Chapter 13: SI Prefixes
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Because physical quantities can range from the astronomically large (the distance to a star) to the vanishingly small (the diameter of an atomic nucleus), SI provides a standard set of prefixes that can be attached to any base or derived unit to indicate a multiple or sub-multiple of ten. This avoids having to write out long strings of zeros and gives every branch of science a common shorthand — for instance, a kilometre is 1000 metres, a millimetre is one-thousandth of a metre, and a nanosecond is one-billionth of a second. These prefixes are one of the most frequently and directly tested topics in SSC/RRB general science sections, often appearing as direct "what does X prefix mean" or "convert X to Y" questions.
13.1 Complete List of SI Prefixes
Prefix | Symbol | Factor | Value |
|---|---|---|---|
yotta | Y | 10²⁴ | 1,000,000,000,000,000,000,000,000 |
zetta | Z | 10²¹ | 1,000,000,000,000,000,000,000 |
exa | E | 10¹⁸ | 1,000,000,000,000,000,000 |
peta | P | 10¹⁵ | 1,000,000,000,000,000 |
tera | T | 10¹² | 1,000,000,000,000 |
giga | G | 10⁹ | 1,000,000,000 |
mega | M | 10⁶ | 1,000,000 |
kilo | k | 10³ | 1,000 |
hecto | h | 10² | 100 |
deca | da | 10¹ | 10 |
— (base unit) | — | 10⁰ | 1 |
deci | d | 10⁻¹ | 0.1 |
centi | c | 10⁻² | 0.01 |
milli | m | 10⁻³ | 0.001 |
micro | µ | 10⁻⁶ | 0.000001 |
nano | n | 10⁻⁹ | 0.000000001 |
pico | p | 10⁻¹² | 0.000000000001 |
femto | f | 10⁻¹⁵ | 0.000000000000001 |
atto | a | 10⁻¹⁸ | 0.000000000000000001 |
zepto | z | 10⁻²¹ | 0.000000000000000000001 |
yocto | y | 10⁻²⁴ | 0.000000000000000000000001 |
13.2 Commonly Used Prefixes and Practical Examples
Prefix + Unit | Meaning | Typical Use |
|---|---|---|
kilogram (kg) | 1000 grams | The SI base unit of mass itself |
kilometre (km) | 1000 metres | Road distances |
kilowatt (kW) | 1000 watts | Power rating of motors, appliances |
megawatt (MW) | 10⁶ watts | Power plant capacity |
gigabyte (GB) | 10⁹ bytes (in SI decimal sense) | Data storage (computing uses 2³⁰ informally, but SI prefix is decimal) |
millimetre (mm) | 0.001 metre | Precision length measurement, rainfall |
millisecond (ms) | 0.001 second | Fast timing in electronics |
microsecond (µs) | 10⁻⁶ second | Very fast electronic switching |
micrometre / micron (µm) | 10⁻⁶ metre | Size of cells, fine particles, wavelength of light (in hundreds of nm) |
nanometre (nm) | 10⁻⁹ metre | Wavelength of visible light, size of molecules |
picofarad (pF) | 10⁻¹² farad | Small capacitor values in electronics |
angstrom (Å, non-SI but common) | 10⁻¹⁰ metre | Atomic radii, X-ray wavelengths |
13.3 Exam Tip — Remembering the Order
A helpful way to remember the order of the common positive-power prefixes is the sequence deca, hecto, kilo, mega, giga, tera, peta, exa, zetta, yotta (each 1000 times larger than the one three steps below it after kilo, except deca and hecto which are only ×10 and ×100). For negative powers, the sequence deci, centi, milli, micro, nano, pico, femto, atto, zepto, yocto runs the same way in reverse. Notice that from kilo onward (and from milli downward), each named prefix jumps by a factor of 1000 (10³), while deca/hecto and deci/centi are the odd ones out, stepping only by 10 and 100. Questions frequently ask students to convert directly between two of these — for example, how many micrometres make one centimetre (answer: 1 cm = 10⁴ µm) — so practising a few such conversions is worthwhile.