Size a rainwater collection setup
How many liters does a roof actually deliver? Area times rainfall times a runoff coefficient — then check it against daily use.
Tools used in this guide
The job
A millimetre of rain on a square metre of roof is one litre of water — before losses. Gutters, splash, and the first dirty flush of a storm all steal some; the runoff coefficient folds those losses in.
Area × rain × efficiency
Enter the roof's plan footprint (not the slope area), rainfall in mm, and an efficiency — presets cover metal (~90%), asphalt (~85%), tile and concrete (~80%). Output is litres and gallons, plus days covered at a given daily use.
Worked example
A 120 m² metal roof in a 30 mm storm at 90% efficiency collects 3,240 L ≈ 856 US gallons. At 200 L/day of garden use that storm alone covers about 16 days — before your tank's capacity cuts it off.
Scope
Area 1–100,000 m², rainfall 0.1–5,000 mm, efficiency 1–100%. First-flush diverters and tank overflow are not modeled — the number is what the roof offers, not what your tank kept.