Pipe Marking Requirements: ASME A13.1 Color Codes, Letter Sizes, and Placement
ASME A13.1 is the standard nearly every U.S. facility uses to identify piping — and the one OSHA points to when it cites unlabeled pipes under the general duty to keep employees informed of hazards.
Important Disclaimer
This article is provided for general informational purposes only and does not constitute legal, safety, or professional compliance advice. Regulations, standards, and codes are subject to change and may be interpreted differently by authorities having jurisdiction (AHJ). Always consult with qualified safety professionals, licensed engineers, or legal counsel to determine the specific requirements applicable to your facility. TagBuilder is not responsible for any actions taken based on this information.
The A13.1 Color Code
| Label colors | Contents |
|---|---|
| Black text on yellow | Flammable or oxidizing fluids |
| White text on red | Fire-quenching fluids (sprinkler, fire water) |
| Black text on orange | Toxic or corrosive fluids |
| White text on green | Potable, cooling, boiler feed & other water |
| White text on blue | Compressed air |
| White text on brown | Combustible fluids (user-defined range) |
Flow-direction arrows are part of the marker, not an accessory — a label without an arrow tells half the story.
Letter Height by Pipe Size
| Pipe OD (incl. insulation) | Min letter height |
|---|---|
| 3/4″ to 1-1/4″ | 1/2″ |
| 1-1/2″ to 2″ | 3/4″ |
| 2-1/2″ to 6″ | 1-1/4″ |
| 8″ to 10″ | 2-1/2″ |
| Over 10″ | 3-1/2″ |
Where Labels Go
Mark piping adjacent to valves and flanges, at changes of direction, at both sides of wall and floor penetrations, and at intervals on straight runs — 25 to 50 feet is the common practice. Labels belong where someone actually looks: below the horizontal centerline on overhead pipe, above it on low runs.
Valve Tags Finish the System
Pipe markers identify the line; valve tags identify the device. A numbered engraved valve tag ties each valve to the valve schedule and the P&ID, so "close V-107" means something at 2 a.m. Match tag colors to the A13.1 service colors for instant recognition — yellow for gas, green for water, blue for air. See our guides on valve tag color codes and valve numbering systems.
Engraved valve tags in every A13.1 service color, numbered and shipped in days.
Photo credits: "District heating pipes in London" by RichardAsh1981 via Wikimedia Commons (CC BY-SA 2.0); "Nefelejcs Straße Kraftwerk" by Globetrotter19 via Wikimedia Commons (CC BY-SA 4.0). Product images: TagBuilder.
