True Position - GD&T Position Tolerance Check
Use this true position calculator to check a measured feature against its nominal location per ASME Y14.5 with MMC, LMC, and RFS modifiers.
True Position
Results
What Is True Position?
A true position calculator is a quick way to check whether the measured center of a hole, pin, or slot falls inside the cylindrical tolerance zone from the drawing. True position is the GD&T geometric tolerance that locates a feature relative to its true location, and it controls where fasteners and mating parts sit on a machined part.
- • Inspect a hole on a CNC bracket: Measure the X and Y with a CMM, then compare against the drawing's basic dimensions and stated tolerance.
- • Verify a pattern of bolt holes: Check several holes in a flange or hub against the same nominal pattern, especially when bonus tolerance from MMC is allowed.
- • Read a feature control frame on a print: Convert a callout like |pos tol 0.3 MMC|A|B|C into numeric pass or fail.
- • Document a first-article inspection report: Generate a clear deviation breakdown for QA paperwork.
ASME Y14.5 defines true position as a cylindrical tolerance zone around the theoretically exact point or axis. The actual feature axis must lie inside that zone, otherwise the part fails the GD&T requirement, even if the dimensions are within size limits. The result is a single pass or fail value plus deviations that show how much room is left.
When the same inspection covers a whole pattern of holes, the deviations feed straight into a Bolt Circle Calculator to lay out the rest of the holes relative to the same nominal pattern.
How the True Position Formula Works
The calculator takes the measured coordinates and the basic (nominal) coordinates from the drawing, computes the radial deviation, and multiplies it by 2 to get the diameter of the cylindrical tolerance zone.
- dx: Signed deviation in X, equal to actual X minus nominal X.
- dy: Signed deviation in Y, equal to actual Y minus nominal Y.
- radial deviation: sqrt(dx^2 + dy^2). The straight-line distance from nominal to actual.
- true position: 2 * radial deviation. The diameter of the cylindrical tolerance zone.
- bonus tolerance: Extra tolerance when the actual feature size departs from MMC or LMC. Equals 0 under RFS.
- total allowed: Stated position tolerance plus bonus tolerance. The feature passes if the true position value is less than or equal to this total.
The factor of 2 reflects the fact that GD&T position tolerance is given as a diameter, not a radius. A radial deviation of 0.1 mm corresponds to a 0.2 mm cylindrical zone. Bonus tolerance only applies when the frame carries an MMC or LMC modifier. Under the default RFS (Regardless of Feature Size) modifier, the stated tolerance is the only budget.
Worked example: a hole 0.05 mm off in X and 0.10 mm off in Y
Actual X = 25.05, Y = 12.10, Nominal X = 25.00, Y = 12.00, Position tolerance = 0.30 mm, Modifier = MMC, Actual size = 6.05, MMC = 6.00, LMC = 6.08.
dx = 0.05, dy = 0.10. Radial = sqrt(0.05^2 + 0.10^2) = 0.1118 mm. True position = 2 * 0.1118 = 0.2236 mm.
True position = 0.2236 mm. Bonus tolerance = 0.05. Total allowed = 0.35 mm. 0.2236 <= 0.35, so the feature passes.
Even with a 0.1118 mm drift, the bonus from the slightly oversized hole keeps the part inside the allowed zone.
According to ASME Y14.5, true position is the diameter of the cylindrical tolerance zone within which the axis or center plane of a feature must lie.
When the inspection drives a clearance decision for a mating fastener, the measured feature size pairs with a Clearance Hole Calculator to confirm the fastener still fits.
Key Concepts in True Position Checking
These four concepts come up on every GD&T drawing that uses true position. A good true position calculator lets the user apply them without a manual table lookup.
Cylindrical tolerance zone
The imaginary cylinder of stated diameter around the basic location. The actual feature axis must lie inside this cylinder for the callout to pass.
Basic dimensions and datums
Boxed numbers on a drawing that give the theoretically exact location of a feature, such as 25.00 and 12.00. The position tolerance frame controls how far the actual feature can drift from those numbers.
MMC and LMC
MMC is the size with the most material, so the smallest hole or the largest shaft. LMC is the opposite. Both are written in a circle next to the tolerance value in the feature control frame.
RFS - Regardless of Feature Size
RFS is the default when no modifier symbol appears next to the tolerance value. Under RFS the actual feature size does not change the allowed tolerance.
Reading the feature control frame correctly is half the work of true position. The format is always a tolerance value, then an optional material condition modifier, then the datum references in priority order, for example |0.3 (M) | A | B | C |.
When the true position callout is on a threaded hole, the feature's basic size and limits can be cross-checked against a Pitch Diameter Calculator so the pitch diameter and tolerance zone stay consistent with the position tolerance.
How to Use the True Position Calculator
Run the calculator once per measured feature. The defaults are loaded for a typical M6 clearance hole, so the page shows a passing case on first load.
- 1 Use mm throughout: All fields are in millimeters, the most common unit for GD&T. For an inch drawing, convert the values to mm before typing, and convert the result back to inches to compare against the print.
- 2 Enter actual X and Y: Type the measured coordinates of the feature axis or center point. For a single hole this comes from a CMM report; for a manual layout it can come from a height gauge or comparator.
- 3 Enter nominal X and Y: Read the boxed basic dimensions off the drawing. These are the theoretically exact location of the feature.
- 4 Type the position tolerance diameter: Take the numeric value from the feature control frame. The number is a diameter, so it goes directly into the field without dividing by 2.
- 5 Add feature size, MMC, and LMC: Enter the measured feature size, the MMC size, and the LMC size. These drive the bonus tolerance when the modifier is MMC or LMC.
- 6 Choose the modifier: Pick MMC, LMC, or RFS to match the symbol on the feature control frame. The result panel updates the bonus tolerance and the total allowed tolerance.
Inspect a flange with four M6 clearance holes. Pick the corner hole, type the measured coordinates, type the 25.00 and 12.00 mm basic dimensions, type 0.30 mm for the position tolerance, and pick MMC if the frame carries the M-in-a-circle modifier. The result panel shows the deviations, the true position value, and a clear pass or fail.
When the same hole is countersunk for a flush fastener, the measured coordinates of the countersink edge can be checked with a Countersink Depth Calculator alongside the true position callout for the through hole.
Benefits of an Automated True Position Check
A good true position calculator avoids the most common errors in true position checking: dropping the factor of 2, ignoring the modifier, or mixing the cylindrical diameter with a radial budget.
- • Catches the diameter-vs-radius mistake: Multiplies the radial deviation by 2 automatically, so the value compared against the feature control frame is always the cylindrical diameter.
- • Applies bonus tolerance correctly: Reads the actual feature size and the MMC or LMC limit, then adds the difference to the stated tolerance only when the modifier calls for it.
- • Works for holes, pins, and slots: Uses the same 2 * sqrt(dx^2 + dy^2) formula for any feature with a 2D axis or center point.
- • Speeds up first-article reports: Returns the deviations and pass or fail in a format a quality team can paste into a first-article inspection.
- • Supports RFS, MMC, and LMC in one tool: Lets the user switch the modifier without re-entering the whole problem.
When the inspected feature is a threaded hole, the measured pitch and tolerance can be cross-checked with a Thread Calculator to confirm the limit dimensions match the GD&T callouts.
Factors That Affect the True Position Result
A clean pass or fail depends on more than plugging numbers into a formula. These factors are the most common reasons a real measurement disagrees with a calculator output.
CMM probe calibration and datum alignment
Probe error and datum alignment errors add to dx and dy. A 0.01 mm probe error alone can move a tightly toleranced hole from pass to fail.
Thermal expansion between drawing and inspection
A 25 mm nominal feature on a steel part moves about 0.003 mm per degree Celsius, which is enough to change a tight callout.
Modifier symbol and feature size limits
The MMC and LMC values must come from the feature size limit, not from the measured size. A drawing that calls out 6.00 to 6.08 has MMC = 6.00.
Whether the callout is RFS or MMC
RFS removes the bonus tolerance entirely, so the same deviation can pass under MMC and fail under RFS. Always check the feature control frame symbol.
Datum reference frame priority
A, B, and C in the feature control frame are read in order. Skipping the primary datum can give a different coordinate.
- • The calculator handles a single 2D feature. Multi-feature composite position tolerances from the ASME Y14.5 composite position callouts are not modeled here.
- • Datum alignment error is not subtracted from the result. The numbers come straight from the CMM or layout report.
- • True position is part of GD&T, not a replacement for size, form, or orientation tolerancing. A part that passes true position can still fail size, flatness, perpendicularity, or runout.
A good practice is to record the input values into the QA report rather than only the pass or fail, so the inspection can be re-checked later.
According to GD&T Basics, the bonus tolerance for position equals the difference between the measured feature size and the MMC size, and a feature that departs from MMC earns extra tolerance that can be used up by position drift.
When the true position result drives a clearance decision for a bolted joint, the same fastener size and grade feed into a Bolt Torque Calculator to size the torque that holds the assembly together.
Frequently Asked Questions
Q: What is true position in GD&T?
A: True position is the geometric tolerance that locates a feature relative to its basic (theoretically exact) location. ASME Y14.5 defines it as the diameter of an imaginary cylindrical zone within which the actual feature axis or center point must lie.
Q: What is the true position formula?
A: True position (diameter) equals 2 * sqrt(dx^2 + dy^2), where dx is the deviation in X between actual and nominal coordinates and dy is the deviation in Y. The result is compared directly against the position tolerance value in the feature control frame.
Q: What is the difference between true position and tolerance?
A: Tolerance is the general word for any allowed variation. True position is a specific geometric tolerance defined by ASME Y14.5, expressed as the diameter of a cylindrical zone. The pass or fail is checked against that diameter, not against a generic tolerance band.
Q: How do you calculate true position for a hole?
A: Measure the X and Y coordinates of the hole axis with a CMM or comparator, subtract the basic X and Y from the drawing to get dx and dy, then compute 2 * sqrt(dx^2 + dy^2). Compare the result against the position tolerance plus any bonus tolerance from the MMC or LMC modifier.
Q: What is bonus tolerance in true position?
A: Bonus tolerance is the extra tolerance the feature is allowed when it departs from MMC or LMC. Under the MMC modifier it equals the difference between the actual feature size and the MMC size. Under the LMC modifier it equals the difference between the LMC size and the actual feature size. RFS removes the bonus entirely.
Q: What is the difference between RFS and MMC in true position?
A: RFS (Regardless of Feature Size) keeps the stated position tolerance fixed no matter what the actual feature size measures. MMC (Maximum Material Condition) lets the tolerance grow as the actual feature size departs from MMC, giving the part a bonus that can be used up by position drift.