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Rainfall Collection

Gallons of roof runoff from a storm or season.

About Rainfall Collection

Each square foot of roof yields roughly 0.6 gallons per inch of rain, discounted for splash and overflow losses. A modest roof captures more irrigation water than tanks can hold, cutting tap-water use for the driest weeks.

Formula

Rainfall Collection

Gallons = area x inches x 0.623 x efficiency

0.623
Gallons caught per square foot per inch of rain

Worked example

With the default values below, Rainfall Collection returns Gallons Collected of 635. Change any input above and every figure here is recalculated in your browser.

Inputs used

Roof footprintminimum 1 sq ft
1,200 sq ft
Rainfallminimum 0 in
1 in
Collection efficiencyaccepts 10–100 %
85 %

Results

Gallons Collected
635 gal
Collected
2,405 L
55 gal Barrels
12

Step by step

  1. 1Values used
  2. 2Rainfall Collection
  3. 3Gallons Collected= 635 gal
  4. 4Collected= 2,405 L
  5. 555 gal Barrels= 12

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Rainfall Collection calculated?

Gallons = area x inches x 0.623 x efficiency Each square foot of roof yields roughly 0.6 gallons per inch of rain, discounted for splash and overflow losses.

Why does Rainfall Collection matter?

A modest roof captures more irrigation water than tanks can hold, cutting tap-water use for the driest weeks.

What values do I need to enter?

This calculator takes 3 inputs: Roof footprint, Rainfall, Collection efficiency. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

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