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.
Alpha is a Greek letter and stands for a quantity — an angle, a coefficient, a significance level. The proportionality sign is an operator and stands between two quantities to say they keep a constant ratio. The drawings differ in that the proportionality sign's right-hand stroke runs out in an open curve rather than closing into the letter's tail.
The becquerel counts nuclear decays — one per second — and says nothing about harm. The sievert weights absorbed energy by the kind of radiation delivering it, and so estimates biological effect. A becquerel of one isotope and a becquerel of another can carry very different risk, which is exactly why exposure limits and medical doses are stated in sieverts and never in becquerels.