How much rain does it take to fill up this rain barrel?
The simple answer is around 0.3 inches of rain.
The total roof area is 240 square feet and the barrel holds 42.5 gallons. (Yes, I should be using metric units, but my mind is hard wired together in “Old English” components.)
That assumes that all the water that falls on the roof finds its way into the barrel.
And I ignored both the rain that falls “directly” into the barrel and evaporation from it – both are minuscule.
Between rainy days, that keeps the water barrel less than full.