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GD&T Basics for Machinists

Guide · Updated October 28, 2026

Geometric dimensioning and tolerancing (GD&T, ASME Y14.5) controls form, orientation, location and runout. On the floor you mostly need to read a feature control frame, find the datums, and know how each tolerance is checked.

Set check heights fast

Many GD&T checks start with a gage stack on the surface plate. Build it free with the Gage Block Calculator.

Step by step

Find the feature control frame: The box attached to a feature lists the symbol, the tolerance zone size, any modifier like MMC, and the datums.
Identify the datums: Datum letters (A, B, C) tell you how to hold the part: A is primary (most contact), then B, then C.
Read the symbol: Form: flatness, straightness, circularity, cylindricity. Orientation: perpendicularity, parallelism, angularity. Location: position. Profile of a line or surface. Circular and total runout.
Note the zone shape: A diameter symbol before the tolerance means a cylindrical zone (common with position); without it the zone is usually two parallel planes or lines.
Check modifiers: M in a circle (maximum material condition) allows bonus tolerance as the feature departs from MMC; L is least material; no symbol means regardless of feature size.
Choose the check: Flatness and parallelism on a surface plate with an indicator; perpendicularity against a square; position from measured X/Y; runout with the part rotating on its datum axis.

Worked example

A hole is called out at X 2.000, Y 1.000 from datums B and C, with position ⌀0.010 to A|B|C. The measured hole center is X 2.003, Y 0.9985.

Deviations: ΔX = 0.003, ΔY = −0.0015. Position = 2 × √(0.003² + 0.0015²) = 2 × 0.00335 = ⌀0.0067, inside the ⌀0.010 zone, so the hole is good.

If the frame had ⌀0.010 at MMC and the hole (MMC 0.250) measured 0.253, you would get 0.003 bonus: allowed ⌀0.013.

Symbols at a glance

⏥ flatness · ⊥ perpendicularity · ∥ parallelism · ∠ angularity · ⌖ position · ○ circularity · ⌭ cylindricity · ⌓ profile of a surface · ⌒ profile of a line · ↗ circular runout · ⌰ total runout. Concentricity and symmetry were removed in the 2018 standard; older drawings may still show them.

Tips

Start with the title block and notes: blueprint reading basics.
Perpendicularity checks: how to check squareness.
Flatness and parallelism: flatness and parallelism with gage blocks.
Runout: runout and TIR basics.
Fits are size, not geometry: use the ISO 286 fit calculator for limits.

FAQ

What is a feature control frame?

The rectangular box that holds a GD&T symbol, tolerance value, modifiers and datum references for one feature.

How do you calculate true position?

Position = 2 × √(ΔX² + ΔY²) from the nominal location, compared to the diameter of the tolerance zone.

What does MMC mean in GD&T?

Maximum material condition: the smallest hole or largest pin. With the MMC modifier you gain bonus tolerance as the feature moves away from MMC.

What is the difference between runout and total runout?

Circular runout checks each cross-section separately; total runout checks the whole surface at once as the indicator travels along it.

Is there a free gage block calculator?

Yes. The Gage Block Calculator at gageblockcalc.com builds any stack, free with no login.

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Questions? Ask a machinist.

General shop information only — follow your tooling, machine and gage manufacturers' instructions and your quality system.