Symbols and Abbreviations

§1Symbols as a language

The value of a symbol is that it is unambiguous and international. A drawing dimensioned in millimetres reads the same in Sydney, Stuttgart and São Paulo — but only if the marks obey shared rules.

Two ideas run through this whole page. First, a written measurement has grammar: quantity symbols and unit symbols are different kinds of word and are set differently (the hero drawing). Second, the marks are case-sensitive and space-sensitive in ways ordinary prose is not — m and M, k and K, a space present or absent, each change the meaning. The rules below are the international conventions (SI, as adopted in the AS/ISO standards); following them is the difference between a specification and a guess.

Contents

§2The grammar of writing SI

A short rulebook prevents almost every unit error seen on real drawings and reports.

The writing rules, with right and wrong
Rule	Right	Wrong
Space between value and unit	25 mm, 10 A	25mm, 10A
Unit symbols are upright (roman)	m, kg, Pa	m, kg
Quantity symbols are italic	F, v, σ	F, v, σ (upright)
No plural “s” on a symbol	5 kg	5 kgs
No full stop (except sentence end)	10 m long	10 m. long
Case matters	kW (kilowatt)	KW, kw
Product: middle dot or space	N·m, N m	Nm (ambiguous), N-m
Quotient: slash or negative power	m/s, m·s⁻¹	m/s/s
Prefix attaches directly	mm, µF	m m, µ F
One prefix only	nm	mµm
Names, unlike symbols, are lower-case even when honouring a person (newton, pascal, kelvin) and do take plurals (newtons) — it is only the symbol (N, Pa, K) that is capitalised and never pluralised.

Contents

§3Base-unit symbols

Seven base units name the seven base quantities; every other unit is built from these (the Measuring Units page).

The SI base units
Quantity	Unit	Symbol
Length	metre	m
Mass	kilogram	kg
Time	second	s
Electric current	ampere	A
Thermodynamic temperature	kelvin	K
Amount of substance	mole	mol
Luminous intensity	candela	cd
The kilogram is the lone base unit that already carries a prefix — a historical quirk, which is why mass prefixes attach to the gram (mg, g, Mg) rather than compounding onto kg.

Contents

§4Prefix symbols

A prefix scales a unit by a power of ten. Engineering favours the powers that are multiples of three, so numbers stay between 0.1 and 1000.

SI prefixes across the engineering range (each a power of ten)
Prefix	Symbol	Factor	Prefix	Symbol	Factor
tera	T	10¹²	deci	d	10⁻¹
giga	G	10⁹	centi	c	10⁻²
mega	M	10⁶	milli	m	10⁻³
kilo	k	10³	micro	µ	10⁻⁶
hecto	h	10²	nano	n	10⁻⁹
deca	da	10¹	pico	p	10⁻¹²
Beyond this range sit peta/exa/zetta/yotta and the 2022 additions ronna and quetta upward, and femto/atto/zepto/yocto with ronto and quecto downward — completing 10³⁰ down to 10⁻³⁰.

Example 1 — prefixes doing their job

A clearance of 0.000021 m is unreadable; written with the micro prefix it is 21 µm — the very IT7 tolerance at Ø25 from the fits page. A pressure of 13 800 000 Pa becomes 13.8 MPa. The prefix earns its place by keeping the significant figures in view and the zeros out of it.

Two cautions: the capital K is kelvin, the lower-case k is kilo — “Kg” and “KW” are both wrong; and the prefix binds tighter than any power, so mm² means (mm)² = 10⁻⁶ m², not m(m²). The four largest and smallest prefixes (quetta/ronna and quecto/ronto) were added in 2022 and rarely surface in mechanical work, but the range they complete is worth knowing exists.

Contents

§5The Greek alphabet in engineering

Greek letters carry the quantities the Latin alphabet ran out of room for — and several appear on nearly every page of this Library.

Greek letters and their common engineering duties
Letter	Name	Typical use
α	alpha	coefficient of thermal expansion; angular acceleration; an angle
β	beta	an angle; a ratio
γ	gamma	shear strain; specific weight
Δ δ	delta	Δ a change or difference; δ a small deflection
ε	epsilon	strain (direct)
η	eta	efficiency; dynamic viscosity
θ	theta	an angle; angle of twist
λ	lambda	wavelength; a roughness cutoff (λc)
μ	mu	coefficient of friction; the micro prefix; dynamic viscosity
ν	nu	Poisson’s ratio; kinematic viscosity
π	pi	3.14159…; the circle constant
ρ	rho	density; radius of curvature; resistivity
Σ σ	sigma	Σ summation; σ direct stress; standard deviation
τ	tau	shear stress
φ	phi	an angle; angle of repose; a diameter in text
ω	omega	angular velocity
Ω	omega (cap.)	the ohm
Context disambiguates the overloaded letters: μ is friction in a statics equation, the micro prefix before a unit, and viscosity in a flow one — the surrounding symbols make which is meant unmistakable.

Contents

§6Quantity and drawing abbreviations

Beyond units, two more vocabularies recur: the italic letters for physical quantities, and the upright abbreviations that populate drawings.

Common quantity symbols and drawing abbreviations
Quantity symbols	Drawing abbreviations
F force · m mass · a acceleration	Ø diameter · R radius · THK thick
v velocity · s distance · t time	TYP typical · REF reference · NTS not to scale
P power · T torque or temperature · E energy	CL centre line · CSK countersink · CBORE counterbore
σ stress · τ shear · ε strain	MATL material · HT heat treat · TOL tolerance
I inertia/current · A area · V volume/voltage	ASSY assembly · DWG drawing · REV revision
Letters are reused across fields — T is torque on the Shafts page and temperature on the Strength page; V is volume, voltage or shear force. Always define the symbols a document relies on, ideally in a nomenclature block, and never let two meanings of one letter share a single equation.

Contents

§7Quick reference

The working core of the page on one card rack.

Grammar

25 mm (space) · N·m (dot)

no plural, no full stop

Typeface

quantity italic · unit upright

Case trap

k kilo · K kelvin

m milli/metre · M mega

Prefixes

prefer steps of 10³

one prefix only

Greek staples

σ stress · τ shear · ε strain

μ friction · ω ang. velocity

Contents

Handbook application: from concept to controlled practice

Purpose. This expanded section turns the original page into a practical handbook. It preserves the supplied material and adds a repeatable way to apply, check and review Symbols and Abbreviations. It does not replace a contract, legislation, a controlled standard, competent engineering judgement or specialist advice.

The operating aim is to carry the subject from function and assumptions through design evidence, verification and controlled release. Read the original explanation first, then use the workflow and checks below to convert knowledge into evidence.

Apply Symbols and Abbreviations by beginning with the duty, not the component or software command. Convert the key ideas—symbols, abbreviations, writing, grammar, base-unit—into measurable requirements and interfaces. Record operating and non-operating environments, duty cycle, expected life, loads, energy sources, human interaction and reasonably foreseeable abnormal conditions. When a value is not a project requirement or verified supplier datum, identify it as an assumption or illustrative value.

Create a calculation and evidence trail that another competent person can audit. Every input should carry a source, unit, revision and uncertainty or tolerance where relevant. Every model should state its boundary conditions and limitations. Keep nominal capacity separate from design capacity, and keep verification margin separate from an arbitrary safety factor. If a code or standard governs the work, confirm the applicable edition and contractual status rather than copying a number from a secondary summary.

Design for manufacture, assembly, inspection, operation and maintenance at the same time. A technically valid geometry can still fail because it cannot be fixtured, measured, cleaned, guarded, reached or replaced. Review process capability, datum or reference strategy, tolerance accumulation, access, error-proofing and changeover. Where people interact with plant, apply the hierarchy of controls and consult those who will operate, clean, maintain and recover the equipment.

Plan verification before release. Define the characteristic, method, equipment, sample or test condition, acceptance criterion, record and responsible person. Validation then asks a different question: whether the resulting system is effective and suitable in the intended use context. A passed drawing check or analysis does not by itself validate usability, maintainability or production performance.

Step-by-step operating method

Define the duty. Capture the required function, interfaces, operating environment, life, loads and unacceptable outcomes.
Establish the model. Identify governing principles, units, material or process data, assumptions and uncertainty.
Develop alternatives. Compare feasible concepts against performance, manufacturability, safety, maintainability and cost.
Verify the design. Use analysis, test, inspection or demonstration with acceptance criteria defined before execution.
Release and learn. Baseline the design, control changes, retain evidence and feed operating results into the next revision.

Illustrative design review record

Illustrative values only. Build a one-page record with the required function, input sources, assumptions, governing load or process condition, failure consequences, selected concept, verification method and acceptance criterion. Mark every numerical input as project requirement, verified supplier data, measured value, calculation output or assumption. Review the weakest evidence first. If an assumption can change safety, compliance, interchangeability or capacity, it must be resolved before release rather than buried in a calculation note.

Evidence class	Question	Release expectation
Requirement	What must the design do and under which conditions?	Approved and traceable
Input	Where did the load, property, tolerance or process limit come from?	Source, unit and revision recorded
Analysis	Which model and assumptions connect input to result?	Checkable calculation or simulation
Verification	How will conformity be demonstrated?	Method and acceptance criterion agreed
Validation	Will the solution work for intended users and conditions?	Representative use evidence

Common failure modes and recovery actions

1. Watch for

Starting detailed design before interfaces and operating limits are agreed.

Recovery: Return to the governing definition or requirement and restate the decision in one sentence.

2. Watch for

Using catalogue or typical values as though they were certified project inputs.

Recovery: Separate evidence from assumption, assign an owner and set a date for validation.

3. Watch for

Checking nominal performance while ignoring tolerances, degradation and foreseeable misuse.

Recovery: Run a small counterexample, boundary test, pilot or independent check before proceeding.

4. Watch for

Confusing verification of requirements with validation of user need.

Recovery: Record the consequence, decision and rationale, then update the controlled baseline.

5. Watch for

Releasing drawings or procedures without configuration, inspection and change controls.

Recovery: Escalate when the issue affects safety, compliance, acceptance, material value or an agreed tolerance.

Review checklist

What function and failure consequence govern this decision?
Which inputs are measured, specified, assumed or illustrative?
How will conformity be demonstrated and recorded?
What change would invalidate the current evidence?
Are mandatory requirements distinguished from recommendations and illustrative values?
Are sources, assumptions, units, dates and versions recorded closely enough to reproduce the decision?
Have safety, legal, ethical, stakeholder and operational consequences been considered at the appropriate level?
Is there a named owner and a trigger for review, escalation, change or retirement?

Questions for deeper application

What is the most important distinction a practitioner must preserve when applying Symbols and Abbreviations?