SI Units & Engineering Prefixes
Also known as: metric prefixes, kilo mega micro nano
9 min read
Quick Answer
SI units are the internationally agreed units of measurement — the volt, ampere, ohm, farad and second among them. An engineering prefix is a single letter glued to the front of such a unit to mean a power of ten: kilo is a thousand, nano a thousandth of a millionth. Prefixes keep electronic quantities readable.
Intuition
Counting in steps of a thousand
Electronics deals in numbers that are absurdly large and absurdly small, often in the same sentence. A radio receiver works at hundreds of millions of cycles per second while the capacitor tuning it holds 22 pF — a value that, spelled out in farads, is 0.000000000022 F. Nobody wants to count those zeros twice, and the notation is arranged so that nobody has to.
Every quantity is written as a plain number, then a unit that says what is being measured (the volt, the ampere, the ohm, the farad, the second), and — where it helps — a prefix: a single letter glued to the front of the unit meaning "multiply by this power of ten".
The prefixes that matter in electronics climb and descend in steps of a thousand. Going up: kilo (k) is a thousand, mega (M) a million, giga (G) a thousand million. Going down: milli (m) is a thousandth, micro (µ) a millionth, nano (n) a thousandth of a millionth, pico (p) a millionth of a millionth.
A prefix never changes what is being measured; it only moves the decimal point. "4.7 kΩ" is not a special kind of ohm, it is 4700 ordinary ohms written in four characters instead of five. Learn the ladder once and the meter displays and datasheets in the rest of the course stop being cryptic.
Practitioner
Reading, writing and converting
Almost every value you will meet is written with one of these:
| Prefix | Symbol | Multiplier | Where you meet it |
|---|---|---|---|
| giga | G | 10⁹ | GHz clock and radio frequencies |
| mega | M | 10⁶ | MΩ pull-ups, MHz |
| kilo | k | 10³ | kΩ resistors, kV insulation ratings |
| — | — | 1 | the base unit itself |
| milli | m | 10⁻³ | mA currents, mV signals, ms delays |
| micro | µ | 10⁻⁶ | µF capacitors, µA sleep currents |
| nano | n | 10⁻⁹ | nF capacitors, ns switching times |
| pico | p | 10⁻¹² | pF capacitors and stray capacitance |
The habits below head off almost every prefix mistake:
- Convert to base units before you calculate — ohms, amperes, volts, farads, seconds — and convert back afterwards, for presentation only. Mixing prefixes mid-calculation is the single most common source of answers that are wrong by a factor of a thousand.
- Respect the case. Lowercase k, m, n and p; uppercase M and G. K is the kelvin, not kilo; M is mega, while m is milli, a thousand million times apart.
- Never stack prefixes. There is no such thing as a millimicrofarad, so use the single prefix that fits.
- Leave a space between the number and the unit, and never pluralise a symbol: five kilohms is 5 kΩ, not 5 kΩs.
You will also meet prefixes used as the decimal point on components and schematics, because a printed dot is easy to lose: 4k7 means 4.7 kΩ, 2µ2 means 2.2 µF, 1n5 means 1.5 nF. The same convention appears in the resistor colour code and in capacitor markings.
Worked example — From marked values to base units
Take three parts labelled the way a schematic would print them: a resistor of 4.7 kΩ, a capacitor of 100 nF, and a current limit of 25 mA.
Before any of them enters a calculation, each goes back to its base unit. The resistor is 4700 Ω, the capacitor is 0.0000001 F, and the current is 0.025 A. Each one is the same component before and after; only the way it is written has changed.
A multimeter does this conversion in front of you: the same resistor reads in kilohms on one range and in ohms on another, and both readings describe the same resistance.
Engineer
The system underneath, and the rules that come with it
The SI is built from seven base units. Electronics uses four of them constantly — the second (s), metre (m), kilogram (kg) and ampere (A) — plus the kelvin (K) whenever temperature enters. Everything else is a derived unit, formed as a product or quotient of base units with no numerical conversion factor anywhere: the volt is one watt per ampere, the ohm one volt per ampere, the farad one coulomb per volt. That coherence is why unit errors in electronics are almost always prefix errors rather than genuine conversion errors: there is no conversion factor left to get wrong.
Since 20 May 2019 the SI has been defined by fixing the exact numerical values of seven physical constants rather than by physical artefacts. The ampere is now defined by fixing the elementary charge, and the kilogram by fixing the Planck constant. No number a working engineer uses moved. The gain is reproducibility: the definitions can now be realised in any adequately equipped laboratory, and constants like the elementary charge are exact by definition rather than measured — a point that matters as soon as you start counting charge carriers in Electric Charge.
Style is part of the specification, and reviewers of schematics and datasheets do enforce it:
- Unit symbols are printed upright, are never pluralised, and take no full stop unless they end a sentence.
- A symbol derived from a person's name is capitalised (V, A, W, Hz, K), while the spelled-out unit name is not (volt, ampere, watt, hertz, kelvin).
- Prefix symbols for 10⁶ and above are capitalised (M, G, T); those for 10³ and below are lowercase (k, m, µ, n, p). Kilo is the prefix most often capitalised by mistake, and it never takes a capital.
- The micro symbol is µ. Plain "u" is a widely tolerated ASCII substitute in schematic capture, BOMs and part numbers, but it is a substitution rather than correct SI, and it is why a search for "10uF" and a search for "10 µF" return different results in a component database.
- A prefix attaches to a unit, never to a bare number, and never to another prefix.
Memory and storage sizes sit outside SI entirely. They use binary prefixes — kibi, mebi, gibi (Ki, Mi, Gi) for steps of 2¹⁰ — standardised in IEC 80000-13 precisely because "kilobyte" was being used for two different numbers. In SI, and therefore in every electrical quantity on this site, kilo means 10³ exactly.
The ladder's most useful property is that prefixes cancel. Take the relationship you will meet properly in Ohm's law:
Worked example — Prefixes that cancel
A current of 1 mA flows through a resistance of 1 kΩ. Multiplying current by resistance gives a voltage of 1.0 V.
Milli is a thousandth and kilo is a thousand, so the two prefixes cancel exactly and the answer lands in plain volts with no decimal-point bookkeeping at all. Milliamperes times kilohms gives volts; microamperes times megohms gives volts too. Used deliberately, those pairings take most of the zero-counting out of everyday design arithmetic.
Where a quantity spans many decades — gain, attenuation, sound level — engineers switch from prefixes to a logarithmic scale instead; that is the subject of the decibel. And when a value carries an uncertainty rather than being exact, how many digits you are entitled to write is a separate question, handled in measurement accuracy.
Common mistakes
- Writing KΩ or Kohm — K is the kelvin. Kilo is a lowercase k, always.
- Confusing M with m — a 1 MΩ resistor and a 1 mΩ resistor differ by a factor of a thousand million. On a hand-drawn schematic, write the word out if there is any doubt.
- Calculating with mixed prefixes — multiplying kilohms by milliamperes works only because those two cancel; kilohms times microamperes does not. Convert to base units unless you have checked the cancellation.
- Reading µ as m on a component — the difference between a 100 µF and a 100 mF capacitor is a thousandfold, and only one of them fits on the board.
- Assuming kilo means 1024 — that is the binary prefix kibi, and it applies to memory sizes, never to volts, ohms or farads.
Frequently asked questions
What does the prefix micro mean?
Micro (symbol µ) means one millionth — a factor of 10⁻⁶. A microfarad is a millionth of a farad and a microampere is a millionth of an ampere.
Why do the common prefixes jump in steps of a thousand?
Because they line up with engineering notation, where exponents move in multiples of three. That keeps the digits before the decimal point between one and nine hundred and ninety-nine, which is the range people read most reliably.
Is it 'kΩ' or 'KΩ'?
kΩ. Lowercase k is the SI prefix kilo; uppercase K is the symbol for the kelvin, a completely different quantity. Prefix symbols only become capitals at mega and above.
Can I write uF instead of µF?
In schematic capture, BOMs and plain-text part numbers, yes — it is universally understood and often unavoidable when the tool is ASCII-only. It is not correct SI, and it means database searches must allow for both spellings.
Do prefixes change what a quantity actually is?
No. A prefix is pure notation: it multiplies the number by a power of ten and leaves the unit — and the physical quantity — untouched.
Knowledge check
How many ohms is 2.2 kΩ? (Show answer)
Express 0.00047 F using an SI prefix. (Show answer)
Which is the larger resistance: 1 MΩ or 1000 kΩ? (Show answer)
Why is it wrong to write a resistance as KΩ? (Show answer)
A datasheet gives a capacitance as 2n2. What does that mean? (Show answer)
References
- Bureau International des Poids et Mesures, The International System of Units (SI), 9th edition — base units, derived units, prefixes and the 2019 constant-based definitions.
- International Electrotechnical Commission, IEC 80000-13: Quantities and units — Information science and technology — the binary prefixes kibi, mebi and gibi.