§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.
| 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. | ||
§3Base-unit symbols
Seven base units name the seven base quantities; every other unit is built from these (the Measuring Units page).
| 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. | ||
§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.
| 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. | |||||
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.
| 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. | ||
§6Quantity and drawing abbreviations
Beyond units, two more vocabularies recur: the italic letters for physical quantities, and the upright abbreviations that populate drawings.
| 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. | |
§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