system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:how would I calculate the rain flow into a water tank off a given corrugated iron roof structure? Say I had a four square metre roof and the given average rain fall (say it was 1000 mm annually)?

I'm building a water tank system for an off the grid property- just for occasional use, so a 1000 litre tank. I want to get the optimum results without massive wastage or shortfall so some ideas would help. Thanks in advance.
gpt
Even if you can say you'll get a consistent 1000mm per year, your figures will only be vague and will vary. Rain doesn't often fall vertically, and your roof may or may not be flat. If your local winds tend to blow rain towards the east, and you're collecting rain off a roof that slopes away from that direction, you won't collect much rain.

If we simplify it by looking at a completely flat roof with vertical rain falling on it, we can just treat it as a volume, that is we just multiply the area by the depth of water we get - so 4m^2 * 1000mm = 4m^3, which is 4000 litres over a year.