GD&T (Geometric Dimensioning and Tolerancing) is a system for defining and communicating engineering tolerances using symbolic language on engineering drawings. It is governed by ISO 1101 (international) and ASME Y14.5 (US).

What are the categories of geometric tolerances?

There are four categories: Form (flatness, straightness, circularity, cylindricity), Orientation (perpendicularity, angularity, parallelism), Location (position, concentricity, symmetry), and Runout (circular runout, total runout).

What is a datum in GD&T?

A datum is a theoretically exact geometric reference point, line, or plane from which toleranced features are located. Datums are identified by letters (A, B, C) in a datum reference frame.

What does MMC and LMC mean in GD&T?

MMC (Maximum Material Condition) is when a feature contains the maximum amount of material (smallest hole, largest shaft). LMC (Least Material Condition) is the opposite. Bonus tolerance may apply at MMC/LMC per ISO 1101.

How do I read a feature control frame?

Free Engineering Tool

GD&T Tolerance Calculator

ISO 1101 geometric dimensioning and tolerancing reference. Select tolerance type, enter value, and set datum frame to see symbol, tolerance zone, and inspection method.

ISO 1101 14 Tolerance Types ASME Y14.5

Tolerance Type

Tolerance Value (mm)

Material Condition Modifier

Datum Frame (e.g. A|B|C)

Quick presets

GD&T Reference

Feature Control Frame

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Tolerance Zone Shape

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Geometric Tolerance Types Summary

Symbol	Name	Category	Datum?	Zone
⏥	Flatness	Form	No	Two parallel planes
⏤	Straightness	Form	No	Two parallel lines / cylinder
○	Circularity	Form	No	Annular ring
⌭	Cylindricity	Form	No	Two coaxial cylinders
⊥	Perpendicularity	Orientation	Yes	Two parallel planes / cylinder
∠	Angularity	Orientation	Yes	Two parallel planes
∥	Parallelism	Orientation	Yes	Two parallel planes / cylinder
⌖	Position	Location	Yes	Cylindrical / between planes
◎	Concentricity	Location	Yes	Cylindrical
↗	Circular Runout	Runout	Yes	Two coaxial circles
↗↗	Total Runout	Runout	Yes	Two coaxial cylinders
⌒	Profile of a Line	Profile	Optional	Two equidistant curves
⌓	Profile of a Surface	Profile	Optional	Two equidistant surfaces

Tip: Form tolerances (flatness, straightness, circularity, cylindricity) never require a datum reference. Orientation and location tolerances always require at least one datum.

GD&T Tolerance Calculator | ISO 1101 | Free Online Tool

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026

GD&T Reference

Geometric Tolerance Types Summary