The number that surprises people
The traditional rule is an inch a week — 25 mm across the growing area, from rain and hose combined. For a bed of 10 m² that is 250 litres, which is about twenty-eight full nine-litre watering cans.
Almost nobody waters that much, and almost everybody believes they are watering thoroughly. A single can over that bed is not quite one millimetre. It wets the surface, looks convincing, and reaches almost nothing.
The consequence is not just underwatering. A light daily sprinkle keeps the top few centimetres damp and the soil below dry, and roots grow where the water is. The plant ends up shallow-rooted and unable to survive a week of neglect, which is the opposite of what a dry summer needs.
Supply, then demand
The rainwater tool answers the other half: what a roof can actually collect. The recurring mistake there is measuring the sloped roof area rather than its footprint. Rain falls vertically, so a pitched roof catches exactly the rainfall of the outline it casts on the ground, no matter how steep it is. Using the slope overestimates by 15 to 40%.
Size a butt for the dry spell rather than the annual total, because storage is limited by the gap between rain events. About three weeks of typical summer use is the working rule — beyond that a bigger butt only helps if there is enough roof to refill it.
The cheapest way to need less water is not a bigger butt at all. A 50–75 mm mulch roughly halves evaporation from a bed, and it costs less than the storage it saves.