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Composite Position Tolerance Calculator (PLTZF / FRTZF)

Check a composite position callout against both tiers — the upper PLTZF (pattern-locating, to the datums) and the lower FRTZF (feature-relating, feature-to-feature). The same true position ⌖ = 2·√(ΔX²+ΔY²) must pass both. Metric and imperial.

Inputs

mm
mm
mm
mm

Results

True position(⌖)⚠
0.2828mm

Fails the lower (FRTZF) tier.

Diameter of the tolerance zone the feature needs: ⌖ = 2·√(ΔX²+ΔY²).

PLTZF margin✓0.2172mm

Within the upper PLTZF tier.

PLTZF − true position. Positive (✓) passes the upper, pattern-locating tier.

FRTZF margin⚠−0.08284mm

Exceeds the lower FRTZF tier.

FRTZF − true position. Positive (✓) passes the lower, feature-relating tier.

  • A composite callout stacks two tiers: the upper PLTZF locates the whole pattern to the datums (larger tolerance); the lower FRTZF controls feature-to-feature spacing and orientation (tighter tolerance).
  • The same true position ⌖ = 2·√(ΔX²+ΔY²) is checked against both — the feature must pass BOTH tiers.
  • The lower (FRTZF) segment never controls location to the datums; any datums in the lower frame refine orientation only.

How it works

A composite position callout stacks two tolerance zone frameworks under one position symbol. The upper PLTZF (pattern-locating tolerance zone framework) locates the whole pattern to the datums with a larger tolerance; the lower FRTZF (feature-relating tolerance zone framework) controls feature-to-feature spacing and orientation with a tighter tolerance. The same true position is checked against both: ⌖ = 2 · √(ΔX² + ΔY²) where ΔX and ΔY are the deviations of the measured axis from its basic location. Because position tolerance is a cylindrical (diametral) zone, the radial deviation is doubled.

Each tier reports a margin of tolerance − true position: a non-negative margin passes that tier. The feature is acceptable only when it passes both — within the upper PLTZF and within the lower FRTZF.

Worked example

A feature is measured 0.1 mm off in X and 0.1 mm off in Y, so its true position is 2 × √(0.1² + 0.1²) ≈ 0.283 mm. Against a 0.5 mm PLTZF the margin is +0.217 mm — the pattern is comfortably located to the datums. But against the tighter 0.2 mm FRTZF the margin is −0.083 mm, so the feature fails the feature-relating tier: it sits where the pattern is allowed to be, but not as tightly spaced to its neighbours as the lower segment requires. The calculator shows exactly this.

Frequently asked questions

What is composite position tolerance?
Composite position tolerance is a single position callout with two stacked tiers: an upper pattern-locating tolerance zone framework (PLTZF) that locates the whole feature pattern to the datums with a larger tolerance, and a lower feature-relating tolerance zone framework (FRTZF) that controls how the features sit relative to each other with a tighter tolerance. A feature must satisfy both tiers.
What is the difference between PLTZF and FRTZF?
The PLTZF (upper segment) is the pattern-locating tolerance zone framework — it locates the entire pattern to the datums, and carries the larger tolerance. The FRTZF (lower segment) is the feature-relating tolerance zone framework — it is tighter and controls only the feature-to-feature spacing and orientation within the pattern.
Why does a composite callout use two tiers?
It lets you locate the pattern loosely to the datums while holding the features tightly to one another. Mating parts often care more about how the holes line up with each other than about exactly where the group sits, so the upper PLTZF can be generous while the lower FRTZF stays tight.
Does the lower (FRTZF) segment control location to the datums?
No. The lower segment never controls location to the datums — only orientation and feature-to-feature spacing. Any datums repeated in the lower frame refine orientation (parallelism, perpendicularity) of the zone framework only; the upper PLTZF remains the sole control of where the pattern is located.
How do I read a composite feature control frame?
A composite frame shares one position symbol for both rows. The top row gives the PLTZF: the larger tolerance plus the primary, secondary and tertiary datums that locate the pattern. The bottom row gives the FRTZF: the tighter tolerance plus, at most, the datums that orient the feature-relating zones — never enough to relocate them.
Does this work in metric and imperial?
Yes — toggle SI/imperial anytime. Deviations and tolerances convert; the math runs the same per ASME Y14.5 composite positional tolerancing.

Method & assumptions

  • Two-dimensional (X-Y) position from the measured deviation, per ASME Y14.5 composite positional tolerancing.
  • The lower (FRTZF) segment controls feature-to-feature orientation and spacing only — never location to the datums; datums in the lower frame refine orientation alone.
  • One feature is evaluated against both tiers; the upper PLTZF and lower FRTZF tolerances come straight from the two rows of the composite feature control frame.

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