Rainwater harvesting (RWH) tank sizing can be an onerous task due to the range of available methods. This paper presents a simple, yet robust, end-user and design criteria-focused decision support tool, based on a comparison of 12 well established methods across four types (simplified, mass balance continuous simulation, cost-function and statistical). Each method was applied to two example households located in Bucaramanga, Colombia and considered uncertainty through integrated representation of demand variability, climate change, overflow and financial scenarios. Quantitative and qualitative criteria were used to assess the performance of each method. Based on the summation of these criteria, a diversified approach constituting mass balance simulation, cost-function and probabilistic methods was most suited to RWH tank sizing for houses in Bucaramanga. Global RWH practitioners now have a decision support tool to assist in selecting the best tank sizing approach for their context.