GD&T Symbol Guide
Flatness
Controls surface waviness or bow by requiring all points of a surface to fit between two parallel planes.
Definition
Flatness is a form control. Every measured point on the controlled surface must fit inside a tolerance zone bounded by two parallel planes.
It refines the shape of one surface without establishing a datum reference frame.
Application
Use flatness on sealing faces, wear pads, fixture bases, and mating surfaces where rocking, leaks, or uneven contact are the concern.
It is useful when the surface must be smooth and stable, but its orientation to another feature is not the functional requirement.
Sealing Plate With Controlled Top Surface
Flatness controls only the top surface form. It does not force the surface to be parallel to the bottom face or to any datum.
3D Tolerance Zone
Two parallel planes separated by the flatness tolerance value.
Inspection Method
Collect points across the full length and width of the surface. A CMM or surface plate setup can evaluate the smallest pair of parallel planes enclosing those points.
Do not clamp the part to a datum and call that flatness; once another surface is used for orientation, you are closer to checking parallelism.
Worked Check: Sealing Face
A pump cover has a top sealing face with flatness 0.08 mm. A scan of the surface shows the highest point is +0.031 mm and the lowest point is -0.026 mm.
Form spread
0.031 - (-0.026) = 0.057 mm
The surface variation is the distance between the two enclosing planes.
Compare
0.057 <= 0.08
The complete surface fits inside the allowed slab.
Functional read
Pass
The surface may still be tilted; flatness does not control orientation.
The sealing face passes the flatness requirement. Add parallelism only if the face must also be oriented to another datum.
Comparison Table
| Control | Controls | Uses Datum | Common Mistake |
|---|---|---|---|
| Flatness | Surface form | No | Treating it like parallelism |
| Straightness | Line element form | No | Sampling only one direction |
| Parallelism | Orientation to a datum | Yes | Using it when only waviness matters |
Notes
Flatness must normally be tighter than the associated size tolerance if it is applied to a feature of size.
Flatness is the surface version of straightness: straightness uses two lines, while flatness uses two planes.