Rainfall and runoff depths were examined for 763 storms on 26 urban basins located in 12 countries. For 17 of the basins, impervious surfaces were the major contributors to storm runoff. These basins were generally smaller than 25 ha and had small to medium storms in the data set. Nine basins had significant amounts of runoff from pervious as well as impervious surfaces. Eight of these basins are located in Australia. For all 26 basins, plots of rainfall and runoff depths were used to estimate the effective impervious area and the impervious area initial loss. The data plotted close to a single straight line on all basins, indicating that the effective impervious area remained constant for all storm sizes. The effective impervious fraction was related to total impervious area and the directly connected impervious fraction estimated from maps. For the basins with pervious runoff, the depth of rain in the storm was the most important factor in determining pervious runoff for rainfalls less than 50 mm, while for larger storms other factors including rainfall intensity and antecedent wetness were also found to be significant.