See the Tolerance Stack-Up calculator to combine several toleranced dimensions into a worst-case or statistical stack.
GDT. 01
True Position
Controls where a feature (almost always a hole) is allowed to fall relative to datums, located with basic (boxed, untoleranced) dimensions. The tolerance zone is a cylinder centered on the true position — the feature's actual axis must land somewhere inside it, in any direction.
⊕
⌀0.014
Ⓜ
A
B
C
⌀.008"Tight
⌀.014"Standard
⌀.020"Loose
A ⌀.014 position tolerance is roughly the same as a ±.005" tolerance under old-style plus/minus coordinate dimensioning. The circular zone circumscribes the corners of that square zone, so it holds about 57% more usable area for the same worst-case deviation (gdandtbasics.com) — divide the position diameter by ≈2.8 to get the equivalent bilateral (±) value per axis. Same ratio gives ⌀.008 ≈ ±.003" and ⌀.020 ≈ ±.007".
GDT. 02
Profile of a Surface
A catch-all, 3D tolerance zone that follows a surface's contour — two offset surfaces, one on each side of the true (basic) profile, that the actual surface must stay between. Works on flat faces, curves, or complex surfaces alike, and can locate/orient a surface when tied to datums.
⌓
0.003
A
B
.003"Typical
Equally disposed by default (±.0015" about the true profile) unless flagged unequal with the "U in a circle" modifier. gdandtbasics.com doesn't publish a standard number here — casting/sheet-metal surfaces commonly run looser (.010–.030"), while sealing or mating faces run tighter.
GDT. 03
Flatness
Controls how flat one surface is, on its own — no datum needed. The entire surface must lie between two parallel planes spaced the tolerance value apart. It's a form control, so it only tightens the surface itself, never its location.
▱
0.003
.003"Typical
gdandtbasics.com's only rule of thumb: flatness should be about half the associated size tolerance, and always looser than the size tolerance, never tighter. .003" is a reasonable default for a standard machined face; tighten toward .001–.0005" for a gasket or optical seating surface.
GDT. 04
Perpendicularity
Controls a 90° relationship to a datum. On a surface, the zone is two parallel planes, perpendicular to the datum, that the face must lie between. On an axis (a hole or pin, called out with ø), the zone is a cylinder instead.
⊥
0.005
A
.005"Typical
.005" is a common default for a standard perpendicular face or pin/hole axis off a primary datum. Tighten for a locating shoulder or bearing bore; loosen for a non-mating face.
