With the advent of Internet, on-line technologies have been adopted for offering e-government services. Recently, information services are also being complemented with participatory platforms, which try to bridge the gap between citizens and government in decision making processes. However, ensuring best decision making becomes crucial to engage citizens with their (national or local) administrations. This paper proposes the application of optimisation techniques to participatory budget allocation processes, which are currently based on vote-counting. In particular, we formalize the problem of selecting the best combination of proposals in terms of their social support and available budget. Moreover, we analyse the case of budget allocation in Decidim Barcelona, an online platform for citizen participation. We encode the problem as a linear program and test it in two local participatory budget allocation pilots. Results illustrate how our optimisation approach outperforms the standard proposal selection method in the total number of gathered supports, the number of selected proposals as well as in the allocated budget.