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

This is the scheme of ASME A13.1-2023. It classifies mostly by hazard, not by fluid: what determines the colour is what the contents would do if they escaped, which is why 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
Fire-quenching fluids	Red White legend	White	Red
Toxic and corrosive fluids	Orange Black legend	Black	Orange
Flammable, combustible and oxidizing fluids	Yellow Black legend	Black	Yellow
Steam, steam condensate, boiler feed water and other hot water	Gray Black legend	Black	Gray
Potable, cooling and other cold or tepid water	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	White Black legend	Black	White
User defined	Brown White legend	White	Brown
User defined	Black White legend	White	Black

What changed from the 2020 edition

Markers bought before 2023 follow an older scheme, and both are in service. In the 2015 and 2020 editions:

combustible fluids had their own colour, white on brown; they are now black on yellow with the flammable fluids;
green covered every kind of water, including boiler feed water; hot water and boiler feed are now black on gray;
steam had no colour of its own; it is now black on gray;
the user-defined colours were purple, black, white and gray, the gray with white lettering; they are now purple, white, brown and black.

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 marker and legend size, 2020 and 2023 editions.
Pipe outside diameter	Length of colour field	Letter height
0.7 – 1.3 in	8 in	0.5 in
1.4 – 2.4 in	8 in	0.7 in
2.5 – 6.7 in	12 in	1.3 in
6.8 – 10.0 in	24 in	2.5 in
Over 10 in	32 in	3.5 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	8 in	0.5 in
NPS 3/4	1.05026.67 mm	8 in	0.5 in
NPS 1	1.31533.40 mm	8 in	0.5 in
NPS 1 1/4	1.66042.16 mm	8 in	0.7 in
NPS 1 1/2	1.90048.26 mm	8 in	0.7 in
NPS 2	2.37560.32 mm	8 in	0.7 in
NPS 2 1/2	2.87573.02 mm	12 in	1.3 in
NPS 3	3.50088.90 mm	12 in	1.3 in
NPS 3 1/2	4.000101.60 mm	12 in	1.3 in
NPS 4	4.500114.30 mm	12 in	1.3 in
NPS 5	5.563141.30 mm	12 in	1.3 in
NPS 6	6.625168.27 mm	12 in	1.3 in
NPS 8	8.625219.07 mm	24 in	2.5 in
NPS 10	10.750273.05 mm	32 in	3.5 in
NPS 12	12.750323.85 mm	32 in	3.5 in
NPS 14	14.000355.60 mm	32 in	3.5 in
NPS 16	16.000406.40 mm	32 in	3.5 in
NPS 18	18.000457.20 mm	32 in	3.5 in
NPS 20	20.000508.00 mm	32 in	3.5 in
NPS 22	22.000558.80 mm	32 in	3.5 in
NPS 24	24.000609.60 mm	32 in	3.5 in
NPS 26	26.000660.40 mm	32 in	3.5 in
NPS 28	28.000711.20 mm	32 in	3.5 in
NPS 30	30.000762.00 mm	32 in	3.5 in
NPS 32	32.000812.80 mm	32 in	3.5 in
NPS 34	34.000863.60 mm	32 in	3.5 in
NPS 36	36.000914.40 mm	32 in	3.5 in

The band is chosen from the outside diameter, not from the NPS number, so NPS 10, which is 10.750 in across, takes the largest marker. NPS 1/8, 1/4 and 3/8 are below 0.7 in across, where the standard gives no field size and recommends a permanently attached tag.

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.

Guides and comparisons for this page

Pipe material selectionA53, A106, A312 and A335: which grade suits which duty.NPS vs DN explainedTwo designators for one pipe, and why neither is a measurement.

Common questions

What do the ASME A13.1 pipe colours mean?

In the 2023 edition six colour combinations are fixed. Red with white lettering is fire-quenching; orange with black is toxic or corrosive; yellow with black is flammable, combustible or oxidizing; gray with black is steam and hot water; green with white is cold water; blue with white is compressed air. Four further combinations — purple, white, brown and black fields — are left for the user to define.

Did the colours change in 2023?

Yes. In the 2015 and 2020 editions brown with white lettering meant combustible fluids, green covered all water including boiler feed, and gray was one of the user-defined colours. The 2023 edition moved combustible fluids into yellow, gave gray to steam and hot water, and made brown user-defined. Markers to the older scheme are still widely installed, so check which edition a site works to.

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.