Per mille (‰): meaning, notation, history and common uses

Notation and conversions

Per mille values appear either with the symbol immediately after a number (for example 5‰) or spelled out as "per thousand." Conversions help compare scales:

• 1‰ = 0.1% (one tenth of a percent)
• 1‰ = 0.001 in decimal form
• 1‰ = 1,000 ppm (parts per million) — see parts per million (ppm) for that scale

History and name

The phrase derives from Latin roots: "per" (for each) and "mille" (thousand). The symbol and concept evolved from notations used in bookkeeping and scientific measurement as practitioners sought compact ways to record ratios smaller than percent. The dedicated symbol gained formal encoding in modern character sets and is supported by standard fonts and Unicode.

Common uses and examples

Per mille is used where thousandths are a convenient unit of measure. Typical examples include:

• Salinity of water (parts per thousand), e.g., ocean average close to 35‰
• Blood alcohol concentration in some countries, expressed in ‰ rather than percent
• Interest rate spreads or fees stated per thousand in finance and insurance
• Geological or materials analyses where composition is small but larger than ppm

For comparison with the more familiar percent (%), remember that percent indicates parts per hundred; per mille indicates parts per thousand, ten times smaller increments.

Usage notes and distinctions

Writers may prefer words ("per thousand") in prose to avoid unfamiliar symbols. Spellings vary by region and style guide. While compact, the ‰ sign is less common in everyday U.S. texts than in scientific or technical contexts. When high precision is required, authors choose between percent, per mille and ppm according to the magnitude of the quantity being reported.