Pipe Marking Color Codes Explained: The ASME A13.1 Standard (Colors, Sizes, Placement)

The Color on the Pipe Is a Sentence

Walk any plant, boiler room, or mechanical space and the pipes are talking to you: a yellow band that says this line burns, an orange one that says this line will hurt you, a green one that says just water. That vocabulary comes from ASME A13.1 — Scheme for the Identification of Piping Systems, the U.S. consensus standard for pipe marking.

This article decodes the full color scheme, the size chart that most facilities get wrong, the flow-arrow rule, and what OSHA actually requires (the honest answer is more nuanced than most sign vendors tell you).

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The Six Standard Colors (Plus Four You Assign Yourself)

A13.1 assigns the color by what the fluid does, not what it's called:

Four more combinations — purple/white, white/black, gray/white, black/white — are user-defined. They exist so a site can flag services the six standard categories don't cover (glycol loops, nitrogen, DI water). If you use one, define it in your written program and post the key where workers can see it.

Yellow vs. brown — the one facilities get wrong

The line between flammable (yellow) and combustible (brown) follows flash point, the same line the fire codes draw. Ignites at ambient temperature → yellow. Must be heated first (diesel, fuel oil, lube oil) → brown. The brown category only arrived in the 2015 revision of A13.1, so plants marked before then often show diesel lines in yellow. It's one of the first things a sharp auditor checks.

The Size Chart — Letters Scale With the Pipe

A marker you can't read from the floor is decoration. A13.1 prescribes the length of the color field and the letter height from the pipe's outside diameter including insulation:

Two consequences people miss:

1. Insulation counts. A 2″ line with 2″ of insulation is a 6″ OD for marking purposes — it needs 1¼″ letters, not ¾″.
2. Pipes under ¾″ don't get a marker at all — they get a permanently attached identification tag (stamped or engraved) at valves and outlets.

Flow Arrows Are Not Optional

Every marker carries an arrow showing which way the contents move. If the line can flow both directions — recirculating loops, header cross-ties — the arrows point both ways. In an emergency, the arrow is the difference between isolating a leak upstream and chasing it downstream.

Where Markers Go

Placement is about the moment someone actually needs the information:

• Adjacent to every valve and flange
• At every change of direction
• On both sides of wall, floor, and ceiling penetrations
• Next to the equipment the line serves
• Every 25–50 feet on straight runs
• Sighted for the viewer: below the centerline on overhead pipe, on both approach sides where workers come from either direction

What OSHA Actually Requires

Here's the honest version most vendors skip. There is no general OSHA standard that says "label every pipe." OSHA's Hazard Communication Standard (29 CFR 1910.1200) explicitly exempts pipes from its container-labeling rules.

But that exemption is nowhere near a free pass:

• Specific standards reference A13.1. Oxygen–fuel gas piping under 1910.253, for example, must be marked per the scheme.
• The General Duty Clause. An unmarked caustic or steam line that injures someone is a recognized hazard OSHA can cite without a pipe-specific standard.
• Everyone downstream expects it. Fire marshals, mechanical codes, insurance auditors, and customer audits (ISNetworld, Avetta) all treat A13.1 as the baseline.

So: not a named OSHA mandate, but the consensus standard your facility will be measured against by everyone who walks the floor with a clipboard.

Build the Marking Schedule First, Then Print

The real work isn't printing one label — it's the marking schedule: every line in the facility, its contents, hazard category, OD, marker size, and placement points. That schedule is what an auditor asks for, and it's what keeps the maintenance team consistent when a line changes service.

Our free pipe marker generator gets each marker's legend, color, letter height, and length right — and if you're marking a whole facility from a chemical inventory, Tellus EHS generates pipe markers, GHS, NFPA, and HMIS labels from the same inventory, so the pipe in the rack and the drum below it never disagree about what's inside.