---
type: "gdt-symbol"
name: "Diameter"
symbol: "⌀"
unicode: "U+2300"
unicode_name: "DIAMETER SIGN"
category: "modifier"
category_name: "Modifiers and material conditions"
datum_required: "never"
aliases: ["diameter symbol gdt", "phi symbol"]
standard: "ASME Y14.5 (cited, not reproduced); ISO 1101"
verified: "August 1, 2026"
url: https://shopmath.org/gdt/diameter
---

# Diameter (⌀) — GD&T Modifier Symbol and Meaning

Inside a feature control frame this makes the tolerance zone cylindrical rather than a pair of parallel planes. Its presence on a position callout is the difference between a round zone and a slab zone, and therefore between a hole pattern that gauges and one that does not.

- **Symbol**: ⌀ (Unicode U+2300)
- **Category**: Modifiers and material conditions
- **Datum reference**: never references a datum
- **Standard**: ASME Y14.5 (cited, not reproduced); ISO 1101

## Diameter at a Glance

| Property | Value |
| --- | --- |
| Characteristic | Diameter |
| Symbol | ⌀ |
| Unicode codepoint | U+2300 |
| Unicode name | DIAMETER SIGN |
| Category | Modifiers and material conditions |
| Datum reference | never references a datum |
| Standard | ASME Y14.5 (cited, not reproduced); ISO 1101 |

## How Diameter Behaves

Diameter belongs to the modifiers and material conditions group. Change how a geometric tolerance is interpreted or verified.

## Other Modifiers and material conditions Controls

The controls in the same category — the ones a drawing is choosing between when it specifies diameter.

| Control | Symbol | Tolerance zone or effect | Datum |
| --- | --- | --- | --- |
| [Maximum Material Condition (MMC)](https://shopmath.org/gdt/maximum-material-condition.md) | Ⓜ | States that the stated tolerance applies when the feature contains the most material — the largest pin or the smallest hole. | never references a datum |
| [Least Material Condition (LMC)](https://shopmath.org/gdt/least-material-condition.md) | Ⓛ | The mirror of MMC: the stated tolerance applies when the feature contains the least material — the smallest pin or the largest hole. | never references a datum |
| [Regardless of Feature Size (RFS)](https://shopmath.org/gdt/regardless-of-feature-size.md) | Ⓢ | The default condition since 1994: the geometric tolerance applies at whatever size the feature is actually produced, with no bonus. | never references a datum |
| [Projected Tolerance Zone](https://shopmath.org/gdt/projected-tolerance-zone.md) | Ⓟ | Zone projected above the feature | never references a datum |
| [Free State](https://shopmath.org/gdt/free-state.md) | Ⓕ | Says the requirement is to be verified with no restraining force applied. | never references a datum |
| [Tangent Plane](https://shopmath.org/gdt/tangent-plane.md) | Ⓣ | Evaluates an orientation control against a plane resting on the high points of the surface rather than against every point of it. | never references a datum |
| [Unequally Disposed Profile](https://shopmath.org/gdt/unequally-disposed-profile.md) | Ⓤ | Profile band split unevenly about the true profile | never references a datum |

## Sources and Method for Diameter

### Diameter definition

- **Source**: Symbols and codepoints: Unicode Character Database (Unicode License v3). Characteristic list cross-checked against NASA MSFC GD&T Basics (public domain). Descriptions written from scratch; no ASME Y14.5 text reproduced.
- **Method**: compiled
- **Verified against**: Unicode Character Database codepoint check; 98/98 structural checks passed

### Assumptions

- Descriptions are original prose. No ASME Y14.5 definitional text, worked example or figure is reproduced — the standard is cited, not copied.
- Symbol glyphs and codepoints are from the Unicode Character Database; a drawing may render the same characteristic with a different font.
- Dataset scope: EXCLUDED: all ASME Y14.5 definitional text, worked examples and figures. Every description on this site is original prose.

**Data last verified**: August 1, 2026

## Common Questions About Diameter in GD&T

### What does Diameter mean in GD&T?

Inside a feature control frame this makes the tolerance zone cylindrical rather than a pair of parallel planes. Its presence on a position callout is the difference between a round zone and a slab zone, and therefore between a hole pattern that gauges and one that does not.

### What is the GD&T symbol for diameter?

Diameter uses the ⌀ symbol — Unicode U+2300 (DIAMETER SIGN). It sits in the first compartment of the feature control frame.

### Does Diameter need a datum?

Diameter never references a datum. It qualifies how another control is interpreted rather than establishing a tolerance zone of its own.

## Related pages

- Previous control: [Unequally Disposed Profile](https://shopmath.org/gdt/unequally-disposed-profile.md) (Ⓤ)
- All symbols: [GD&T symbol reference](https://shopmath.org/gdt.md)
- Tolerances: [ISO 286 fit reference](https://shopmath.org/fits.md)

---

*Source: [ShopMath](https://shopmath.org/gdt/diameter) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*