What does darcy mean?
A darcy is a unit of measurement used to express permeability — how readily a fluid flows through a porous substance such as sandstone, limestone, or packed soil. Named after the French engineer Henry Darcy, whose 1856 experiments on water flowing through sand established the foundational law of groundwater movement, the unit quantifies how much fluid passes through a given area under a given pressure gradient. Although it is not part of the International System of Units, the darcy remains standard vocabulary among hydrogeologists and petroleum engineers, who more often speak in millidarcies because most natural rocks are far less permeable than one full darcy. A high darcy value signals a material through which oil, gas, or water moves easily, making the term central to discussions of aquifers, oil reservoirs, and filtration. Outside technical fields it appears mainly in writing about energy and groundwater science.
nounA darcy is a non-SI unit of permeability, measuring the ease with which a fluid can flow through a porous material such as rock or sand. It equals the permeability that permits a flow rate of one cubic centimetre per second of a fluid of one centipoise viscosity through one square centimetre under a pressure gradient of one atmosphere per centimetre.
- A unit of permeability used in geology and petroleum engineering to quantify how easily fluids pass through porous material.
"The sandstone's permeability of several hundred millidarcies made it an excellent reservoir rock."
"Core samples from the field showed permeabilities ranging from fifty to two hundred darcies."
"Because most reservoir rocks are far less permeable, engineers usually report values in millidarcies rather than darcies."
Regular English plural; used when stating numerical values greater than one, though measurements are often left uninflected after numerals in technical tables.
"The formation varies from ten darcies near the surface to a fraction of a darcy at depth."
The darcy is one of the few everyday-ish units named after a man who never saw it coined — Henry Darcy died decades before petroleum engineers honored his 1856 water-flow experiments with his own name.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of darcy
The unit is named after Henry Darcy (1803–1858), the French engineer whose careful experiments on water percolating through sand led to Darcy's law, published in 1856 and still the basis of groundwater hydraulics. The term was adopted by later scientists and engineers as a tribute to his work. It should not be confused with the surname Darcy of literary fame, which derives separately from the Norman-French place name d'Arcy.
Related word forms
How darcy is actually used
The darcy is used almost exclusively in hydrogeology, soil science, and petroleum engineering; it is not part of the SI system, so formal scientific publications often prefer SI units (square metres) for permeability. In practice the millidarcy (mD) is far more common, since intact rock rarely reaches a full darcy. The word takes a plural in ordinary use ("darcies").
Easily confused with darcy
"Darcy" capitalized is most familiar as the surname of Mr. Fitzwilliam Darcy, Jane Austen's hero in Pride and Prejudice (and a family name), while "darcy" lowercase is a scientific unit of permeability named for French engineer Henry Darcy.