<p>Rooftop rainwater harvesting is regaining attention as a sustainable solution to water scarcity and climate change. However, financial feasibility depends on the cost of the storage tanks. Accurate estimates of tank sizes require rainfall data, where long-term, high-resolution records capture rainfall variability but are often unavailable. This study shows how rainfall data characteristics affect tank size. A continuous simulation approach was applied to daily rainfall data (1987–2018) from five stations in Jordan and five in Germany, selected for similar annual rainfall but with different seasonal patterns. Results suggest that at least a 30-year rainfall record is needed for reliable tank sizing in both regions. When the collected rainwater exceeds demand and frequent overflows occur, the impact of rainfall variability on tank size is less, and a shorter time series suffices. Using monthly instead of daily rainfall data may yield an underestimation of tank size, particularly for Jordanian sites, when the collection and use of rainwater occur in the same time period. For Jordan, the required tank size to provide supply during the dry period (62–70 m³) was nearly twice that for year-round supply. However, harvested rainwater from 150 m² roofs can meet up to 100% of demand during the dry period at two stations. In Germany, much smaller tank sizes (&lt; 2 m³) can satisfy up to 95% of demand from a 150 m² roof. These findings highlight the effects of rainfall variations on tank design and recommend using long-term high-resolution data for tank sizing.</p>