects the simplified bonus result, not the stated tolerance compartment.
Bonus tolerance
0.08 mm
Simplified departure from MMC based on actual feature size.
Simplified total diameter zone
0.28 mm
Stated tolerance plus simplified bonus tolerance, when a material condition is selected.
One-direction plus/minus intuition
+/- 0.14 mm
Half the diameter zone. Useful for intuition along one direction only.
Conservative X/Y plus/minus box
+/- 0.09899 mm
A square coordinate box that fits inside the circular or cylindrical zone.
Examples
Approximation rules
What the converter is simplifying
These rules are intentionally simplified. They can be useful when explaining a tolerance zone to someone who thinks in plus-minus values, but they do not preserve the full meaning of the GD&T control.
| GD&T control | Approximation | What it misses |
|---|---|---|
| Position at RFS | Diameter zone / 2 for one-direction intuition, or diameter zone / (2 * sqrt(2)) for a conservative X/Y coordinate box. | True position uses a circular or cylindrical zone, not a square bilateral coordinate tolerance. |
| Position at MMC | Stated position tolerance plus simplified departure from MMC, then the same position approximation. | This ignores datum mobility, virtual condition details, projected tolerance zones, composite position, and inspection strategy. |
| Position at LMC | Stated position tolerance plus simplified departure from LMC, then the same position approximation. | LMC is often about minimum wall or stock protection. A plus-minus value does not capture that design intent. |
| Profile | Total profile tolerance / 2 as a normal-direction plus/minus value when the zone is assumed centered. | Profile may be unequally disposed, all-around, datum-related, composite, or applied across complex surfaces. |
| Perpendicularity, axis | Treat the cylindrical zone like position: diameter / 2 for one direction or diameter / (2 * sqrt(2)) for conservative X/Y intuition. | This does not model the actual inspection axis, datum simulator, or full 3D orientation requirement. |
| Perpendicularity, surface | Total zone / 2 for centered offset intuition, plus atan(total zone / evaluation length) for a rough tilt visualization. | The angle is only a teaching aid. Perpendicularity is controlled by a tolerance zone, not by a simple angular note. |
Why approximate?
Plus-minus values can help explain scale, but not design intent
GD&T is usually better than plus-minus coordinate tolerancing because it describes tolerance zones, datums, material conditions, orientation, location, and functional requirements more directly. The approximation here is only a translation aid for conversation.
For example, a position tolerance creates a circular or cylindrical tolerance zone. A plus-minus X/Y coordinate tolerance creates a square zone. That means there is no exact universal conversion. The converter shows both a simple half-zone value and a more conservative coordinate-box value to make that tradeoff visible.
FAQ
GD&T plus-minus approximation questions
Is this a correct way to convert GD&T to plus-minus tolerances?
No. This is explicitly not an exact or standards-compliant conversion. It is a reference approximation to help people understand the rough size of GD&T tolerance zones.
Why is position not the same as plus-minus X and Y?
Position commonly controls a circular or cylindrical tolerance zone. A plus-minus X/Y coordinate tolerance creates a square zone. The converter shows a one-direction half-zone and a conservative X/Y box, but neither fully represents true position.
How does the MMC example calculate bonus tolerance?
The simplified MMC example adds departure from MMC to the stated tolerance. For a hole, a larger actual hole adds bonus. For a pin, a smaller actual pin adds bonus. Real drawings can involve more requirements than this simplified calculation.
How does the LMC example calculate bonus tolerance?
The simplified LMC example adds departure from LMC to the stated tolerance. For a hole, a smaller actual hole departs from LMC. For a pin, a larger actual pin departs from LMC.
Can this be used for released drawings or inspection plans?
No. Use the controlling standard, your company drawing practices, a qualified GD&T reviewer, and the actual inspection method for released requirements. This tool is only for explanation and rough review intuition.