Rho

The seventeenth Greek letter, used for density and resistivity.

Also known as: Greek letter rho, Rho symbol, Density symbol, ρ

ScientificMathematicalEngineering & technicalStandard profile

Meanings

Rho is the conventional letter for density — mass per unit volume in physics, and by extension charge density and electrical resistivity. It is one of the more consistent letters: wherever it appears, something is being measured per unit of something else.

Common modern usageVerified fact
Is a letter of the Greek alphabet before it is a symbol: what it denotes is fixed by the field it appears in, not by the character itself. The uses below are the documented conventions, not competing definitions.
Contexts: Mathematics
Common modern usageVerified fact
In physics, denotes density — mass per unit volume.
Contexts: Mathematics
Common modern usageVerified fact
In electromagnetism, denotes electrical resistivity, or charge density.
Contexts: Mathematics
Common modern usageVerified fact
In statistics, denotes a population correlation coefficient.
Contexts: Mathematics
Common modern usageVerified fact
In polar coordinates, denotes the radial distance from the origin.
Contexts: Mathematics
Common modern usageVerified fact
Is drawn almost identically to the Latin letter p and is frequently mistyped as one, which silently changes the meaning in any formula that also uses p for pressure or probability.
Contexts: Mathematics

Visual description

A rounded bowl on the right of a vertical stroke that descends below the writing line, like a lowercase p.

Symmetry: none
Orientation: upright

Opposite of: Sigma
Electrical resistivity and conductivity are reciprocals of one another: rho is one divided by sigma.

Confidence

IdentityHigh confidence: High confidence because the identity is defined by a current official standard, but multiple independent sources agree.
Historical originModerate confidence: Moderate confidence because the historical origin is documented in encyclopedic references, but multiple independent sources agree, but early or supporting evidence is limited, but the regional or cultural match to this specific claim is not precise.
Current useHigh confidence: High confidence because the current use is documented in encyclopedic references, but multiple independent sources agree.
Visual matchHigh confidence: High confidence because the visual match is defined by a current official standard, but multiple independent sources agree.

Sources

Unicode Code Chart: Greek and Coptic (U+0370–U+03FF)
The Unicode Consortium · retrieved 2026-07-30
Authoritative for the character's code point, name and block only. Unicode assigns identity; it does not define what a symbol means in use.
Greek alphabet
Wikipedia · retrieved 2026-07-31
Tertiary reference. Used for the widely-agreed uses of the letter across fields and for its history. Not used to assert that any one use is the letter's definitive meaning — no body defines that, and the record says so.
Greek letters used in mathematics, science, and engineering
Wikipedia · retrieved 2026-07-31
Tertiary reference. Used for the widely-agreed uses of the letter across fields and for its history. Not used to assert that any one use is the letter's definitive meaning — no body defines that, and the record says so.
Rho
Wikipedia · retrieved 2026-07-31
Tertiary reference. Used for the widely-agreed uses of the letter across fields and for its history. Not used to assert that any one use is the letter's definitive meaning — no body defines that, and the record says so.

Last reviewed 2026-07-31