Flatness (⏥)

Flatness is a geometric tolerance that controls how flat a surface is, independent of its height or angle relative to any other feature.

What it means for the calculation

A tight flatness requirement, especially on a sealing or mounting face, usually needs a finishing pass (grinding, lapping or fly-cutting) after milling, since plain milling can leave a surface within a coarser flatness than the spec demands.

Where it appears on the drawing

In a feature control frame with the ⏥ symbol and a tolerance value; no datum reference, since flatness controls the surface's own form.

Common mistakes

Confusing flatness with parallelism (which does need a datum), or assuming a flatness callout also controls the height or position of the surface, which it does not.

Does flatness need a datum?

No, flatness only controls the surface relative to itself, with no reference to another feature.

Is flatness the same as parallelism?

No, parallelism also requires the surface to align with a datum plane; flatness only limits its own high-low variation.

Can a surface be flat but still out of position?

Yes, flatness says nothing about where the surface sits, only how flat it is.