What is the difference between flatness and parallelism?

Flatness is a form control — the surface is compared to itself with no datum, so it only limits how much the surface deviates from a plane. Parallelism is an orientation control that measures deviation from a datum plane, so it also constrains the angle. A surface can be perfectly flat and still not be parallel to anything.

When is a datum required?

Always, except for form controls. Straightness, flatness, circularity and cylindricity are self-referencing and take no datum. Profile, orientation, location and runout all reference at least one datum.

What does MMC mean in GD&T?

Maximum Material Condition — the state where the feature contains the most material: the largest shaft or the smallest hole. Applied to a position tolerance, it grants bonus tolerance as the feature departs from MMC, which is why a hole at MMC gets only the stated tolerance while a larger hole gets more.

What is the difference between circular and total runout?

Circular runout is measured on one cross-section during rotation and detects out-of-roundness and eccentricity. Total runout is measured across the entire surface during rotation, so it also catches taper, bow and axial variation. Total runout is the stricter control.

What is true position?

The theoretically exact location of a feature, defined by basic dimensions from the datums. A position tolerance defines a zone centred on true position within which the feature's axis or centre must lie.

What does the circled M symbol mean?

Maximum Material Condition. Placed after a tolerance in a feature control frame, it makes the tolerance apply at MMC and grants bonus tolerance as the feature departs from it. It can also follow a datum letter to allow datum shift.

GD&T Symbols Chart

Every symbol in ASME Y14.5 — the five categories of geometric control and the modifiers that change how a tolerance applies.

Data verified 2026-09-29 · based on ASME Y14.5-2018

Quick Answer

GD&T has five categories of control: form (no datum needed), profile, orientation, location and runout. Knowing the category tells you whether a datum reference is required — form controls never need one, everything else does.

Find a symbol

Geometric Dimensioning and Tolerancing Symbols

Symbol^[5]	Name^[5]	Category^[5]	What it controls^[5]
⏤ #	Straightness	Form	How much a line element deviates from a perfectly straight line. Applied to a surface or an axis.
⏥ #	Flatness	Form	How much a surface deviates from a perfectly flat plane. No datum is required — it is self-referencing.
○ #	Circularity (Roundness)	Form	How much a cross-section deviates from a perfect circle. Also called roundness.
⌭ #	Cylindricity	Form	Combined roundness and straightness of a cylindrical surface — a single control covering both.
⌒ #	Profile of a Line	Profile	Deviation of a cross-sectional line from its true profile. Usually used on curved surfaces.
⌓ #	Profile of a Surface	Profile	Deviation of a whole surface from its true profile. The most powerful control, and usually the most expensive to inspect.
∥ #	Parallelism	Orientation	How much a feature deviates from being parallel to a datum. Requires a datum.
⊥ #	Perpendicularity	Orientation	How much a feature deviates from 90° to a datum. Requires a datum.
∠ #	Angularity	Orientation	How much a feature deviates from a specified angle to a datum.
⌖ #	Position	Location	Deviation of a feature's location from its true position. The most widely used geometric control — it defines a tolerance zone centred on true position.
◎ #	Concentricity	Location	How much the median points of a feature deviate from a datum axis. Rarely used — position or runout usually controls the same thing more robustly.
⌯ #	Symmetry	Location	How much the median points of a feature deviate from a datum plane or axis.
⌰ #	Total Runout	Runout	Composite control applied over the entire surface during one revolution — covers both circular runout and any axial variation.
⌭ #	Circular Runout	Runout	Deviation of a single cross-section during one full revolution about a datum axis.
⌀ #	Diameter	Modifier	Placed before a dimension to indicate a diameter.
Ⓢ #	Regardless of Feature Size (RFS)	Modifier	The default condition — the tolerance applies regardless of the feature's actual size. No symbol is required; it is implied.
Ⓜ #	Maximum Material Condition (MMC)	Modifier	The tolerance applies when the feature is at its maximum material size (largest shaft, smallest hole). Adds bonus tolerance as the feature departs from MMC.
Ⓛ #	Least Material Condition (LMC)	Modifier	The tolerance applies at minimum material size. Used mainly to guarantee a minimum wall thickness.
Ⓟ #	Projected Tolerance Zone	Modifier	Extends the tolerance zone beyond the feature, used for press-fit or threaded holes where a mating part occupies the zone.
⌮ #	Free State	Modifier	The tolerance applies to the part in its free, unrestrained condition — used for thin or flexible parts.
Ⓣ #	Tangent Plane	Modifier	The tolerance zone is bounded by a plane tangent to the high points of the surface.
Ⓕ #	Free State (alternate)	Modifier	Alternate symbol for the free state condition.
↗ #	Statistical Tolerance	Modifier	Indicates a statistical tolerance applies. Rare in practice.
⌴ #	Counterbore	Feature	Indicates a counterbored hole.
⌵ #	Countersink	Feature	Indicates a countersunk hole.
↧ #	Depth	Feature	Indicates a depth dimension.
⌴ #	Spotface	Feature	Indicates a spotfaced surface.

Symbols are grouped by category. Form controls are self-referencing and take no datum; profile, orientation, location and runout all require a datum reference frame.

The Five Categories

The category determines what a control can and cannot do, and whether a datum is needed:

Form — straightness, flatness, circularity, cylindricity. Self-referencing: the feature is compared against itself, so no datum is required. Form tolerance must always be tighter than the size tolerance.
Profile — profile of a line and of a surface. Controls a feature against a true profile, and can control size, form, orientation and location simultaneously. Requires a datum when locating.
Orientation — parallelism, perpendicularity, angularity. Controls the angle of a feature relative to a datum, but not its location.
Location — position, concentricity, symmetry. Controls where a feature is. Position is by far the most used, and defines a cylindrical or rectangular tolerance zone centred on true position.
Runout — circular and total. Composite controls measured by rotating the part about a datum axis, covering form, orientation and location together.

Material Condition Modifiers

These change how much tolerance is available depending on the feature actual size:

RFS (regardless of feature size) — the default. The stated tolerance applies whatever the feature measures.
MMC (maximum material condition) — the tolerance applies when the feature has the most material (largest shaft, smallest hole). As the feature departs from MMC, bonus tolerance opens up by the same amount. Widely used on bolt holes because it makes assembly easier without loosening the part.
LMC (least material condition) — tolerance applies at minimum material. Used mainly to guarantee a minimum wall thickness.

MMC is the modifier that most often confuses people reading a drawing: a position callout at MMC does not mean one fixed tolerance — the acceptable deviation grows as the hole gets larger.

Frequently Asked Questions

What is the difference between flatness and parallelism?: Flatness is a form control — the surface is compared to itself with no datum, so it only limits how much the surface deviates from a plane. Parallelism is an orientation control that measures deviation from a datum plane, so it also constrains the angle. A surface can be perfectly flat and still not be parallel to anything.
When is a datum required?: Always, except for form controls. Straightness, flatness, circularity and cylindricity are self-referencing and take no datum. Profile, orientation, location and runout all reference at least one datum.
What does MMC mean in GD&T?: Maximum Material Condition — the state where the feature contains the most material: the largest shaft or the smallest hole. Applied to a position tolerance, it grants bonus tolerance as the feature departs from MMC, which is why a hole at MMC gets only the stated tolerance while a larger hole gets more.
What is the difference between circular and total runout?: Circular runout is measured on one cross-section during rotation and detects out-of-roundness and eccentricity. Total runout is measured across the entire surface during rotation, so it also catches taper, bow and axial variation. Total runout is the stricter control.
What is true position?: The theoretically exact location of a feature, defined by basic dimensions from the datums. A position tolerance defines a zone centred on true position within which the feature's axis or centre must lie.
What does the circled M symbol mean?: Maximum Material Condition. Placed after a tolerance in a feature control frame, it makes the tolerance apply at MMC and grants bonus tolerance as the feature departs from it. It can also follow a datum letter to allow datum shift.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	Source	Type	Revision / method
[1]	ASME B4.1 — Preferred Limits and Fits for Cylindrical Parts	standard	ASME B4.1-1967 (R2022)
[2]	ASME B94.11M — Twist Drills	standard	ASME B94.11M-1993 (R2017) — source
[3]	Values computed from the defining relationship	derived	Decimal inch values are the defining dimensions of each drill series; millimetre values are inch × 25.4. Fraction-to-decimal values are exact arithmetic (numerator ÷ denominator).
[4]	Standard twist drill size series (number, letter and fractional)	standard	n/a — long-established industry series, standardised in ASME B94.11M — source
[5]	ASME Y14.5 — Dimensioning and Tolerancing	standard	ASME Y14.5-2018 — source

Data Sources

Standard	Revision	What it covers on this page
ASME Y14.5 — Dimensioning and Tolerancing	ASME Y14.5-2018	every symbol, its category and its meaning on this page

Cross-checked against:

Symbol set and categories cross-checked against ASME Y14.5 training material
The rule that form controls require no datum verified against the standard's definition of self-referencing controls

This page explains what each symbol controls. It does not cover how to inspect a control, how to calculate bonus tolerance, or the rules for composite feature control frames — those are in the standard, and GD&T is a field where a training course pays for itself.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "GD&T Symbols." https://refmanual.com/charts/gdt-symbols/ (accessed 2026-09-29). Data from ASME Y14.5 — Dimensioning and Tolerancing, ASME Y14.5-2018.

BibTeX

@misc{gdt-symbols,
  title        = {GD&T Symbols},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/gdt-symbols/}},
  note         = {Accessed: 2026-09-29},
  source       = {ASME Y14.5 — Dimensioning and Tolerancing, ASME Y14.5-2018}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - GD&T Symbols
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/gdt-symbols/
PY  - 2026
DA  - 2026-09-29
N1  - Source: ASME Y14.5 — Dimensioning and Tolerancing, ASME Y14.5-2018
ER  -

Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.