P&ID Editor — ISA-5.1-Style Instrument Symbols & Identification

ISA-5.1 — Instrumentation Symbols and Identification

Page last updated 8 October 2026

ANSI/ISA-5.1-2009 (Instrumentation Symbols and Identification) is the governing standard for Piping and Instrumentation Diagrams across the process industries — oil and gas, chemical, power generation, water treatment, and pharmaceuticals. It defines two interdependent systems: a library of graphical symbols for every class of process equipment and measurement instrument, and an alphanumeric identification system that encodes the measured variable, function class, and loop number into a unique instrument tag (e.g. FIC-101 for Flow Indicating Controller, loop 101). Together, they give every engineer, operator, and reviewer a shared visual language that is unambiguous across organisations and national borders.

The standard is maintained by the International Society of Automation (ISA) and was first published in 1949; the current edition (ISA-5.1-2009) superseded the 1984 revision. It is adopted as an American National Standard by ANSI and is widely accepted internationally, including within IEC and EN practice where ISA notation is the dominant convention for P&IDs. MechanixCalc's P&ID Editor is a browser drafting tool that provides ISA-5.1-style bubbles for field and panel instruments, equipment and valve symbols, and tag nodes in the ISA letter-and-loop-number style (e.g. FT-101, PCV-305A) with a format check. It is not a full ISA-5.1 implementation: DCS and PLC function shapes and off-sheet connectors are not provided, and the connection lines use their own dash patterns (see below).

Calculators that implement ISA-5.1

P&ID EditorBrowser-based drafting tool with 44 ISA-5.1-style symbols (instrument bubbles for field and control-room only; no DCS/PLC shapes or off-sheet connectors), nine line types with their own dash patterns (four are conventions), ISA-5.1-style loop-number tags, multi-sheet project management, BOM and equipment / valve / instrument / line lists (CSV and Excel), and PDF, SVG, PNG and DXF export.

Video walkthrough (4:02)

Read the transcript

A pumped transfer line, with its flow control loop. Drawn as a P&ID, in the browser.

And the same drawing is also your equipment, valve, instrument and line list.

This is the MechanixCalc P&ID editor. ISA-5.1-style symbols, and an ISO 10628-2 pack with registration numbers. Let's build one, step by step.

Step one: the drawing setup. Pick the symbol set, ISA 5.1 or ISO 10628. Then the diagram type: a P&ID, a process flow diagram, or a block diagram.

Start from a template. Pumped transfer with flow control: a tank, strainer, pump, check valve, orifice, control valve and a receiver.

Its line numbers are already on the drawing. 100-PW-1001 on the suction side.

Step two: the library. Filter it by industry pack: bulk solids, chemical, valves, instruments, dust collection and process aids.

Search by name, or by ISO registration number. X8062 finds the closed chain conveyor.

ISO symbols show their registration number. Select one, and the panel shows its level: an official sample, or cross-checked. And the conv. badge marks a convention, not ISO.

Step three: tags. Every symbol gets an ISA-style tag, like FT-101. Type it wrong, and the format check tells you.

Add the service and, for instruments, the mounting. They go straight into the instrument list.

Step four: an instrument loop. Place a pressure transmitter. It is tagged PT-101 automatically.

Choose the signal line type, and draw from the transmitter to the controller. Signal lines are green and dashed. Process lines are solid.

Field instruments are plain circles. The controller has a line through it, because it sits in the control room.

Step five: the line spec. Click a line. Line number P-120. Size, 2 inch. Then the service, design pressure, temperature and insulation.

The label builds itself: 2 inch, P-120. Or type your own. From and To are read from the drawing. And every field goes into the line list.

Step six: arrange. Press R to rotate, and the lines follow. Drag a line end onto another port. Any port can join any port.

Step seven: the lists. The BOM tab counts equipment, valves and instruments, across every sheet.

Under Export, each list is its own CSV. Or take one Excel workbook, with a tab per list.

Step eight: the ISO 10628-2 pack. The bulk solids template opens as a process flow diagram. Bag filter and fan, rotary valve, chain conveyor, silo, weigh hopper and dosing screw.

Each symbol keeps its registration number and its level. So the bill of materials says which ones are official samples, cross-checked, or conventions.

Step nine: save and export. Save to cloud, and the drawing opens on any device. From the Cloud Library, you can share a read-only link.

The PDF drawing is A3, in a zoned frame, with a title block. One page per sheet. Then the equipment and line lists.

Plus a DXF with a layer for every line type, and a vector SVG.

One honest note. The editor calculates nothing. The lists are your data, organised for you.

That's a P&ID, drawn and listed. Try it on your own plant, on MechanixCalc.com.

What ISA-5.1 covers

Graphical symbol library — circles for instruments, equipment shapes for vessels/pumps/compressors/heat exchangers/columns/tanks, valve body symbols, and actuator symbols, ISA-5.1-style shapes in the editor (the standard defines a larger library than the tool provides)
Alphanumeric identification system — instrument tag structure: first letter(s) encode the measured variable (F = Flow, T = Temperature, P = Pressure, L = Level, A = Analytical…) and subsequent letters encode function class (I = Indicator, C = Controller, T = Transmitter, R = Recorder…) followed by a unique loop number and optional suffix
Line-type conventions — the process line is solid and heavy; a PNEUMATIC signal is a solid line crossed by DOUBLE HASH MARKS (—//—); an ELECTRIC signal is a plain DASHED line; a hydraulic signal is a solid line marked with the letter L; capillary tubing is a solid line crossed with X marks; and a software or data link is a solid line marked with small circles. ⚠️ The MechanixCalc P&ID editor has nine line types: five (process, instrument, electrical, signal, pneumatic) are distinguishable DASH PATTERNS in the ISA style rather than these exact graphical conventions — its electrical line is dot-dash and its pneumatic line is dotted, where ISA-5.1 specifies dashed and double-hash respectively — and four (solids / pneumatic conveying, compressed air, dust / extraction duct, by others / existing) are bulk-solids drawing conventions that are neither ISA nor ISO line symbols. The lines are unambiguous within a drawing produced by the editor, but they are not ISA-5.1's own glyphs; relabel them before issuing a drawing into an ISA-governed workflow
Instrument bubble conventions — ACCESSIBILITY and LOCATION are shown by the HORIZONTAL LINE through the bubble, not by fill or hatching: no line = field-mounted; a solid line = primary/central control room, accessible to the operator; a dashed line = behind the panel, not normally accessible; a double solid line = a local or auxiliary panel; a double dashed line = inaccessible in that local panel. The ENCLOSING SHAPE carries the function: a plain circle is a discrete instrument, a circle inside a square is a shared display / shared control (DCS) function, a hexagon is a computer function, and a diamond inside a square is a PLC function. ⚠️ The MechanixCalc editor provides ISA-5.1-style bubbles with no line (field-mounted: indicator, transmitters, analyser) or one solid line (control room: controller, recorder, flow meter) only; the DCS, computer and PLC function shapes are not provided
Cross-referencing and off-sheet continuations — conventions for referencing tags and process streams that continue on other P&ID sheets. Off-sheet connectors and tie-line symbols are not provided in the MechanixCalc editor; use annotation text for sheet references
Bill of materials traceability — every placed symbol (by type and tag) can be enumerated from the P&ID into equipment, valve, instrument and line lists, as an organised inventory of your own data (the editor calculates nothing) to support HAZOP studies, management-of-change reviews, and instrument index documents

Frequently asked questions

What is ISA-5.1 used for?

ISA-5.1 defines the standard graphical symbols and alphanumeric instrument identification (tag) notation used on Piping and Instrumentation Diagrams (P&IDs). It is the universal drawing language for process-industry engineering: symbols placed on a P&ID — vessel, pump, heat exchanger, control valve, pressure transmitter — and instrument tags (e.g. PT-201 for Pressure Transmitter, loop 201) are drawn and written to the ISA-5.1 conventions, allowing engineers, operators, safety reviewers, and maintenance teams to read diagrams without ambiguity.

What is the ISA-5.1 instrument tag structure?

A tag is built as: <variable letter(s)><function letter(s)>-<loop number>[suffix]. The first letter identifies the measured variable (F = Flow, T = Temperature, P = Pressure, L = Level, A = Analytical, etc.). The next letter(s) identify the function (I = Indicator, C = Controller, T = Transmitter, R = Recorder, V = Valve, etc.). The loop number is a unique numeric identifier for the control loop. An optional suffix (A, B, …) distinguishes duplicate functions in the same loop. For example: FIC-101A = Flow Indicating Controller, loop 101, first of two.

How does ISA-5.1 differ from ISO 10628 or IEC standards for P&IDs?

ISA-5.1 is the dominant convention for P&ID symbols and tag notation in North America and much of the international process industry. ISO 10628 (formerly DIN/EN 28000) is the ISO series for flow diagrams of process plants; its part 2 defines graphical symbols, each with a registration number, but it does not define the instrument tag alphanumeric system that ISA-5.1 specifies. MechanixCalc offers an ISO 10628-2 pack of 56 symbols that shows each symbol's registration number and verification level (A = official sample, B = cross-checked against independent references, conv. = a convention, not ISO 10628-2). In practice, ISA-5.1 symbols and tags are widely used on P&IDs worldwide, including in projects that reference ISO or IEC documentation standards.

Does the MechanixCalc P&ID Editor produce fully ISA-5.1 compliant drawings?

No — it is a drafting tool with ISA-5.1-style symbols, not a conformance checker. The editor provides instrument bubbles for field and panel instruments, equipment shapes (vessels, pumps, heat exchangers, columns, compressors, tanks), valve bodies and actuators in the ISA-5.1 style; DCS/PLC function shapes and off-sheet connectors are not provided. The nine connection line types are drawn as distinguishable dash patterns rather than ISA-5.1's own glyphs — ISA specifies a double-hash (—//—) pneumatic line and a plain dashed electric line, where the editor uses dotted and dot-dash, and four of the nine (solids conveying, compressed air, dust duct, by others) are conventions with no ISA or ISO equivalent. They are unambiguous within a drawing but not ISA-5.1 line-style conventions. Instrument tags use the ISA-style structure (variable letter + function letter + loop number, e.g. FT-101, PCV-305A) with a format check; this is not a full implementation of the ISA-5.1 identification system. The resulting PDF carries a title block with title, sheet, date and a revision field; whether that satisfies a company's management-of-change procedure is for that company to decide.

Is the P&ID editor free?

You can use the editor during a free 30-minute preview with no sign-up required. A free 14-day account trial (no credit card) unlocks every tool at Pro level, including unlimited sheets, cloud save, and the full editor symbol library. The branded PDF engineering report with title block, saved projects, and share links are included in the free 14-day trial and in the paid Pro plan.

Related standards

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