Summer numeracy interventions have become standard educational practice across a range of jurisdictions, but there is a paucity of evaluations of those interventions across international settings. We employ a quasi-experimental evaluation of voluntary summer numeracy programs for 569 attendees and 2,193 comparison students in grades 1-3, conducted in 2012-13 in Ontario, Canada. Using multi-level logistic regression models, we find that students with prior academic challenges were most likely to attend the program. Estimating causal effects using entropy balancing to achieve balance, we find that attendees gained about 1.5 months more numeracy learning than did the comparison group (Cohen's d = 0.19). These findings are comparable to US studies and add to an international stock of knowledge on summer interventions.