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The same lower-case letter, claimed by a constant and by a multiplier of a thousand.
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.
Both are a bare lower-case k, and both appear in physical formulae.
| Boltzmann constant symbol | Kilo prefix symbol | |
|---|---|---|
| What it is | A physical constant | A multiplier of one thousand |
| Stands alone? | Yes | No — always before a unit |
| Careful form | Subscripted as k_B | Bare k, always lower case |
| Also claimed by | Spring and rate constants | Nothing — but capital K is the kelvin |
| Relationship (from Symbolwise's records) | Commonly confused with Kilo prefix symbol — Both are a bare lower-case k, and both appear in physical formulae. | Commonly confused with Boltzmann constant symbol — A third claimant to the letter k, alongside the kelvin. |
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol denotes in ordinary scientific practice and for which fields reuse it, never for a claim that any body assigns the letter its meaning.
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.
CODATA Internationally Recommended Values of the Fundamental Physical Constants
NIST / CODATA Task Group on Fundamental Constants · retrieved 2026-08-01
Authoritative for the recommended value and uncertainty of a physical constant, and for whether that value is exact by definition or measured. It fixes values; it does not decide which letter denotes which quantity, and no record cites it for a symbol's meaning.
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.
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.
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.