The leakage reactance calculations performed apply to a single phase unit or to one phase of a 3-phase transformer. The phase can have multiple windings interconnected in such a way that only 2 or 3 terminals external or buried result. The calculations for windings that are interconnected are based on 2-winding leakage reactances. This chapter discusses the general theory of inductance calculations. Reactances are obtained from the inductances by multiplying by the angular frequency under sinusoidal conditions, which are assumed here. The simple reactance calculation assumes that the amp-turns are uniformly distributed along the windings. It also treats the windings as if they were infinitely long solenoids insofar as the magnetic field is concerned, although a correction for fringing at the ends is included in the final formula. The chapter considers a single phase ideal transformer with 2 or more windings per phase.