Specific quantity

In the natural sciences, including physiology and engineering, the qualifier specific or massic typically indicates an intensive quantity obtained by dividing an extensive quantity of interest by mass.¹² For example, specific leaf area is leaf area divided by leaf mass. Derived SI units involve reciprocal kilogram (kg⁻¹), e.g., square metre per kilogram (m²⋅kg⁻¹); the expression "per unit mass" is also often used.

In some fields, like acoustics, "specific" can mean division by a quantity other than mass.¹

Named and unnamed specific quantities are given for the terms below.

Per unit of mass (short form of mass-specific):

• Specific absorption rate, power absorbed per unit mass of tissue at a given frequency
• Specific activity, radioactivity in becquerels per unit mass
• Specific energy, defined as energy per unit mass
  • Specific internal energy, internal energy per unit mass
  • Specific kinetic energy, kinetic energy of an object per unit of mass
• Specific enthalpy, enthalpy per unit mass
• Specific enzyme activity, activity per milligram of total protein
• Specific force, defined as the non-gravitational force per unit mass
• Specific heat capacity, heat capacity per unit mass, unless another unit is named, such as mole-specific heat capacity, or volume-specific heat capacity
• Specific latent heat, latent heat per unit mass
• Specific leaf area, leaf area per unit dry leaf mass
• Specific modulus, a materials property consisting of the elastic modulus per mass density of a material
• Specific orbital energy, orbital energy per unit mass
• Specific power, per unit of mass (or volume or area)
• Specific relative angular momentum, of two orbiting bodies is angular momentum divided by the reduced mass, or the cross product of the relative position and the relative velocity
• Specific surface area, per unit of mass, volume, or cross-sectional area
• Specific volume, volume per unit mass, i.e. the reciprocal of density
• Mass ratio, "ratio by mass", mass per unit mass, massic mass or specific mass: the ratio of two mass values (in kilogram per kilogram)

Per unit of other types.

• Brake-specific fuel consumption, fuel consumption per unit of braking power
• Thrust-specific fuel consumption, fuel consumption per unit of thrust
• Specific acid catalysis, in which the reaction rate is proportional to the concentration of the protonated solvent molecules
• Specific acoustic impedance, ratio of sound pressure to particle speed at a single frequency
• Specific capacity of a water well, quantity of water produced per (length) unit of drawdown
• Specific detectivity of a photodetector
• Specific gas constant, per molar mass
• Specific gravity, also called relative density, is a dimensionless quantity defined as the ratio of the density (mass divided by volume) of a substance to the density of a given reference material
• Specific humidity, mass of water vapour per unit mass dry air
• Specific impulse, impulse (momentum change) per unit mass of propellant
• Specific modulus, elastic modulus per mass density
• Specific speed, unitless figure of merit used to classify pump impellers (pump-specific) and turbines (turbine-specific). Ratio of performance against reference pump that needs one unit of speed to pump one unit volume per one unit hydraulic head pressure. For a turbine, it is performance measured against a reference turbine that develops one unit of power per one unit speed per one unit of hydraulic head.
• Specific storage, specific yield, and specific capacity, quantify the capacity of an aquifer to release groundwater from storage per unit decline in hydraulic head pressure
• Specific strength, material strength (pressure required at failure) per unit material density
• Specific surface area, per unit of mass, volume, or cross-sectional area
• Specific thrust, thrust per unit mass of air intake rate