How much rainwater can I collect?

Count it like this: 1 mm of rain on 1 m² of roof gives 1 litre, and a good roof returns about 90 % of it. A 50 m² roof fills a 400-litre butt from a single 10 mm shower. Measure the roof from above (its footprint, not the slope), divert the first dirty minutes of rain, and keep the butt covered against mosquitoes. Draw your roofs on the plot drawing and the app counts the litres for you.