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The prediction of absorption loss in the ionosphere and, hence, of the expected field strength is frequently necessary when new h.f. communication circuits are being planned. Various methods have been proposed for making these calculations. The paper describes a slide rule based on the absorption-loss equation given by Piggott, with which the calculations may easily and quickly be made. The parameters are arranged on the rule so that the computation of the diurnal variation of absorption loss on a particular circuit may be carried out with the minimum of slide-rule resetting. It has also been possible to include on the rule the calculation of other ionospheric variables that depend mainly on the solar zenith distance. These are the E-region and the F1-region maximum usable frequencies (m.u.f.s) and the E-region screening frequency.