How to use this calculator
- Probe the surface. Measure point heights across the surface relative to a best-fit or reference plane.
- Enter the readings. Enter up to five point heights; leave a point blank if unused, or enter just your highest and lowest readings.
- Enter the flatness tolerance. Enter the flatness tolerance from the feature control frame — the allowed width of the parallel-plane zone.
- Read pass/fail. Read the flatness (max − min), the highest and lowest points, and whether the surface is within tolerance.
How it works
Flatness is the total range of a surface — the gap between the two parallel planes that just contain every measured point: flatness = max(points) − min(points) The flatness tolerance zone is the space between those two parallel planes; the surface passes when every point lies inside it. Because flatness is a form control, it references no datum — it constrains the surface only to itself, so the result does not change with how the part is oriented.
Probe several points across the surface relative to a best-fit or reference plane and enter their heights. The flatness is simply the difference between the highest and lowest readings; the surface is acceptable when that difference is within the flatness tolerance.
Worked example
Verified against the live calculator
Five points are probed across a surface at heights of
0, 0.02, 0.05, 0.01, 0.03 mm. The highest is 0.05 mm and the lowest is
0 mm, so the flatness is 0.05 − 0 = 0.05 mm — comfortably within a
0.10 mm flatness tolerance, so it passes. The calculator returns
exactly this.
Frequently asked questions
How do you calculate flatness?
Flatness is the total range of your measured surface heights: flatness = highest point − lowest point. Probe several points across the surface relative to a best-fit or reference plane; the difference between the maximum and minimum reading is the flatness.
What is the flatness tolerance zone?
It is the space between two parallel planes. The surface is flat enough when every point lies between those two planes, and the distance between the planes equals the flatness tolerance from the feature control frame.
Is flatness related to a datum?
No. Flatness is a form control, so it is datum-independent — there is no datum reference in a flatness callout. It only constrains the surface to itself, not its orientation or location relative to anything else.
How many points should I measure?
More points give a better picture of the surface — a CMM may take hundreds. This calculator takes up to five; if you only have the extremes, enter your highest and lowest readings, since flatness depends only on the range.
What is the difference between flatness and parallelism?
Flatness controls a single surface to itself and needs no datum. Parallelism is an orientation control: it holds a surface parallel to a datum, so it references a datum and also limits how the surface is angled, not just its form.
Does it work in metric and imperial?
Yes — enter the point heights and the flatness tolerance in mm or inches, and the flatness, highest and lowest values are reported in the same units. Toggle SI/imperial anytime.
Method & assumptions
- Enter point heights relative to a best-fit or reference plane; flatness depends only on their range (max − min), not the chosen zero.
- With only a few points the true flatness may be underestimated — the more of the surface you sample, the more reliable the result.
- CMM software fits a minimum-zone (or least-squares) plane; this uses the simple range of the points you enter, which matches the minimum zone in the limit of dense sampling.