A method is demonstrated for arriving at simple formulae by which the resultant reactive power of overhead lines under any load condition can be quickly calculated, and in which the only line constant necessary to be known is the inductance. Tables of practical overhead-line inductances are given. The method is extended to lines containing series capacitors and also to low-voltage lines. The accuracy of the method, applied both to electrically short and long lines, is discussed fully in the Appendix.