The proportion of time that river catchment soils are wet, PROPWET, is an integral part of calculating flood risk in hydrological modelling. Currently a single, constant PROPWET value is available for each river catchment in the Flood Estimation Handbook however increasingly it is becoming important to understand how different land use and climate factors will affect soil moisture in the developing research field of natural flood management. A regression-based equation for calculating PROPWET was developed and tested on 43 selected catchments across Scotland and the north-east of England. In this study we expanded on this previous analysis and developed the equation for use on every gauged river catchment in Scotland and two in the north-east of England. The equation was found to be broadly suitable for use across the study area in all but one catchment tested which exhibited very low annual rainfall and high evapotranspiration relative to the other catchments across the study area. It is recommended that further analysis is undertaken to develop the proposed equation using river catchments across the UK, in particular those with lower rainfall values.