The International System of Units, universally abbreviated as SI (from the French Système International d’Unités), is the modern metric system of measurement. It is maintained by the General Conference on Weights and Measures (CGPM) and serves as the standard framework for scientific, industrial, and commercial transactions globally. The system is built upon seven precisely defined base units, an array of derived units, and a standard set of decimal prefixes.
SI Base Units and International Symbols
The SI system defines seven fundamental dimensions of the physical world. Each dimension is assigned a unique base unit and a standardized symbol.
| Base Quantity | Base Unit Name | Unit Symbol | Physical Dimension Symbol |
| Length | meter | m | [L] |
| Mass | kilogram | kg | [M] |
| Time | second | s | [T] |
| Electric Current | ampere | A | [I] or [A] |
| Thermodynamic Temperature | kelvin | K | [Θ] or [K] |
| Amount of Substance | mole | mol | [N] |
| Luminous Intensity | candela | cd | [J] |
Technical Rules for Writing SI Units and Symbols
To maintain absolute uniformity in scientific literature, the CGPM has established strict syntax rules for using SI symbols. Missteps in these rules often form the basis of conceptual questions in competitive exams.
Rules for Capitalization
- Unit symbols named after an individual scientist are written with an initial capital letter. Examples: N for Isaac Newton, J for James Prescott Joule, W for James Watt, and V for Alessandro Volta.
- Unit symbols not named after a person are always written in lowercase. Examples: m (meter), kg (kilogram), and s (second).
- When a unit name is spelled out completely in prose, it must always begin with a lowercase letter, even if it is named after a scientist. Examples: newton, joule, ampere, and meter. The only exception is Celsius.
Rules for Plurals and Punctuation
- Unit symbols are mathematical entities, not abbreviations. Therefore, they never take a plural form. For example, 50 kilograms is written strictly as 50 kg, never 50 kgs.
- A symbol must never be followed by a period or full stop unless it falls at the very end of a sentence. Writing 5 m. inside a sentence is incorrect; it must be written as 5 m.
Rules for Spacing and Formatting
- A single space must always separate the numerical value from the unit symbol. Example: 25 kg is correct; 25kg is incorrect.
- This spacing rule applies to temperature measurements as well (e.g., 298 K, 25 °C). The only exceptions are the non-SI geometric units for angles: degree (°), minute (’), and second (”), where no space is left between the number and the symbol.
- Unit symbols must always be printed in Roman (upright) type, whereas variables representing physical quantities are printed in italics. Example: The equation for force is written as F = 10 N, where F is italicized as a variable, but N remains upright as a unit.
SI Prefixes: Multiples and Sub-multiples
SI prefixes are standardized syllables prefixed to unit names to express extremely large or small quantities in a concise, decimal-based manner. They scale the base unit by powers of $10$.
Prefixes for Multiples (Positive Powers of 10)
These prefixes are used to quantify macroscopic and astronomical dimensions, such as computing storage capacities, terrestrial distances, and stellar masses.
| Prefix | Symbol | Multiplier (Power of 10) | Value / Meaning |
| quetta | Q | 1030 | One nonillion |
| ronna | R | 1027 | One octillion |
| yotta | Y | 1024 | One septillion |
| zetta | Z | 1021 | One sextillion |
| exa | E | 1018 | One quintillion |
| peta | P | 1015 | One quadrillion |
| tera | T | 1012 | One trillion |
| giga | G | 109 | One billion |
| mega | M | 106 | One million |
| kilo | k | 103 | One thousand |
| hecto | h | 102 | One hundred |
| deca | da | 101 | Ten |
Prefixes for Sub-multiples (Negative Powers of 10)
These prefixes quantify microscopic, atomic, and subatomic dimensions, such as semiconductor gate widths, cellular structures, and light wavelengths.
| Prefix | Symbol | Multiplier (Power of 10) | Value / Meaning |
| deci | d | 10-1 | One-tenth |
| centi | c | 10-2 | One-hundredth |
| milli | m | 10-3 | One-thousandth |
| micro | μ | 10-6 | One-millionth |
| nano | n | 10-9 | One-billionth |
| pico | p | 10-12 | One-trillionth |
| femto | f | 10-15 | One-quadrillionth |
| atto | a | 10-18 | One-quintillionth |
| zepto | z | 10-21 | One-sextillionth |
| yocto | y | 10-24 | One-septillionth |
| ronto | r | 10-27 | One-octillionth |
| quecto | q | 10-30 | One-nonillionth |
Critical Rules for Combining Prefixes
- No Double Prefixes: Compound prefixes formed by juxtaposing two SI prefixes are strictly prohibited. For instance, 10-9 meters must be written only as 1 nm (one nanometer), never as 1 mμm (one milli-micrometer).
- The Kilogram Exception: The base unit for mass—the kilogram (kg)—already contains the prefix “kilo”. Because double prefixes are not allowed, prefixes for mass are applied directly to the unit gram (g). Example: 10-6 kg is written as 10-3 g, which equals 1 mg (one milligram), never 1μkg.
- Exponent Interaction: When an exponent is applied to a prefixed unit symbol, the exponent scales both the unit and the prefix. For example: 1 cm2 = (10-2 m)2 = 10-4 m2, not 10-2 m2.
UPSC Prelims High-Yield Facts and Trivia
Recent Expansions to the Prefix Scale
In late 2022, the 27th CGPM introduced four new prefixes to accommodate data volumes and cosmic dimensions: ronna (1027), quetta (1030), ronto (10-27), and quecto (10-30). This was driven primarily by advancements in global data sciences, where terms like “yottabyte” were becoming insufficient to describe global digital storage.
Important Non-SI Units Accepted for Use with SI
Several practical units do not belong to the official SI catalog but are officially sanctioned by the BIPM for usage due to their widespread global relevance:
- Time: Minute (min = 60 s), Hour (h = 3600 s), Day (d = 86,400 s).
- Volume: Liter (L or l = 1 dm3 = 10-3 m3).
- Mass: Tonne or Metric Ton (t = 103 kg).
- Pressure: Bar (bar = 105 Pa).
- Energy: Electronvolt (eV ≈ 1.602 × 10-19 J).