In 5 km and 300 K, the letters look almost the same, but they do completely different jobs. Lowercase k is the SI prefix kilo, or 103; uppercase K is the unit symbol for kelvin.
That one change of case is part of the notation itself. Treating unit strings like ordinary prose—lowercasing or title-casing them for consistency—can change their meaning in a way that is invisible at a glance.
Read the case before the unit
| Mark | What it stands for | How it behaves | Example in context |
|---|---|---|---|
| k | kilo = 10³ | SI prefix | km, kHz, kΩ |
| K | kelvin | SI base unit | 300 K |
k is a prefix, not a unit
For kilo, the prescribed prefix symbol is lowercase k, representing a factor of 103. You see it only as part of a larger unit token: km, kHz, kΩ.
The prefix symbol is not a standalone unit. Writing 5 k does not, by itself, specify a complete SI unit. The following unit symbol is what tells you what has been multiplied by one thousand.
K is already a complete unit symbol
K is already a complete unit symbol. A thermodynamic temperature such as 300 K therefore has a space between the number and K, just as other numerical values are separated from their unit symbols.
Kelvin is written without a degree sign. 300 K is current notation; 300 °K is historical. For the reason Celsius keeps ° while kelvin does not, see Celsius °C vs Kelvin K.
Case cannot be normalized away
SI prefix and unit symbols are case-sensitive. In measurement symbols, k is deliberately lowercase for kilo, while mega is M and giga is G. Kelvin is uppercase K because the unit is named after William Thomson, Lord Kelvin.
Changing the case is therefore not a cosmetic edit. km is kilometre. Km is not an alternative capitalization for kilometre, and a lowercase k cannot replace the kelvin symbol in a temperature.
Where the special Kelvin sign K fits
Unicode also contains U+212A KELVIN SIGN K. It is a legacy letterlike character that is canonically equivalent to the ordinary capital letter K. Unicode recommends the regular letter for current text, and Unicode normalization can map the special Kelvin sign to K.
The separate guide Kelvin Sign K vs Letter K covers that encoding issue in detail. It is a different question from k vs K: one comparison is about Unicode character identity, while this one is about SI case and meaning.
Position tells you a lot
Position is a strong clue. In kPa, kW, and kV, lowercase k modifies the unit that follows. In 273.15 K, uppercase K is already the complete unit. There is no SI prefix K that can be substituted for kilo.
Compare 5 kW with 5 K. In the first expression, k cannot be read on its own; it scales the watt that follows. In the second, K is the complete unit. That structural difference is often more informative than the letter shape itself.
This positional pattern is useful for parsers as well as readers. Instead of asking what the letter means in isolation, first identify the complete unit token and then interpret its prefix and unit components.
Why auto-capitalization breaks measurements
Spreadsheet cleanup, OCR correction, title-casing, and database normalization sometimes change letter case automatically. Applied to measurement data, that can corrupt valid unit strings. A rule designed for ordinary prose should not transform kW, MW, mW, or K.
The same problem appears with m vs M, where lowercase m can represent metre or milli and uppercase M means mega.
Reading k and K in context
First decide whether the text is actually a measurement expression. A K in a person’s name or a k in an identifier has nothing to do with units. Inside a unit expression, preserve case exactly.
Then look at structure: k + unit is kilo; K as the unit after a temperature value is kelvin. That simple rule handles most real-world cases without guessing from shape alone.
No space inside a prefixed unit
A prefix and its unit form one symbol, so km and kΩ stay closed up. The space comes before that whole token: 5 km, 10 kΩ. Splitting the prefix from the unit would misrepresent the structure of the notation.
A bare k is therefore not a complete SI unit. Informal writing may use “k” to mean one thousand in prices or counts, but that convention is separate from the SI prefix attached to a unit symbol.
SI symbols do not follow ordinary capitalization rules
Editorial capitalization rules stop at the edge of a unit token. A heading may begin with km or kW without changing the prefix simply because the expression happens to appear first.
Likewise, kelvin remains K wherever it appears. Read these strings as notation with prescribed case, not as words waiting to be title-cased.