GD and T for buyers: the four symbols that do most of the work
July 31, 2026
GD and T, geometric dimensioning and tolerancing, is a language for saying what a part has to do rather than only how big it is. Buyers do not need to write it fluently. Knowing four symbols and one concept is enough to read a drawing and to have a useful conversation about it.
The concept: datums
A datum is a reference. It says “measure from here”. Most parts need three, usually a primary face, a secondary edge or hole and a tertiary feature, which together lock the part in space the way it sits in your assembly.
The single most valuable thing you can add to a mediocre drawing is a datum scheme that matches how the part is used. If the part bolts to a plate on face A and locates on two dowel holes B and C, then A, B and C are your datums, and every functional tolerance should reference them.
Without datums, everything else is guesswork dressed up as precision.
Position
Position is the workhorse. It says a feature must lie within a circular zone of a stated diameter relative to the datums.
Position 0.1 mm to A, B, C
That means the axis of the hole must sit inside a 0.1 mm diameter cylinder centred on the theoretically exact location. Compare that with a linear ±0.05 mm in X and Y, which creates a square zone and is both harder to inspect and less forgiving in the corners.
Position also accepts modifiers. Maximum material condition, written as an M in a circle, allows extra positional tolerance when the hole is larger than its smallest allowed size. On clearance holes this is free money: the assembly still works and the shop gets more room.
Flatness, parallelism, perpendicularity
These control form and orientation.
- Flatness applies to one surface on its own. It does not reference a datum.
- Parallelism controls one surface relative to a datum surface.
- Perpendicularity does the same at 90 degrees.
They matter on mounting faces, sealing faces and anything that will be clamped. A flatness callout on a large thin part is a serious request, because the part can be flat on the machine and relax after unclamping. If you need flatness on a thin plate, say whether it is measured free standing or constrained.
Profile
Profile of a surface is the most general control. It creates a tolerance band that follows the shape of the surface as modelled. On curved or organic geometry it is often the only sensible control, and it plays well with parts that are inspected by CMM against the model.
If your part is mostly 3D surfaces, a profile callout plus a datum scheme communicates more than dozens of linear dimensions.
Runout, for anything that spins
Circular and total runout control how much a surface wobbles relative to a datum axis. If your part carries a bearing, a seal or a pulley, runout is the callout that says what you actually care about. It is easy for a shop to measure with an indicator and a pair of centres, so it is a low friction request.
What GD and T asks the shop to do
Each callout implies a measurement method, and that is where cost comes from:
| Callout | Usual measurement | Inspection effort |
|---|---|---|
| Position on holes | CMM, or a gauge for high volume | Moderate |
| Flatness | Surface plate and indicator, or CMM | Moderate |
| Parallelism | Indicator over a surface plate | Low |
| Runout | Indicator between centres | Low |
| Profile | CMM against the model | Higher |
A shop with no CMM can still make a part with position callouts. It cannot prove it the way you may expect. That is the question to ask before the order, not after.
Three practical habits
- Reference the same datums throughout. Mixed datum schemes on one drawing are the main source of confusion.
- Use maximum material condition on clearance holes. It costs nothing functionally and widens the supplier pool.
- Ask for the inspection plan. Not a full report, just which features the shop intends to measure and how. It takes one email and tells you whether your intent came through.
Translating function into callouts
Most buyers know what the part has to do and are unsure how to say it. These four translations cover a large share of real parts.
| What you mean | What to write |
|---|---|
| This face bolts flat against a plate | Flatness on the face, and it becomes datum A |
| These four holes must line up with a mating pattern | Position, diameter 0.2 mm, to A, B, C, at maximum material condition |
| This bore takes a bearing and must not wobble | Diameter with a fit, plus circular runout to the shaft axis |
| This curved surface must match the model | Profile of a surface, 0.2 mm, to A, B, C |
Notice what is missing: long lists of linear dimensions with tight bands. Functional callouts are usually fewer and looser, and they give the shop room where the function allows it.
Three mistakes that cost money
Tolerancing the same feature twice. A hole with a linear position and a position callout is over-defined, and the shop has to satisfy both. Pick one.
Datums that do not match assembly. If the part sits on a face but the drawing datums from an edge, the parts can pass inspection and still rock in the fixture.
Profile everywhere. Profile is powerful and it implies CMM inspection. Applying it to surfaces that nobody cares about turns a simple part into a metrology exercise.
The tolerances guide covers the linear side of the same subject, and the guide on first article inspection covers what the shop does with these callouts once parts exist. When your drawing is marked up, send it for quotes.
Questions we get on this
Do I need GD and T on every drawing?
No. Simple parts are fine with linear dimensions and a general tolerance note. GD and T earns its place when features have to relate to each other, when a part mounts to something else or when you want a tolerance that follows function rather than a box.
What happens if my drawing has no datums?
The shop chooses them, usually whichever faces are convenient to clamp and measure from. That may be the opposite of what your assembly needs, and the parts can be within tolerance on paper and still not fit.
Does GD and T make parts more expensive?
It makes inspection more explicit, which can add time. It often makes parts cheaper overall, because position tolerance with material condition modifiers gives the shop more room on features where the function allows it.
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