True Position Calculator (GD&T)
Actual true position from X/Y deviations plus MMC bonus tolerance — instant accept/reject against the feature control frame.
The factor of 2 trips up every newcomer: position tolerance defines a DIAMETER zone, so a radial miss of 0.05 mm uses 0.10 mm of tolerance. The MMC modifier is the escape hatch — every micron the hole grows past its smallest legal size is a micron of free extra position tolerance, because a bigger hole still swallows the mating fastener. That is why parts that look out on a naive X-Y check can be perfectly good.
Formula
Note: Bonus shown is for an internal feature (hole) with the Ⓜ modifier. RFS callouts get no bonus, and datum-shift from datum-feature modifiers is a separate allowance this simple check does not include.
Disclaimer: This tool is for general informational and estimation purposes only and is not professional financial, tax, accounting or legal advice. All figures are estimates — verify with a qualified professional before making decisions. Read the full disclaimer.
Actual true position from X/Y deviations plus MMC bonus tolerance — instant accept/reject against the feature control frame. A free machinist & fabrication essentials tool — no sign-up, no upload, instant results in your browser.
About True Position Calculator (GD&T)
True Position Calculator (GD&T) computes the governing relationship TP = 2·√(ΔX² + ΔY²) ; allowed = tol@MMC + (D_actual − D_MMC) live as you type. The factor of 2 trips up every newcomer: position tolerance defines a DIAMETER zone, so a radial miss of 0.05 mm uses 0.10 mm of tolerance. The MMC modifier is the escape hatch — every micron the hole grows past its smallest legal size is a micron of free extra position tolerance, because a bigger hole still swallows the mating fastener. That is why parts that look out on a naive X-Y check can be perfectly good. Defaults are pre-filled with realistic values for this exact scenario, and the worked example substitutes your numbers step by step so the math is never a black box.
How to use True Position Calculator (GD&T)
- 1Enter your values — X deviation from nominal, Y deviation from nominal, Position tolerance (at MMC), Hole MMC size (smallest) and more (sensible defaults are pre-filled).
- 2Read the live results: Actual true position, Bonus tolerance, Total allowed, Margin.
- 3Check the "with your numbers" line to see TP = 2·√(ΔX² + ΔY²) ; allowed = tol@MMC + (D_actual − D_MMC) substituted step by step.
- 4Adjust inputs until the scenario matches yours, then copy or share the result.
Why use True Position Calculator (GD&T)?
- ✓Instant, free and private — every calculation runs client-side in your browser; nothing is uploaded
- ✓Built on the stated formula TP = 2·√(ΔX² + ΔY²) ; allowed = tol@MMC + (D_actual − D_MMC) with authoritative sources cited on the page (ASME Y14.5-2018 — Dimensioning and Tolerancing; Machinery's Handbook, 31st ed.)
- ✓The factor of 2 trips up every newcomer: position tolerance defines a DIAMETER zone, so a radial miss of 0.05 mm uses 0.10 mm of tolerance.
- ✓SI ⇄ Imperial toggle converts your inputs in place, so you can work in the units your drawings use
Frequently asked questions
What formula does the true position calculator (gd&t) use?+
It evaluates TP = 2·√(ΔX² + ΔY²) ; allowed = tol@MMC + (D_actual − D_MMC), exactly as published. Sources: ASME Y14.5-2018 — Dimensioning and Tolerancing; Machinery's Handbook, 31st ed.. The substituted worked example on the page lets you verify every step against the textbook.
How should I read the result — and how far can I trust it?+
The factor of 2 trips up every newcomer: position tolerance defines a DIAMETER zone, so a radial miss of 0.05 mm uses 0.10 mm of tolerance. Bonus shown is for an internal feature (hole) with the Ⓜ modifier. RFS callouts get no bonus, and datum-shift from datum-feature modifiers is a separate allowance this simple check does not include.
When is this calculator the right tool for the job?+
Actual true position from X/Y deviations plus MMC bonus tolerance. The MMC modifier is the escape hatch — every micron the hole grows past its smallest legal size is a micron of free extra position tolerance, because a bigger hole still swallows the mating fastener. For neighbouring scenarios, the related tools below cover the same engine with different presets.
Does it support both metric and imperial units?+
Yes — the SI ⇄ Imperial toggle converts the values already in the fields, preserving the physical quantity, so you can flip mid-calculation without re-entering anything.
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