How Much Water Can You Collect From a Roof?

Measure the footprint, not the slope

This is the first thing to get right, and it's counterintuitive enough that plenty of guides get it wrong.

Rain falls vertically. So the catchment is the area your roof covers seen from above — its footprint — not the surface area of the sloped roof.

A steep roof and a flat roof with the same footprint catch exactly the same water. The steep one just presents that same area at an angle. Using the sloped surface overstates your catchment, and on a steeply pitched roof it overstates it badly.

Pace the building out at ground level and include the overhang — water landing on the overhang still runs into the gutter.

The conversion is exact

There's no rule of thumb here. One inch of rain on one square foot is 144 cubic inches of water, and a US gallon is defined as exactly 231 cubic inches.

So: 144 ÷ 231 = 0.6234 gallons, per square foot, per inch of rain.

The metric version is tidier still — one millimetre of rain on one square metre is exactly one litre, because a litre is exactly a thousand cubic centimetres and the geometry lands on a whole number.

Multiply that by your Catchment area and you have the answer before losses.

The numbers are bigger than people expect

A 40 × 30 ft footprint is 1,200 square feet — an ordinary house. One inch of rain on it is 748 gallons before any losses, and about 636 after a realistic 85%.

What your barrels actually hold is 55 of that. Overflow is 581.

That's not a defect in the barrel. It's the scale of a roof. And it's why Barrels to catch all of it reads 12 — which is the honest answer to "should I add another barrel", even if it isn't the one anyone wants.

Overflow is a plumbing decision, not a disappointment

Every barrel fills, usually in the first serious rain of the season. The question isn't whether it overflows — it's where the excess goes.

Directed back into the downpipe, or to a soakaway, that's fine. Pooling against your foundation is a real problem that costs far more than the water saved.

Size the overflow route for the full flow off the roof, not for the trickle people picture. On that 1,200 sq ft example, a heavy hour can push several hundred gallons through it.

Efficiency is yours to measure

The Collection efficiency field defaults to 85%, and that's a placeholder rather than a finding.

Water is lost to splash-out at the gutter, to the first flush carrying roof debris, to evaporation on a hot roof, and to gutter overshoot in heavy rain when the guttering simply can't carry the flow. How much depends on your roof material, your gutters and how hard it's raining — and published figures vary widely enough that no single number is honest.

To get your own: note a rainfall total, measure what actually arrived in the barrel, divide. The printable has a table for exactly that.

Size storage to your use, not your roof

Catching every drop takes a startling number of barrels and is almost never the goal.

The better question is how many weeks of watering you want held between rains — which is what Weeks of watering from what you caught answers. At the defaults, one barrel and 20 gallons a week is about 2.8 weeks of cover.

Work back from your use and the dry gap you want to bridge. That turns the storage figure from "impossible" into something you'd actually buy.

Two practical cautions

Water is heavy. A US gallon weighs about 8.34 lb, so a full 55-gallon barrel is roughly 460 lb plus the barrel itself. Raising it on blocks for tap clearance is normal, but the base has to be rated for the filled weight — and it's the filled one that falls over.

This is garden water. Roof runoff picks up dust, bird droppings, moss and sometimes material shed by the roofing. Drinking it is a separate subject with real health consequences, and one for your local health authority rather than a calculator. Water off the roof, in litres is there for anyone working in metric, not as a hint that it's potable.

Worth a five-minute check too: a few places regulate rainwater capture, and some rules cover where the overflow may discharge. Cheap insurance before you buy several hundred litres of storage.