Most of the 4000 composting sites in North America employ open windrow composting techniques in which rainfall often comes into contact with the compost. The resulting runoff can leach contaminants from the compost, making the runoff unsuitable for direct release into receiving waters. Stormwater from these facilities is usually collected in a detention pond and treated prior to release. There is little guidance for determining the appropriate size of these runoff detention ponds. One barrier to modelling the hydrology of composting sites is a lack of basic information regarding rainfall–runoff relationships for compost. This note describes the results of laboratory and field experiments designed to estimate the fraction of rainfall incident on a compost windrow that leaches from the windrow onto the composting pad and into the detention pond. The results indicate that approximately 68% of the rainfall incident on a saturated compost windrow will eventually run off. Key words: solid waste management, composting, runoff, detention pond.