Rainwater catchment

What a roof actually yields — and the first-flush standpipe that keeps the worst of it out of the tank.

Result

Roof footprint1000 sq ft

Rainfall1 inches

Collection efficiency85 %

Water collected

530 gallons

One inch on 1,000 sq ft is 623 gallons before losses.

Weight of that water

4,416 lb

Worth knowing before you build the stand. Water is heavy.

First-flush to divert

10.0 gallons

About 10 gallons per 1,000 sq ft of roof — more under trees.

Standpipe length (4-inch PVC)

15.3 ft

4-inch Schedule 40 holds about 0.65 gal per foot.

What this will not do

First flush is not treatment. It removes the dust, droppings and grit that collected since the last rain. Roof water still needs filtration and disinfection before anybody drinks it.
Check your state law. Rainwater harvesting is encouraged in most states and restricted in a few. Look it up before you plumb a system.
The drip hole in the standpipe clogs, and a diverter full of old water does nothing at all — it sits there full while the dirty first flush runs straight past into your tank. Check it twice a year.

The full build and the reasoning are in Chapter 12 of the guide.