Thedarcy (ordarcy unit) andmillidarcy (md or mD) areunits ofpermeability, named afterHenry Darcy. They are notSI units, but they are widely used inpetroleum engineering andgeology. The unit has also been used in biophysics and biomechanics, where the flow of fluids such as blood through capillary beds and cerebrospinal fluid through the brain interstitial space is being examined.^[1] A darcy hasdimensions oflength².

Permeability measures the ability offluids toflow through rock (or other porous media). The darcy is defined usingDarcy's law, which can be written as:

${\displaystyle Q=\frac{Ak\mathrm{\Delta }P}{\mu \mathrm{\Delta }x}}$

where:

${\displaystyle Q}$	is the volumetric fluid flow rate through the medium
${\displaystyle A}$	is the area of the medium
${\displaystyle k}$	is thepermeability of the medium
${\displaystyle \mu }$	is the dynamicviscosity of the fluid
${\displaystyle \mathrm{\Delta }P}$	is the appliedpressure difference
${\displaystyle \mathrm{\Delta }x}$	is the thickness of the medium

The darcy is referenced to a mixture of unit systems. A medium with a permeability of 1 darcy permits a flow of 1 cm³/s of a fluid withviscosity 1 cP (1 mPa·s) under a pressure gradient of 1 atm/cm acting across an area of 1 cm².

Typical values ofpermeability range as high as 100,000 darcys for gravel, to less than 0.01 microdarcy for granite. Sand has a permeability of approximately 1 darcy.^[2]

Tissue permeability, whose measurementin vivo is still in its infancy, is somewhere in the range of 0.01 to 100 darcy.^[1]

The darcy is named afterHenry Darcy.^[1] Rock permeability is usually expressed in millidarcys (md) because rocks hosting hydrocarbon or water accumulations typically exhibit permeability ranging from 5 to 500 md.

The odd combination of units comes from Darcy's original studies of water flow through columns of sand.Water has a viscosity of 1.0019 cP at about room temperature.

The unit abbreviation "d" is not capitalized (contrary to industry use).^{[clarification needed]} The American Association of Petroleum Geologists^[3] uses the following unit abbreviations and grammar in their publications:

• darcy (plural darcys, not darcies): d
• millidarcy (plural millidarcys, not millidarcies): md

Converted toSI units, 1 darcy is equivalent to9.869233×10⁻¹³ m² or 0.9869233 μm².^[4] This conversion is usually approximated as 1 μm². This is the reciprocal of 1.013250—the conversion factor from atmospheres tobars.^[1]

Specifically in the hydrology domain, permeability of soil or rock may also be defined as theflux of water underhydrostatic pressure (≈ 0.1 bar/m) at a temperature of 20 °C. In this specific setup, 1 darcy is equivalent to 0.831 m/day.^[5]

• Richard Selley's "Elements of Petroleum Geology (2nd edition)," page 250.

• Units of measurement
• Hydraulics
• Hydraulic engineering
• Hydrology
• Hydrogeology
• Soil mechanics
• Soil physics

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