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The case is the entire difference, and it separates a unit from a multiplier of a thousand.
A capital K is the kelvin, a unit of temperature. A lower-case k is the prefix kilo, meaning a factor of one thousand and never standing alone. So kW is a kilowatt and KW is not a real unit symbol at all. NIST's guide lists exactly this kind of case error among the standard mistakes in applying the SI.
Capital K is the kelvin; lower-case k is the prefix kilo. The case is the whole distinction.
| Kelvin symbol | Kilo prefix symbol | |
|---|---|---|
| What it is | A unit of thermodynamic temperature | A prefix meaning one thousand |
| Case | Always capital | Always lower case |
| Stands alone? | Yes — 300 K is a complete quantity | No — it must precede a unit symbol |
| Named after a person? | Yes, Lord Kelvin, which is why it is capitalised | No, so it stays lower case |
| Relationship (from Symbolwise's records) | Commonly confused with Kilo prefix symbol — Capital K is the kelvin; lower-case k is the prefix kilo. The case is the whole distinction. | Commonly confused with Kelvin symbol — Lower-case k is the prefix kilo; capital K is the kelvin. The case is the whole distinction. |
The International System of Units (SI), 9th edition
Bureau International des Poids et Mesures · retrieved 2026-07-31
The defining document for SI unit names and symbols, including the capitalisation and spacing rules. It defines units; it says nothing about how a symbol is encoded in a character set, and nothing about non-SI units except where it lists those accepted for use with the SI.
Guide for the Use of the International System of Units (SI), NIST Special Publication 811
National Institute of Standards and Technology · retrieved 2026-07-31
A national metrology institute's guidance on applying the SI, including symbol formatting. Authoritative for usage rules in the United States; it interprets the SI rather than defining it.
Wikipedia · retrieved 2026-07-31
Tertiary reference. Used for origin narrative and for units the SI does not define, never for an official-definition claim.
Look at what follows. A prefix never stands alone — it is glued to a unit symbol, as in kW or kg. A constant stands by itself in an equation. Careful physical writing subscripts the constant as k_B for exactly this reason, and the kelvin's capital K is a third claimant to the same letter.
A change of one kelvin and a change of one degree Celsius are exactly the same size, so a temperature *difference* may be stated in either. The scales start in different places: 0 K is absolute zero and 0 °C is the freezing point of water, 273.15 K. The written forms differ too — the kelvin takes no degree sign, so 300 K is right and 300 °K is not.
Look for the horizontal line. Fire is a plain upward triangle; air is the same triangle with a bar drawn across its upper part. Dropping the bar is the commonest error in reproductions of the element set, and it turns air into fire.
The same rule as fire and air, inverted. Water is a plain downward triangle; earth is the same triangle with a bar across its lower part. The four element signs are one designed set: two point up, two point down, and within each pair the bar is the only difference.