GD&T Calculator

Quick Answer

• True Position Error = 2 × √(X² + Y²)
• Bonus Tolerance = Actual Size − MMC
• Total Allowed = Position Tolerance + Bonus

Core Formulas

True Position Error

TP = 2sqrt

MMC & LMC (Internal Feature)

MMC = D_, quad LMC = D_

MMC & LMC (External Feature)

MMC = D_, quad LMC = D_

Bonus Tolerance (MMC Modifier)

Bonus = D_ – D_

Total Position Tolerance Allowed

TP_ = TP_ + Bonus

How to Use

1. Select Feature Type
   • Internal (hole)
   • External (shaft)
2. Enter Nominal Size
3. Input Size Tolerance
   • Upper tolerance (+)
   • Lower tolerance (−)
4. Choose Modifier
   • MMC (Most Common)
   • LMC
   • RFS (Regardless of Feature Size)
5. Enter Listed Position Tolerance
6. Input Measured Values
   • X deviation
   • Y deviation
   • Actual measured size

👉 The calculator will output:

• True position tolerance
• Bonus tolerance
• Total allowed tolerance
• Position error
• Pass / Fail result

Engineering Meaning

Position Error

• Actual deviation of feature from true position
• Must be within allowed tolerance

Bonus Tolerance

• Extra tolerance gained when feature departs from MMC
• Only applies with MMC modifier

Total Allowed Tolerance

• Base tolerance + bonus
• Defines acceptance zone

Pass / Fail

• PASS → Part meets GD&T requirement
• FAIL → Part out of tolerance

Example

Nominal Size: 2.000 mm
Tolerance: ±0.005 mm
Measured Size: 2.004 mm
X = 0.009 mm
Y = 0.005 mm
Position tolerance = 0.014 mm

Step 1: Calculate Position Error

TP = 2sqrt

Step 2: Calculate Bonus

Bonus = 2.004 – 1.995 = 0.009

Step 3: Total Allowed

TP_ = 0.014 + 0.009 = 0.023

👉 Result: PASS

GD&T Modifier Explanation

MMC (Most Common)

• Tightest material condition
• Allows bonus tolerance

LMC

• Least material condition
• Used for clearance-critical parts

RFS

• No bonus tolerance
• Strict geometric control

Practical Engineering Tips

When to Use MMC

• Assembly fit critical
• Clearance needed
• Cost reduction via tolerance flexibility

Common Mistakes

• Forgetting factor of 2 in position formula
• Wrong MMC definition (hole vs shaft)
• Ignoring bonus tolerance

Inspection Tips

• Use CMM for accuracy
• Verify datum alignment
• Check actual size before position

Applications

• Precision holes in CNC machining
• Aerospace components
• Automotive assemblies
• Medical devices

FAQ

What is true position?

The distance between actual and theoretical position of a feature.

Why multiply by 2?

Because GD&T defines a diametrical tolerance zone.

What is bonus tolerance?

Extra tolerance gained when feature departs from MMC.

Does RFS allow bonus tolerance?

No.

What is the most common modifier?

MMC.