The capacitance of a parallel-plate capacitor (or condenser) is given in SI units by , where is the area of each plate, is the spacing between plates, is the dielectric constant (relative permittivity), and is the permittivity of free space, farad/meter. If is expressed in and in , then microfarads (). Capacitance determines the quantity of positive and negative charges that can be held on the plates by a voltage , such that . In an air-gap capacitor, in which there is no dielectric layer, .