Pipe Color Coding — ASME A13.1

The six fixed colour combinations, the four user-defined ones, and the legend and letter size required at every pipe diameter.

The six fixed colour combinations

ASME A13.1 classifies by hazard, not by fluid. What determines the colour is what the contents would do if they escaped, which is why steam, hot oil and natural gas do not each get a colour of their own.

ASME A13.1 pipe marker colour scheme.
Contents	Marker	Legend colour	Field colour
Flammable and oxidizing fluids	Yellow Black legend	Black	Yellow
Combustible fluids	Brown White legend	White	Brown
Toxic and corrosive fluids	Orange Black legend	Black	Orange
Fire-quenching fluids	Red White legend	White	Red
Water — potable, cooling, boiler feed and other	Green White legend	White	Green
Compressed air	Blue White legend	White	Blue

Colours shown are indicative on screen only. Specify marker stock against the standard, not against this page.

The four user-defined combinations

These carry no assigned meaning. If a site uses them it must document what they mean and post the key where the pipes are — an undocumented purple line tells a stranger nothing.

ASME A13.1 user-defined combinations.
Contents	Marker	Legend colour	Field colour
User defined	Purple White legend	White	Purple
User defined	Black White legend	White	Black
User defined	White Black legend	Black	White
User defined	Gray Black legend	Black	Gray

Marker size

The size of the colour field and the letter height both follow the pipe diameter, so a marker legible on a 2 in line is not compliant on a 12 in header.

ASME A13.1 Table 1 — marker and legend size.
Pipe outside diameter	Length of colour field	Letter height
0.75 – 1.25 in	8 in	0.50 in
1.5 – 2 in	8 in	0.75 in
2.5 – 6 in	12 in	1.25 in
8 – 10 in	24 in	2.50 in
Over 10 in	32 in	3.50 in

The standard states these against the outside diameter of the pipe or its covering. An insulated line is sized on the lagging, not the pipe, so it often moves up a band.

Marker size by pipe size

The same table applied to every ASME B36.10M size, with the outside diameter alongside so an insulated line can be checked against the band above.

Dimensions are given in inches, with the millimetre equivalent beneath in grey.

ASME A13.1 marker size for each B36.10M pipe size, bare pipe.
Size	Outside diameter	Colour field length	Letter height
NPS 1/8	0.40510.29 mm	tag instead	—
NPS 1/4	0.54013.72 mm	tag instead	—
NPS 3/8	0.67517.14 mm	tag instead	—
NPS 1/2	0.84021.34 mm	tag instead	—
NPS 3/4	1.05026.67 mm	8 in	0.50 in
NPS 1	1.31533.40 mm	8 in	0.50 in
NPS 1 1/4	1.66042.16 mm	8 in	0.50 in
NPS 1 1/2	1.90048.26 mm	8 in	0.75 in
NPS 2	2.37560.32 mm	8 in	0.75 in
NPS 2 1/2	2.87573.02 mm	12 in	1.25 in
NPS 3	3.50088.90 mm	12 in	1.25 in
NPS 3 1/2	4.000101.60 mm	12 in	1.25 in
NPS 4	4.500114.30 mm	12 in	1.25 in
NPS 4 1/2	5.000127.00 mm	12 in	1.25 in
NPS 5	5.563141.30 mm	12 in	1.25 in
NPS 6	6.625168.27 mm	12 in	1.25 in
NPS 7	7.625193.67 mm	tag instead	—
NPS 8	8.625219.07 mm	24 in	2.50 in
NPS 9	9.625244.47 mm	24 in	2.50 in
NPS 10	10.750273.05 mm	24 in	2.50 in
NPS 11	11.750298.45 mm	32 in	3.50 in
NPS 12	12.750323.85 mm	32 in	3.50 in
NPS 14	14.000355.60 mm	32 in	3.50 in
NPS 16	16.000406.40 mm	32 in	3.50 in
NPS 18	18.000457.20 mm	32 in	3.50 in
NPS 20	20.000508.00 mm	32 in	3.50 in
NPS 22	22.000558.80 mm	32 in	3.50 in
NPS 24	24.000609.60 mm	32 in	3.50 in
NPS 26	26.000660.40 mm	32 in	3.50 in
NPS 28	28.000711.20 mm	32 in	3.50 in
NPS 30	30.000762.00 mm	32 in	3.50 in
NPS 32	32.000812.80 mm	32 in	3.50 in
NPS 34	34.000863.60 mm	32 in	3.50 in
NPS 36	36.000914.40 mm	32 in	3.50 in

Below NPS 3/4 A13.1 gives no field size: the standard calls for a permanently attached tag rather than a wrapped marker.

Colour is not the identification. Every A13.1 marker needs the legend in words and an arrow for flow direction. Colour classifies the hazard; the words identify the line. Bidirectional lines get arrows both ways.

Common questions

What do the ASME A13.1 pipe colours mean?

Six colour combinations are fixed by hazard, not by contents. Yellow with black lettering is flammable or oxidizing; brown with white is combustible; orange with black is toxic or corrosive; red with white is fire-quenching; green with white is water; blue with white is compressed air. Four further combinations — purple, black, white and gray fields — are left for the user to define.

Is the colour enough on its own?

No. A13.1 requires the legend — the name of the contents in words — plus an arrow showing flow direction. The colour field is there to make the hazard class readable from a distance; the words are what identify the line. A pipe marked only by colour does not comply.

How often do markers have to be repeated?

A13.1 asks for markers wherever the line is read: at valves and flanges, at changes of direction, on both sides of a wall, floor or ceiling penetration, at entry and exit points, and at intervals on long straight runs — commonly taken as every 25 to 50 ft. Place them so a marker is visible from the normal approach to the pipe.

Does A13.1 apply outside the United States?

No. A13.1 is a US voluntary standard, adopted by reference in many specifications and by OSHA in practice rather than by rule. The UK and much of Europe use BS 1710 and its EN companions, which have a different and incompatible colour scheme — green for water there too, but the rest does not map across. Marine and shipbuilding practice differs again.