Building Code Glossary
Diameter: Building Code Definition
How the IRC 2009 define diameter, where the wording differs, and which chapters depend on the term.
On this page
Last updated September 7, 2026 · Definitions quoted from the 2009 International Codes as transcribed in this site's building-codes collection, with the source chapter linked for each quotation. Usage counts are computed over the chapters hosted here, one copy per code. The [ICC code library](https://codes.iccsafe.org/codes) publishes the current model text and the [ICC adoption maps](https://www.iccsafe.org/advocacy/code-adoption-maps/) show which edition a jurisdiction has adopted; those sources control.
Direct answer
Diameter is a defined term in the International Residential Code (IRC) 2009, in Definitions. The code's own words are: “Unless specifically stated, the term "diameter" is the nominal diameter as designated by the approved material standard.” No other I-Code in this corpus defines it, so the IRC 2009 definition is the one to work from. The term is used in 72 chapters of this corpus, most often in Soils and Foundations (IBC 2009), Fuel Gas (IRC 2009), Wall Construction (IRC 2009). These pages transcribe the 2009 model text; verify the definition against the edition and amendments your authority having jurisdiction has adopted. Record the jurisdiction, the adopted edition, the application date and any local amendment that rewrites the definition before you use it on a live project, because a term redefined by amendment changes every requirement that depends on it.
Content and cited inputs reviewed by Scott Barrington, Harth founder and architectural technology specialist · Published 2026-09-07 · Updated 2026-09-07.
What does this term mean?
Diameter — IRC 2009, Definitions:
Unless specifically stated, the term "diameter" is the nominal diameter as designated by the approved material standard.
What must be verified before using this guidance?
Diameter is defined once in this corpus, in the Definitions chapter of the International Residential Code (IRC) 2009:
Unless specifically stated, the term "diameter" is the nominal diameter as designated by the approved material standard.
Where diameter is used in the code
The term appears in 72 chapters of the 2009 I-Codes hosted here. The chapters that lean on it hardest are the ones to read next.
| Chapter | Code | Mentions |
|---|---|---|
| Soils and Foundations | IBC 2009 | 110 |
| Fuel Gas | IRC 2009 | 75 |
| Wall Construction | IRC 2009 | 50 |
| Means of Egress | IBC 2009 | 40 |
| Wood | IBC 2009 | 34 |
| Sanitary Drainage | IPC 2009 | 33 |
| Means of Egress | IFC 2009 | 32 |
| Motor Fuel-dispensing Facilities and Repair Garages | IFC 2009 | 30 |
| Sanitary Drainage | IRC 2009 | 29 |
| Fire and Smoke Protection Features | IBC 2009 | 22 |
| Building Planning | IRC 2009 | 22 |
| Wall Covering | IRC 2009 | 22 |
60 further chapters mention the term.
The term in use
How the code actually writes about diameter, quoted from the 2009 text hosted here:
Cast-in-place deep foundation elements with a permanent casing shall have a nominal outside diameter of not less than 8 inches (203 mm).
Where the vertical vent has a larger diameter than the vent connector, the vertical vent diameter shall be used to determine the minimum vent capacity, and the connector diameter shall be used to determine the maximum vent capacity.
When piping or ductwork is placed in or partly in an exterior wall or interior load-bearing wall, necessitating cutting, drilling or notching of the top plate by more than 50 percent of its width, a galvanized metal tie not less than 0.054 inch thick (1.37 mm) (16 ga) and 1½ inches (38 mm) wide shall be fastened across and to …
The terms diameter is built out of
The definition above leans on 1 other defined term. Each carries its own code definition, and a local amendment to any of them changes what diameter covers:
- Approved — IBC 2009: Acceptable to the code official or authority having jurisdiction.
Diameter: what to open, and in what order
Every row is a document this term depends on. The ICC code library and the ICC adoption maps cover the middle rows.
| Check | Where | Confirmed |
|---|---|---|
| Read the definition in the adopted IRC | Definitions | |
| Re-check the requirement that uses it | Soils and Foundations (IBC 2009) | |
| Re-check the requirement that uses it | Fuel Gas (IRC 2009) | |
| Re-check the requirement that uses it | Wall Construction (IRC 2009) | |
| Re-check the requirement that uses it | Means of Egress (IBC 2009) | |
| Confirm the dependent definition | Approved | |
| Local amendment to any of the above | Jurisdiction's amendment list |
A research aid, not a code determination. The authority having jurisdiction decides which edition and which amendments apply.
Where can you explore related guidance?
- Explore all glossary guidance
- Approved: Building Code Definition
- Building: Building Code Definition
- Construction Documents: Building Code Definition
- Registered Design Professional: Building Code Definition
Sources and method
Definitions quoted from the 2009 International Codes as transcribed in this site's building-codes collection, with the source chapter linked for each quotation. Usage counts are computed over the chapters hosted here, one copy per code. The ICC code library publishes the current model text and the ICC adoption maps show which edition a jurisdiction has adopted; those sources control.
Source links
Frequently asked questions
What is the code definition of diameter?
The International Residential Code (IRC) 2009 defines it in Definitions as: “Unless specifically stated, the term "diameter" is the nominal diameter as designated by the approved material standard.” The 2009 model text is reproduced here; the definition that binds your project is the one in the edition your jurisdiction has adopted, as amended locally.
Which parts of the code depend on diameter?
In this corpus the term is used across 72 chapters, led by Soils and Foundations in the IBC 2009. Those are the sections whose scope moves if the definition is amended locally, so read them together with Chapter 2 rather than on their own.
What is the definition of a corridor in the 2018 IBC Code?
Corridor is a separate defined term. The IBC 2009 defines it as: “An enclosed exit access component that defines and provides a path of egress travel to an exit.” That is not the same as diameter, which the IRC 2009 defines as “Unless specifically stated, the term "diameter" is the nominal diameter as designated by the approved material standard.” — read both before applying either, and confirm the wording in the edition your jurisdiction has adopted.
What is the definition of building area according to the IBC?
Building is a separate defined term. The IBC 2009 defines it as: “Any structure used or intended for supporting or sheltering any use or occupancy.” That is not the same as diameter, which the IRC 2009 defines as “Unless specifically stated, the term "diameter" is the nominal diameter as designated by the approved material standard.” — read both before applying either, and confirm the wording in the edition your jurisdiction has adopted.
What does the diameter code 5002 mean?
In the 2009 I-Codes hosted here it is a defined term rather than a description. The IRC 2009 states, in Definitions: “Unless specifically stated, the term "diameter" is the nominal diameter as designated by the approved material standard.” It is defined in IRC 2009. Because it is a defined term, whatever the adopted edition says it means is what every section using it means — so check Chapter 2 of that edition and any local amendment before relying on this wording.
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