Abstract There is a well known but comparatively little-studied relationship between crown diameters (K) and stem diameters (d) of trees. Between about 20 and 50 cm dbh the relationship is very close to being linear, with an r2 value higher than 0.8. The investigation described in this paper was conducted to establish the relationships for 11 broadleaved species that are commonly grown in Britain. The results indicated that all species have higher K/d ratios when they are young, but the ratio reduces as stem diameter increases, beginning to stabilize around 30 cm dbh. Of the species investigated, walnut (Juglans regia) had by far the highest ratio when young, but other strongly light-demanding species, such as birch (Betula pendula), had very low ratios. There was no strong suggestion that the K/d ratio could be used to predict the tolerance of species, with high ratios for the more shade tolerant species, as indicated by Shallenberger et al. (1986) . The possible applications and uses of a knowledge of the K/d ratio are discussed, including implications for decisions on spacings, basal areas per hectare, the prediction of desirable stocking levels for any given mean stem diameter, and thinning regimes. Also discussed, are spacings that might be appropriate in mixed species stands, the management of overstories in shelterwood systems, genetic selection in breeding programmes and prediction of volumes of branchwood for fuel.