ISA 5.1.

ISA 5.1 is the North American standard defining instrument symbols and tag identification on P&IDs. The current revision is ANSI/ISA 5.1-2024. Every tag carries a first letter for the measured variable, succeeding letters for the function, and a loop number.

Upload a drawing Pricing

Part ofGlossary

What ISA 5.1 means.

ISA 5.1 originated in 1949 as the first American standard for instrument identification on process drawings. The current revision, ANSI/ISA 5.1-2024, supersedes the 2009 baseline and the 2022 interim and adds clarifications for digital communication links, multi-variable transmitters, and the shared-display function symbology used by modern DCS HMIs. The standard codifies three things at once. The symbol library drawn on a P&ID, circles, hexagons, squares. Field-mounted, panel-mounted, accessible-to-operator, embedded-in-shared-system. The tag identifier, first letter measured variable, succeeding letters function, plus a loop number that ties members of one control loop together. And the line conventions for connection types, electrical, pneumatic, hydraulic, capillary, software signal. It is paired with ISA 5.2 for binary logic diagrams and ISA 5.3 for graphic symbols in shared-display and shared-control systems. Outside North America the parallel standards are ISO 14617, DIN 19227, and IEC 81346, each of which approaches the same problem with a different alphabet.

How are ISA 5.1 tags read.

Take FIC-101. Letter F flow. Letter I indicator. Letter C controller. So FIC-101 is a flow indicating controller, on loop 101. FT-101, same loop is the flow transmitter. FV-101A is the flow valve, redundant A. The first letter is the measured, initiating variable. The second letter, and any modifier letters describe the function. The number is the loop. The convention scales from 3-letter tags on small skids to 6 plus character tags on tightly-coded enterprise standards.

What does ISA 5.1 actually standardize.

Symbols, the circles, lines, and squares on a P&ID, tag identification letters, line conventions for instrument signal types, electric, pneumatic, hydraulic, fiber, software, and panel-mounting conventions. It does not standardize tag-numbering schemes, every operating company has their own area-loop-suffix convention or what constitutes a redundant pair. Most companies layer their own internal standard on top of ISA 5.1.

What do ISA 5.1 first-letter codes mean.

The first letter identifies what the instrument measures. The dozen most common cover roughly 90% of field tags. F flow, P pressure, T temperature, L level, A analysis or composition, M moisture, Q quantity or totalizer, S speed, V vibration, W weight, X unclassified, Z position. Less common but standard. B burner, D density, E voltage, H hand-actuated, I current, J power, K time or schedule, N user-defined, O user-defined, R radiation, U multivariable, Y event. Plant-specific extensions go into the project tag standard, not into ISA 5.1 itself.

What do ISA 5.1 succeeding letters mean.

Letters after the first identify what the instrument does. T transmit, FT, PT, TT. C control with setpoint, FC, TC, LC. V valve, the final element, TCV, FV, XV. I indicate, a local readout, PI, TI. S switch, a discrete state change at a threshold, PSH, LSL, TSL. A alarm action on the same threshold, PAH, TAL. Y relay, compute, or interlock, FY, TY. E primary element, sensor before the transmitter, FE, TE. K time-scheduled controller, KIC. Z driver or position actuator, ZSO, ZSC. Two and three letters chain. TIC temperature indicating controller, FQI flow quantity indicator, PDT pressure differential transmitter.

What ISA 5.1 symbol shapes appear on a P&ID.

ISA 5.1 specifies four base bubble shapes. A plain circle is a field-mounted single-function instrument. A transmitter near the equipment, a switch on the line. A hexagon is a computing function. Derived signal, calculation, or interlock relay. A square with an inscribed circle is panel-mounted, operator-accessible. A diamond is shared-display, shared-control inside a DCS or SCADA. An open diamond is operator-accessible, a half-shaded diamond is restricted. A solid horizontal line through the symbol is field location. A single dashed line is remote panel. Double horizontal lines are shared display.

ISA 5.1 first-letter quick reference

The first letter of an ISA 5.1 tag identifies the measured or initiating variable. These twelve cover roughly 90% of field tags.

Letter	Measured variable	Example tag	Meaning
A	Analysis, composition	AT-301	Analyzer transmitter
F	Flow	FIC-101	Flow indicating controller
L	Level	LT-201	Level transmitter
M	Moisture	MIT-401	Moisture indicating transmitter
P	Pressure	PT-501	Pressure transmitter
Q	Quantity, totalizer	FQI-101	Flow quantity indicator
S	Speed, frequency	SI-601	Speed indicator
T	Temperature	TIC-701	Temperature indicating controller
V	Vibration	VT-801	Vibration transmitter
W	Weight, force	WIT-901	Weight indicating transmitter
X	Unclassified, multiple	XV-101	Shutoff valve, on, off
Z	Position, dimension	ZSO-101	Position switch, open

ISA 5.1 succeeding-letter, function reference

Letters after the first identify what the device does. Two and three letters chain, TIC temperature indicating controller.

Letter	Function	Example tag	Notes
T	Transmit	FT, PT, TT	Sends signal upstream
C	Control with setpoint	FC, TC, LC	Receives setpoint, modulates output
V	Valve, final element	TCV, FV, XV	Throttling or on, off valve
I	Indicate	PI, TI	Local readout, no transmission
S	Switch	PSH, LSL, TSL	Discrete state change at a threshold
A	Alarm	PAH, TAL	Alarm action at a threshold
Y	Relay, compute, interlock	FY, TY	Derived signal, calculation
E	Primary element	FE, TE	Sensor before the transmitter
K	Time-scheduled controller	KIC	Schedule or time-based control
Z	Driver, position actuator	ZSO, ZSC	Open, closed position switch

Common questions

Is ISA 5.1 mandatory.

It is the de facto standard in North American oil and gas, petrochemicals, refining, power, and water. Other geographies use ISO 14617 or DIN equivalents. Practically. If your project documents reference ISA 5.1, every tag must follow it.

What changed in ISA 5.1-2022.

The 2022 revision added consistent symbology for digital, network-attached, and shared display devices reflecting modern DCS, SIS architectures. It also clarified the use of dotted-line panel boundaries and computing relays.

How does ISA 5.1 handle instruments from non-ISA identification standards.

ISA 5.1 does not govern non-ISA tag formats. Plants using KKS, IEC 81346-based hierarchical coding, DIN 19227, or NORSOK tag patterns operate under those standards and are not ISA 5.1 compliant by default. Many multinationals define a project-specific standard that maps KKS or NORSOK tags alongside an ISA 5.1 symbol set, carrying two tag identifiers for the same instrument.

What is the difference between ISA 5.1 and ISA 5.06.

ISA 5.1 standardizes how an instrument symbol is drawn and how the tag is written. ISA 5.06 is the parallel standard for terminal-block documentation and loop wiring conventions. It picks up where ISA 5.1 stops. A loop sheet drawn to ISA 5.06 references ISA 5.1 tags but adds wire numbers, terminal locations, junction boxes, and panel-mount detail. Larger projects also reference ISA 5.4 for installation details and ISA 5.5 for graphic-symbol conventions used in DCS displays.

Does ISA 5.1 cover safety instrumented systems.

ISA 5.1 covers the symbology for any instrument including SIS-rated devices, but it does not govern the design or independence requirements of an SIS. Those live in ANSI/ISA 84.00.01, which is the U.S. Adoption of IEC 61511. ISA 5.1 contributes the visual convention. SIS-rated bubbles carry separate functional designations, SDV for safety shutdown valve, XV for emergency block valve under SIS interlock and are often drawn with a heavier outline or shaded border per the project standard layered on top of ISA 5.1.

Get the ISA 5.1 letter-code cheat sheet.

First letters, modifiers and succeeding letters on one page, with the tag patterns each combination produces.

Get the ISA 5.1 cheat sheet Upload a drawing