How to Use This Tool
Enter the footprint your roof covers seen from above, the rainfall for the period you care about, and a collection efficiency. Pitch is deliberately absent.
The one time the pitch multiplier is wrong
If you have just estimated shingles you will have used a pitch multiplier: a 1,600 sq ft footprint at 6/12 is 1,789 sq ft of roof surface, and at 12/12 it is 2,263. That is correct for materials, because shingles cover the slope.
It is wrong for water. Rain falls vertically, so what a roof intercepts is its horizontal projection — the footprint. Both of those roofs catch exactly the same rain. A steeper roof costs more to cover and collects not a drop more.
1 sq ft x 1 inch = 144 cubic inches 144 / 231 per gallon = 0.6234 gallons 1,500 sq ft footprint, 1 inch of rain theoretical 1500 x 1 x 0.6234 = 935 gal x 85% efficiency = 795 gal same footprint at 4/12 and at 12/12 = the same 795 gal roof surface to shingle = 1,581 vs 2,121 sq ft
Where the missing 15 per cent goes
Collection efficiency is the term most calculators drop, and it is worth a sixth of your yield. The losses are: the first flush diverted to waste to keep debris out of the tank, the water absorbed wetting a dry roof before any runoff starts, evaporation from a warm surface, splash-out at the gutter edge, and overshoot when rain arrives faster than the gutter can carry it.
Smooth metal runs at the top of the range, around 90 per cent. Asphalt shingle is nearer 85, concrete tile lower again because it is porous, and a gravel or green roof lower still because it is designed to retain water rather than shed it.
Sizing the tank is a dry-spell question
The instinct is to size a tank against annual rainfall, but a tank does not store a year — it bridges the gaps between storms. The size that matters is daily use times the longest dry spell you want to ride out. Twenty-one days at 40 gallons a day is 840 gallons, and a bigger tank than that buys resilience you will rarely use.
The other constraint is that the tank has to refill. If a single typical storm cannot fill a meaningful fraction of it, the extra capacity is dead weight.
What this does not cover
Gutter and downspout sizing depends on rainfall intensity rather than total, first-flush diverter sizing depends on roof area, and potability depends entirely on treatment and on local rules. Many places restrict or regulate rainwater collection, and some roofing materials are not suitable for anything but irrigation.
