GD&T for Buyers: 6 Key Symbols

First, the Mental Model

Plus/minus dimensions (±0.1) tell the shop how big a feature is. GD&T (Geometric Dimensioning and Tolerancing) tells the shop how a feature relates to other features — is the bore centred on the face, is the mating surface flat, does the pin run true. For parts that have to fit together, that relationship is usually what matters more than the absolute size. The full numeric reference is in our CNC tolerance chart; this page is about the symbols themselves.

The single most expensive mistake we see is calling GD&T on features that don't need it. So this guide tells you both when to use each symbol and when to skip it.

Symbol	Name	Controls	Use it when…
⌗ / ⊕	Position (True Position)	Where a feature's centre sits, vs datums	Bolt patterns, press fits, locating holes
▱	Flatness	How flat a single face is	A face must seal, mate or sit flush
◎	Concentricity / Runout	How true a feature is about an axis	Shafts, rotating parts, sealing seats
∥	Parallelism	One face parallel to a datum	Sliding surfaces, stack-ups
⊥	Perpendicularity	One face square to a datum	Bores square to a mounting face
⌀ in a frame	Circular Runout	Wobble of a surface as it turns	Sealing diameters, bearing seats

Note: symbols render as approximate glyphs here for readability; on a real drawing they are the ASME/ISO standard symbols.

1. Position (⊕) — the one you'll use most

This ties a feature's location to your datums instead of to a chain of plus/minus dims that accumulate error. For a four-hole bolt pattern, position lets all four holes share one tolerance zone referenced to the part's datums — so the pattern always fits the mating part even if each hole drifts a little. Call it on any hole pattern, pin location or press fit.

Skip it on a hole that's just for access or weight reduction. Position there buys nothing.

2. Flatness (▱) — pay for the face that matters

Flatness controls a single face. It's worth calling when that face has to seal, mate across its whole area, or sit against another part. It tells the shop "plane or grind this one, leave the rest as-machined" — which is exactly how you save money. Calling flatness on a face that only needs to look flat is wasted cost.

3. Concentricity / Runout (◎) — for anything that turns

If the part rotates, or a diameter has to seal, runout is what keeps it from wobbling or leaking. For a shaft, a sealing seat or a bearing bore, this is the symbol that earns its inspection cost. Our Ra chart pairs with this — a true-running seat still needs the right surface finish to seal.

4. Parallelism (∥) and Perpendicularity (⊥)

These control the angle relationship between two features referenced to a datum. Parallelism matters on sliding surfaces and in stack-ups; perpendicularity matters when a bore must be square to a mounting face. Use them where the relationship is functional, not decorative.

5. Circular Runout — the sealing-diameter check

For a sealing diameter or bearing seat that turns, circular runout is the practical control: it limits the wobble of that surface as the part rotates. It's cheaper to inspect than full concentricity and catches the same real-world problem for most buyers.

Datums: the part nobody explains

Every GD&T symbol references datums — the surfaces the part is measured from. The primary datum is the one that locates the part in its real assembly. Pick the wrong primary datum and the part measures fine but won't go together. A good shop confirms datums before cutting. If you're not sure which surface should be primary, say so on the RFQ — that conversation is free and prevents the expensive kind of scrap. Our cost-reduction guide covers why talking before the design is frozen pays back hardest.

Mistakes That Cost Money

Four GD&T Errors We See on Real Drawings

Mistake	What it costs	Better
GD&T on every feature	Full inspection on all of it	Call only fit/seal features; rest to ISO 2768
Concentricity where runout suffices	Harder, slower inspection	Use circular runout for turning seats
No datum callout	Shop guesses the reference	State primary/secondary/tertiary datum
Tight geo tol with loose size	Conflicting, scrap-prone	Make size and geo tol consistent

FAQ

GD&T Questions Buyers Ask

Do I need GD&T on every dimension?

No — and over-calling GD&T is one of the most common ways to pay more than you should. GD&T exists for the features that control fit, alignment or sealing. Everything else is fine at the ISO 2768 default. A drawing where every feature carries a geometric tolerance forces the shop to inspect everything to that level, which adds time and cost with no functional return. For the broader set of design rules, see our CNC machining design guide.

What is the difference between position and true position?

They are the same callout — 'true position' is the full name, ⊕ is the symbol. It controls where a feature's centre must sit within a tolerance zone relative to datums. For bolt-hole patterns and press fits it is the single most useful symbol, because it ties location to your reference surfaces instead of to a chain of plus/minus dims that accumulate.

When does calling flatness actually save money?

When a face has to seal, mate or sit against another part across its whole area. A flatness callout lets the shop know that face is the priority, so they can plane or grind it and verify it — while leaving non-critical faces at the cheaper as-machined finish. Calling flatness on a face that just needs to look flat is wasted money.

Why did my shop ask which datum is primary?

Because the datums define the coordinate system the part is measured in, and the primary datum is the surface everything is built from. Pick the wrong primary datum and the part measures fine but won't assemble; pick the one that actually locates the part in its mating situation and inspection becomes meaningful. A good shop will confirm datums before cutting, not after.

Unsure if your drawing is over- or under-specified?

Send it over. We'll flag any GD&T that's driving cost without adding function, and confirm the datums before we cut — not after you've paid for scrap.

Send Drawing for Review

New to GD&T terms? The CNC machining glossary defines every symbol and tolerance word in plain language.

GD&T for Buyers: The 6 Symbols That Actually Matter on a CNC Drawing