ISA 5.1 symbols

ANSI/ISA-5.1 is the standard for instrumentation and control symbols on P&IDs. It defines the instrument bubbles, the identification letters inside them, the signal lines between instruments, and the actuators and final control elements. It is the default for instrumentation on US-style P&IDs and on many international projects.

PDF chart (ISA and US legend)All symbols as SVG (.zip)

Full title: ANSI/ISA-5.1-2024, Instrumentation and Control: Symbols and Identification
Latest edition: 2024. Earlier editions 2022, 2009 and 1984
Published by: ISA, the International Society of Automation, approved as an American National Standard (ANSI)
Covers: Instrument bubbles and locations, identification letters and tag numbers, signal lines, actuators and final control elements, primary elements
Does not cover: Equipment, piping and plain valves. These come from the project piping legend, often based on PIP PIC001 or ISO 10628-2
Used with: PIP PIC001, which uses ISA 5.1 for its instrumentation

Control and actuated valves ISA 5.1

About control and actuated valves

Safety devices ISA 5.1

About safety devices

Line types ISA 5.1

About line types

What changed in ISA-5.1-2024

The 2024 edition added "Control" to the title, to show that control symbols are part of the standard. It reorganises the text with more consistent definitions, adds symbols for newer automation technology and a loop instrument diagram symbol table.

The guidance that used to sit in the annexes moved into two technical reports: ISA-TR5.1.02 for the identification system and ISA-TR5.1.03 for graphic symbols. Many plants still work to the 2009 edition or older, so check which edition the project legend cites.

Instrument bubbles and where they are

The shape of the bubble tells you what kind of function it is. The line through it tells you where the operator can reach it.

Shape	Meaning	No line	Solid line	Dashed line
Circle	Discrete instrument	Field mounted	Main control room, accessible	Main control room, behind the panel
Circle in a square	Shared display, shared control (DCS)	Field	Control room	Not accessible to the operator
Hexagon	Computer function	Field	Control room	Not accessible
Diamond in a square	PLC function	Field	Control room	Not accessible

A double line means a local panel or auxiliary location in the plant. Interlocks are drawn as a diamond with a letter inside.

ISA 5.1 identification letters

The first letter is the measured or initiating variable. The letters after it are the functions: what the instrument reads out or does. A PIC is a pressure indicating controller, an LT a level transmitter, an FV a flow control valve.

Letter	As first letter	As succeeding letter
A	Analysis	Alarm
B	Burner, combustion	User choice
C	User choice (usually conductivity)	Control
D	User choice (usually density)	Difference (modifier)
E	Voltage	Sensor, primary element
F	Flow	Ratio (modifier)
G	User choice	Glass, gauge, viewing device
H	Hand	High (modifier)
I	Current	Indicate
J	Power	Scan (modifier)
K	Time, schedule	Control station
L	Level	Light, low (modifier)
M	User choice	Middle, intermediate (modifier)
N	User choice (often torque)	User choice
O	User choice	Orifice, restriction
P	Pressure	Point, test connection
Q	Quantity	Totalize, integrate
R	Radiation	Record
S	Speed, frequency	Switch
T	Temperature	Transmit
U	Multivariable	Multifunction
V	Vibration, mechanical analysis	Valve, damper, louver
W	Weight, force	Well, probe
X	Unclassified (often on/off valve, XV)	Unclassified
Y	Event, state, presence	Relay, compute, convert
Z	Position, dimension	Driver, actuator, final control element

User choice letters must be defined on the project legend sheet. The same letter can mean different things in IEC 62424, see ISA vs ISO and IEC.

Signal lines

Process lines are drawn heavy. Signals between instruments are thin, with a mark that shows the medium: two slashes for pneumatic, dashes for electrical, an L shape for hydraulic, crosses for capillary, small circles for a software or data link and an S-curve for electromagnetic signals. See P&ID line types.

Frequently asked questions

What is ISA 5.1?

ANSI/ISA-5.1 is the standard for instrumentation and control symbols and identification. It defines the bubbles, letters, signal lines and actuators used on P&IDs. The latest edition is ANSI/ISA-5.1-2024.

What is the latest version of ISA 5.1?

ANSI/ISA-5.1-2024, Instrumentation and Control: Symbols and Identification. It replaced the 2022 edition. The guidance annexes now sit in ISA-TR5.1.02 and ISA-TR5.1.03.

Does ISA 5.1 cover pumps and vessels?

No. ISA 5.1 covers instrumentation and control. Equipment, piping and plain valve symbols come from the project legend, which is often based on PIP PIC001 in the US or ISO 10628-2 internationally.

What does a line through an instrument bubble mean?

A solid line means the function is in the main control room and accessible to the operator. A dashed line means it is behind the panel. No line means it is mounted in the field. A double line means a local panel.

Is ISA 5.1 the same as ISA S5.1?

Yes. ISA S5.1 is the older name of the same standard. The S was dropped in later editions.

The symbols on this page were drawn by Deskely to the forms of the standard and are free to use. Standards are revised and every project issues its own legend sheets, which take precedence.

Whichever standard your P&IDs follow.Deskely Extract reads ISA and ISO P&IDs alike, finds each tag with its type, system and subsystem, and hands your engineers a register to check instead of a spreadsheet to type.

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Other standardsStandardISO 10628 P&ID symbols StandardPIP P&ID symbols (PIC001)StandardDIN P&ID symbols (R&I-Fließschema)StandardIEC 62424 P&ID symbols ComparisonISA vs ISO P&ID symbols LegendAll P&ID symbols