The limited coral population sizes of degraded reefs and small outplanted patches can negatively affect coral reproduction; for example, insufficient sperm concentrations may increase the risk of fertilisation failure. To assess the influence of adult patch characteristics on fertilisation success, we conducted a series of manipulated field experiments using three common broadcasting Acropora species at two distinct locations (One Tree Island, GBR; and Nikko Bay, Palau). At One Tree Island, 8 colonies of A. tenuis spawned with an intercolonial distance of 2.0 m, and 15 A. digitifera spawned with an intercolonial distance of 1.1 m. At Nikko Bay, 20 A. hyacinthus colonies spawned with an intercolonial spacing of 1.0 m. Each patch resulted in low but notable fertilisation success, ranging from 4 to 7%. In situ sperm concentrations were heterogenous and ranged from 0 to 104 sperm mL-1, with the upper range of concentrations known to cause successful fertilisation in laboratory experiments. We developed a mechanistic coral fertilisation model using independent data, with fertilisation predictions closely aligning with the empirical data. Our results indicate that small-scale patches of outplanted corals can produce viable embryos, but consideration must be given to patch size and density when conducting coral outplant interventions.