# Flatness

Interpreting planar flatness requirements and evaluating them without confusing datum orientation, sampled form, or restraint.

Reviewed: 2026-11-01

Tags: parts-and-design, specifications, metrology, gd-and-t

A surface flatness zone lies between two parallel planes separated by the stated tolerance. Evaluate the form without a datum reference. [KEYENCE, flatness and parallelism](https://www.keyence.com/products/3d-measure/roughness-measure/applications/a-method-for-highly-accurate-and-instantaneous-measurement-of-flatness.jsp)

Suppose a measured point set has signed normal deviations from a selected least-squares plane ranging from **−0.005 to +0.007 mm**. The enclosing width about that particular plane is `0.007 − (−0.005) = 0.012 mm`, or **12 µm**. It is a candidate enclosing width; it does not prove that 12 µm is the smallest attainable width after reorientation. Nor does it bound defects between sampled points. This example isolates geometry and omits instrument error, filtering, restraint and the decision rule.

## Establish the specification before measuring

Read the drawing or model's governing standard, edition, units, controlled extent, and supplementary indications. ASME lists **Y14.5-2018 (R2024)**; ISO lists **ISO 1101:2017**, edition 4, confirmed in 2022. Those catalog pages establish the applicable publications, not permission to transfer every default or modifier between systems. Reaffirmation or confirmation is not a newly rewritten technical edition. [ASME Y14.5](https://www.asme.org/codes-standards/find-codes-standards/y14-5-dimensioning-tolerancing/2018), [ISO 1101:2017](https://www.iso.org/standard/66777.html)

Distinguish a physical surface from a derived feature. In its February 12, 2021 teaching example, GD&T Basics contrasts a callout attached to a surface with one associated with a feature-of-size dimension: the latter controls the **derived median plane**, obtained from the opposing surfaces. Inspecting only one face does not establish that median feature's form. Conversely, controlling the median feature does not independently specify the flatness of each face. Resolve the attachment and any material-condition indication under the drawing's standard before choosing the measurement construction. [GD&T Basics, surface versus feature-of-size flatness](https://www.gdandtbasics.com/flatness-surface-vs-feature-of-size/)

This reference primarily addresses surface flatness. **ISO 12781-1:2011** explicitly scopes its vocabulary and parameters to individual complete integral features. It describes surface form independently of a datum and notes that extraction and filtering affect the specification operator. Do not use that surface-assessment scope as a substitute for interpreting every derived-feature callout. [ISO 12781-1:2011, introduction and scope](https://www.iso.org/obp/ui?_escaped_fragment_=iso%3Astd%3Aiso%3A12781%3A-1%3Aed-1%3Av1%3Aen)

## Flatness, parallelism, and surface finish

| Requirement | Question answered | Inspection implication |
| --- | --- | --- |
| Surface flatness | Can this surface fit between sufficiently close parallel planes? | Allow the enclosing zone to orient independently of other features. |
| Surface parallelism to a datum | Can the surface fit within a zone oriented parallel to the specified datum? | Establish the datum and preserve that orientation during evaluation. |
| Surface texture | What irregularities exist at the specified measurement scale? | Apply the specified texture parameter and filtering rather than relabeling a roughness result as flatness. |

The first two distinctions follow KEYENCE's description. The texture distinction matters because ISO 12781-1 warns that the extraction/filtering operator influences the feature being assessed. Neither a smooth appearance nor a low roughness value alone establishes whole-face flatness. [KEYENCE](https://www.keyence.com/products/3d-measure/roughness-measure/applications/a-method-for-highly-accurate-and-instantaneous-measurement-of-flatness.jsp), [ISO 12781-1](https://www.iso.org/obp/ui?_escaped_fragment_=iso%3Astd%3Aiso%3A12781%3A-1%3Aed-1%3Av1%3Aen)

A surface plate supplies a measurement reference; it does not automatically become a datum in a flatness requirement. Resting the opposite face on a plate and reporting the top face's height range can include the two faces' lack of parallelism. The setup and evaluation must separate rigid-body tilt from the target face's form. GD&T Basics illustrates the need to level the inspected face for an indicator-based surface-flatness evaluation. [GD&T Basics, flatness inspection](https://www.gdandtbasics.com/flatness/)

## Association and software settings

An enclosing minimum-zone solution seeks the smallest separation of parallel planes containing the evaluated surface data. A least-squares plane instead minimizes squared point deviations; the range of deviations about that plane need not equal the minimum-zone result. ISO 12781-1 distinguishes these reference constructions. Record which one produced the reported number. [ISO 12781-1, reference planes](https://www.iso.org/obp/ui?_escaped_fragment_=iso%3Astd%3Aiso%3A12781%3A-1%3Aed-1%3Av1%3Aen)

For a concrete implementation, **PC-DMIS 2023.2**, help dated August 30, 2023, offers DEFAULT minimum-zone and LSQ tolerance-zone math. Hexagon describes LSQ as producing a larger value than minimum-zone evaluation. That command accepts planar features with surface data and lists no allowed modifiers; these are implementation limits, not universal statements about all flatness specifications. Its per-unit option also requires unit size, shape, and orientation: a local requirement is not an average of the overall result. [Hexagon, PC-DMIS 2023.2 Flatness](https://docs.hexagonmi.com/pcdmis/2023.2/en/helpcenter/mergedProjects/core/geometric_tolerances/Flatness.htm)

## Sampling and the physical setup

A CMM or optical system evaluates acquired data, not every point on an ideal continuous surface. Hexagon cautions that point number, location, and measurement uncertainty affect the reported value. Sparse data can miss form, while measurement errors can also inflate the result. More displayed decimal places do not recover unmeasured regions. [Hexagon, measured value](https://docs.hexagonmi.com/pcdmis/2023.2/en/helpcenter/mergedProjects/core/geometric_tolerances/Flatness.htm)

NIST's **1992 NISTIR 4897** compares CMM flat-surface inspection plans and sample sizes against engineering tolerances. Its scope supports treating coverage as a measurement-planning problem, rather than selecting a universal point count. Include likely bow, twist, edge roll, and localized process marks in the plan. For example, four corner readings on a plate can all agree while a center bulge remains unmeasured; this is an illustrative sampling failure, not a prescribed five-point inspection. [NIST, comparative study](https://www.nist.gov/publications/comparative-study-cmm-flat-surface-inspection-plans)

Specify support and restraint as part of the inspection agreement. For a flexible face, clamping can change the geometry being evaluated. Distinguish a result in an agreed free or supported condition from a result under assembly restraint; do not flatten the part in a fixture and silently report that as its unrestrained condition. Record support positions, clamp/contact forces where relevant, cleanliness, stabilization, and measurement orientation. These are practical controls for reproducibility, not a universal support pattern or a replacement for drawing-specific restraint instructions.

## Original calculation: tilt is not flatness

Consider an ideal, perfectly planar rectangular face with coordinates in millimetres:

`z = 0.001 x`, for `0 ≤ x ≤ 100`.

Its height range relative to a horizontal measurement reference is `0.001 × 100 = 0.100 mm`. Nevertheless, its mathematical surface flatness is **zero**: two coincident planes can contain it. A horizontal-reference height-span check therefore cannot be relabeled as a 0.100 mm flatness error. A parallelism evaluation to a horizontal datum would retain that orientation difference.

## Information to retain with a result

Retain the controlled surface or derived-feature identity, governing edition, tolerance and units, measured extent, support condition, acquisition coverage, filtering, association method, software version, and reported uncertainty or decision rule. A useful result explains what was measured and how it was evaluated. A bare number labeled “flatness” cannot resolve disagreements caused by different features, fixtures, sampled regions, or fitting algorithms.

## Related reference

[Dimensions, tolerances and specifications](index.md)