Diameter symbol

A circle with a diagonal stroke, marking a dimension as a diameter.

Also known as: Circle with a slash, Zero with a line

Sometimes confused with: Phi symbol

Engineering & technicalMathematicalStandard profile

Meanings

⌀ before a number means the dimension is a diameter, not a radius or a width. It is a distinct character from the empty set and from the Scandinavian Ø, and substituting them is common and wrong.

Common modern usageVerified fact
States that the dimension it precedes is a diameter measured across a circular feature, not a radius and not a width.
Contexts: Technical drawing
Common modern usageVerified fact
Is a distinct character, U+2300 DIAMETER SIGN, from the empty set, the Scandinavian letter Ø and the Greek phi — all of which are routinely substituted for it.
Contexts: Technical drawing, Software
Common modern usageVerified fact
Is written before the value rather than after, so a reader knows what kind of quantity is coming before they read the number.
Contexts: Technical drawing

Visual description

A circle crossed by a single straight diagonal stroke running from lower left to upper right and projecting slightly beyond the circle at both ends.

Symmetry: point
Orientation: upright

Commonly confused with: Empty set sign
Near-identical characters from unrelated fields, substituted for each other constantly.
Used alongside: Radius symbol
The two ways of dimensioning a circular feature.

Compare: Diameter symbol (⌀) vs empty set (∅) →

Confidence

IdentityHigh confidence: High confidence because the identity is documented in encyclopedic references, but multiple independent sources agree.
Historical originModerate confidence: Moderate confidence because the historical origin is documented in encyclopedic references, but multiple independent sources agree, but early or supporting evidence is limited, but the regional or cultural match to this specific claim is not precise.
Current useHigh confidence: High confidence because the current use is documented in encyclopedic references, but multiple independent sources agree.
Visual matchHigh confidence: High confidence because the visual match is documented in encyclopedic references, but multiple independent sources agree.

Sources

Engineering drawing
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol instructs and which standard governs it. Where a standard is named it is reported by this source rather than read, so no meaning is typed `official` on that basis.
ISO 128, Technical drawings — General principles of presentation
International Organization for Standardization · retrieved 2026-08-01
Paywalled and NOT READ. Registered so records can state which standard governs a symbol — a checkable fact a reader can act on — and for nothing else. It governs line types, views, sections and projection conventions. No meaning in any record is typed `official` on the strength of this entry. See docs/paywalled-standards.md.
Technical drawing
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol instructs and which standard governs it. Where a standard is named it is reported by this source rather than read, so no meaning is typed `official` on that basis.

Last reviewed 2026-07-31

Part of

Technical drawing notation

Help improve this page

Meanings

⌀ before a number means the dimension is a diameter, not a radius or a width. It is a distinct character from the empty set and from the Scandinavian Ø, and substituting them is common and wrong.

Common modern usageVerified fact

States that the dimension it precedes is a diameter measured across a circular feature, not a radius and not a width.

Contexts: Technical drawing

Common modern usageVerified fact

Is a distinct character, U+2300 DIAMETER SIGN, from the empty set, the Scandinavian letter Ø and the Greek phi — all of which are routinely substituted for it.

Contexts: Technical drawing, Software

Common modern usageVerified fact

Is written before the value rather than after, so a reader knows what kind of quantity is coming before they read the number.

Contexts: Technical drawing

Confidence

IdentityHigh confidence: High confidence because the identity is documented in encyclopedic references, but multiple independent sources agree.
Historical originModerate confidence: Moderate confidence because the historical origin is documented in encyclopedic references, but multiple independent sources agree, but early or supporting evidence is limited, but the regional or cultural match to this specific claim is not precise.
Current useHigh confidence: High confidence because the current use is documented in encyclopedic references, but multiple independent sources agree.
Visual matchHigh confidence: High confidence because the visual match is documented in encyclopedic references, but multiple independent sources agree.

Sources

Engineering drawing
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol instructs and which standard governs it. Where a standard is named it is reported by this source rather than read, so no meaning is typed `official` on that basis.
ISO 128, Technical drawings — General principles of presentation
International Organization for Standardization · retrieved 2026-08-01
Paywalled and NOT READ. Registered so records can state which standard governs a symbol — a checkable fact a reader can act on — and for nothing else. It governs line types, views, sections and projection conventions. No meaning in any record is typed `official` on the strength of this entry. See docs/paywalled-standards.md.
Technical drawing
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol instructs and which standard governs it. Where a standard is named it is reported by this source rather than read, so no meaning is typed `official` on that basis.

Last reviewed 2026-07-31

Meanings

Visual description

Related & commonly confused symbols

Meanings

Visual description

Related & commonly confused symbols