True Position
Calculate true position deviation, position variation, bonus tolerance, and total tolerance for GD&T position tolerancing with MMC and LMC support. Free online engineering tool with offset summary and charts.
Feature Control Frame
True Position (Basic Dimensions)
Measured Dimensions
About This Calculator
Our true position calculator is a comprehensive GD&T (Geometric Dimensioning and Tolerancing) tool that helps engineers, machinists, and quality inspectors determine whether a manufactured part's feature lies within the acceptable tolerance zone. It computes the position variation between the true position specified in the engineering drawing and the actual measured position on the part, accounting for material conditions like MMC (Maximum Material Condition), LMC (Least Material Condition), or RFS (Regardless of Feature Size).
The core formula for position variation is PV = 2 × √((TB − MB)² + (TC − MC)²), where TB and TC are the basic dimensions from datums B and C describing the true position, and MB and MC are the measured dimensions on the actual part. When MMC or LMC is specified, a bonus tolerance is calculated based on the deviation of the actual feature size from its maximum or least material condition size, allowing more positional variation when the feature size provides additional clearance.
Position tolerancing creates a cylindrical tolerance zone around the true position, giving 57% more usable area than traditional coordinate tolerancing with square zones. This allows inspectors to accept more functional parts while maintaining design intent. The calculator outputs the offset from each datum, the overall position variation, bonus tolerance, total tolerance, and tolerance excess or deficit, helping you make quick pass/fail decisions on the shop floor.
Frequently Asked Questions
What is true position in GD&T?
True position is the exact, theoretically perfect location of a feature on a part as specified by basic dimensions in an engineering drawing. In GD&T, position tolerancing creates a cylindrical tolerance zone around the true position, defining an acceptable area where the actual feature center may be located.
How do you calculate true position deviation?
Position variation is calculated using the formula PV = 2 x sqrt((TB - MB)2 + (TC - MC)2), where TB and TC are the true position distances from datums B and C, and MB and MC are the measured distances on the actual part. The result is compared against the total position tolerance to determine if the part passes inspection.
What is the difference between MMC and LMC in true position?
MMC (Maximum Material Condition) gives bonus tolerance when the feature size deviates from its maximum material condition — for holes, this means a larger hole gets more positional tolerance. LMC (Least Material Condition) gives bonus tolerance when the feature size deviates from its least material condition — for holes, a smaller hole gets more tolerance. RFS (Regardless of Feature Size) applies no bonus tolerance.
What does the feature control frame tell you?
The feature control frame contains the GD&T position symbol, the diameter of the cylindrical tolerance zone, the material condition modifier (if any), and the datum references (primary, secondary, tertiary) that define the reference coordinate system for measurement.
How much bonus tolerance does MMC give?
The MMC bonus tolerance equals the absolute difference between the MMC size and the actual measured feature size. For example, if the MMC size of a hole is 0.502 in and the actual diameter is 0.508 in, the bonus tolerance is 0.006 in, which is added to the drawing position tolerance to determine the total allowable tolerance.
What is the difference between coordinate tolerancing and position tolerancing?
Coordinate tolerancing uses square or rectangular tolerance zones defined by X and Y tolerances, which create a diagonal error up to 1.4 times the specified tolerance. Position tolerancing uses a cylindrical tolerance zone specified by a diameter, giving 57% more usable area and equal tolerance in all directions, making it more likely to accept functional parts.
When should you use MMC condition for true position?
MMC is commonly used for clearance holes, bolt patterns, and mating features where a larger hole or smaller shaft provides more assembly freedom. It allows the part to pass inspection when the feature size is farther from MMC, reducing scrap rates while maintaining functional assembly requirements.
Why is true position always a diameter?
True position tolerance is specified as a diameter because the tolerance zone is a cylinder centered on the true position axis. This allows equal positional deviation in any radial direction, unlike coordinate tolerancing which allows different errors in X and Y directions. The diameter symbol in the feature control frame indicates this cylindrical tolerance zone.