In the natural sciences, including physiology and engineering, a specific quantity generally refers to an intensive quantity obtained by the ratio of an extensive quantity of interest by another extensive quantity (usually mass or volume). If mass is the divisor quantity, the specific quantity is a massic quantity. If volume is the divisor quantity, the specific quantity is a volumic quantity. For example, massic leaf area is leaf area divided by leaf mass and volumic leaf area is leaf area divided by leaf volume. Derived SI units involve reciprocal kilogram (kg), e.g., square metre per kilogram (m · kg).