Flatness is a GD&T form control that keeps every point of a surface between two parallel planes set the tolerance value apart. It needs no datum, and the planes are free to tilt to suit the surface, so it controls only how flat the face is, not its angle.
Flatness answers one question: if you could press two perfect, parallel plates against the high and low points of this face, how far apart would they be? It catches bowing, twist, dish and local bumps, all at once, over the whole surface.
Because the planes float, flatness says nothing about whether the face is parallel or square to anything else. A seal face can be perfectly flat and still be tilted 2° to the bore. If the face must also sit true to another surface, add parallelism, perpendicularity or profile with datums.
Flatness is common on sealing faces, mounting pads, gasket faces and the face that becomes datum A, where a rocking part would spoil every later measurement.
The tolerance zone is the space between two parallel planes, the tolerance value apart. All points of the surface must lie in that space. The planes can be oriented in any way that makes the zone fit, which is why a tilted but flat face still passes.
When flatness is applied to a feature of size such as a plate thickness, it controls the derived median plane instead (the plane through the mid-points between the two faces) and the zone is again two parallel planes.
Flatness never references a datum. It is checked against the surface itself. A flat face is often then used as datum A for the other controls on the drawing.
| Modifier | With flatness |
|---|---|
| MMC or LMC | Only for flatness of a derived median plane, where the frame sits under the thickness dimension. Surface flatness cannot take a modifier. |
| Unit area | A per-area value (for example 0.02 per 25 × 25) can be added below or after the overall value to stop sharp local bumps on a large face. |
| Free state | Used for thin sheet or plastic parts that are measured unrestrained. |
Example: a 120 × 80 mm mounting face with a flatness of 0.1 mm
Result: 0.081 mm is within 0.1 mm, so the face passes flatness.
Every GD&T symbol page: GD&T symbols chart
A parallelogram: a four-sided shape with two horizontal sides and two slanted sides. Do not confuse it with parallelism, which is two slanted parallel lines.
No. Flatness is a form control. It compares the surface only with a pair of floating parallel planes, so a datum reference would be a drafting error.
Flatness lets the two planes tilt freely and controls only the shape of the face. Parallelism holds the planes parallel to a datum, so it controls shape and tilt together, and it is always at least as strict as flatness.
Not reliably. Face down, the part rests on its three highest points and you cannot see the low areas underneath. Support it on jacks and measure the face, or use a CMM.
An extra, tighter limit that applies to any square of a stated size on the face, such as 0.02 mm in any 25 × 25 mm area. It stops a sharp local bump that the overall flatness value would allow.
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