Soil pH in southwestern Australia (WA) is usually measured either in water (pHw) or 0.01M CaCl2(pHCa). There are no published relationships to convert one pH to the other. This has been done in this paper for 236 WA soils: pHCa=0.918 pHw‐0.3556, r2=0.9401. It is, therefore, concluded that, for the majority of soils within WA, it is likely that the linear model proposed for all soils would give the best relationship.