GD&T symbols chart
The fourteen geometric characteristics of GD&T (Geometric Dimensioning and Tolerancing), grouped by category, with what each one controls and whether it needs a datum. Use it to read a feature control frame on an engineering drawing.
Geometric characteristics
| Symbol | Characteristic | Category | Controls | Datum |
|---|---|---|---|---|
| Straightness | Form | Straightness of a line element or axis | Not needed | |
| Flatness | Form | How flat a surface is | Not needed | |
| Circularity | Form | Roundness of each cross-section | Not needed | |
| Cylindricity | Form | Combined roundness and straightness of a cylinder | Not needed | |
| Profile of a Line | Profile | Cross-sectional shape of a feature | Optional | |
| Profile of a Surface | Profile | Three-dimensional shape of a surface | Optional | |
| Angularity | Orientation | A feature at a set angle to a datum | Required | |
| Perpendicularity | Orientation | A feature 90 degrees to a datum | Required | |
| Parallelism | Orientation | A feature parallel to a datum | Required | |
| Position | Location | Location of a feature relative to datums | Required | |
| Concentricity | Location | Median points centered on a datum axis | Required | |
| Symmetry | Location | Median points symmetric about a datum | Required | |
| Circular Runout | Runout | Variation at one cross-section during rotation | Required | |
| Total Runout | Runout | Variation across the whole surface during rotation | Required |
Common modifiers
| Modifier | Symbol | Meaning |
|---|---|---|
| Maximum material condition (MMC) | Tolerance applies at the most material, largest pin or smallest hole | |
| Least material condition (LMC) | Tolerance applies at the least material condition | |
| Regardless of feature size (RFS) | — | Default: the tolerance is fixed regardless of feature size |
| Projected tolerance zone | Tolerance zone projected above the surface, used for fasteners |
A feature control frame reads left to right: the geometric symbol, the tolerance value with any modifier, then the datum references. Form controls (straightness, flatness, circularity, cylindricity) need no datum because they control a feature against itself; orientation, location, and runout controls reference one or more datums.
Specifying the surface itself?
Pair tolerances with a surface callout — see the Surface Finish Chart for Ra values and process ranges.
How to read a feature control frame
A feature control frame is the boxed symbol on a drawing that carries a GD&T callout. The first compartment holds the geometric characteristic, the second holds the tolerance zone size and any material-condition modifier, and the remaining compartments list the datum features in order of precedence. Reading them in order tells you exactly how the feature is allowed to vary.
Form, orientation, location, and runout
The fourteen controls fall into families. Form controls limit the shape of a single feature. Orientation controls set the angle of a feature to a datum. Location controls fix where a feature sits, with position being the workhorse. Runout controls limit wobble as a part spins about a datum axis.
FAQ
How many GD&T symbols are there?
Fourteen geometric characteristics, split into form, profile, orientation, location, and runout categories, plus modifiers like maximum material condition.
Which GD&T controls do not need a datum?
The four form controls — straightness, flatness, circularity, and cylindricity — reference the feature against itself, so they need no datum. Profile can be used with or without one.
What does the circled M mean in GD&T?
It is the maximum material condition modifier: the stated tolerance applies when the feature has the most material, and extra tolerance is gained as the feature departs from that size.