Answer first
What this calculator tells you
Estimate the gallons of rainwater a roof can collect from its area and the rainfall. Size a rain barrel or a cistern to the roof that fills it. Formula: Gallons = roof area (sq ft) × rainfall (in) × 0.623 × collection efficiency; 0.623 gallons fall on one square foot per inch of rain. At the worked-example inputs, the gallons collected is 635.5. Holding every other input steady, moving roof catchment area (sq ft) from 960 to 1,440 moves the result from 508.4 to 762.6.
Transparent method
The formula
Size a rain barrel or a cistern to the roof that fills it.
Worked example
Example inputs
How to interpret the result
One inch of rain on one square foot of roof is 0.623 gallons. That single constant turns a roof size and a rainfall into a barrel count. A 1,200 square foot catchment in a 1 inch storm can yield about 635 gallons at 85 percent efficiency, enough to fill more than ten 55 gallon barrels. Small roofs and small storms fill a single barrel quickly, which is the reason overflow needs a plan.
At the worked-example inputs the gallons collected is 635.5. It rises with roof catchment area (sq ft), rainfall (inches) and collection efficiency.
These are material-quantity estimates. Always add a waste and overage margin, and confirm actual coverage with your specific product and supplier before purchasing.
Before you rely on it
What to check
Use the footprint of the roof that drains to your gutters, not the sloped surface, and check local rules on collecting and using the water.
The common error
Where people go wrong with rainwater harvesting calculator
Using the whole roof area when only one slope feeds the downspout. The barrel fills from the part of the roof that drains into it.
Sensitivity evidence
How roof catchment area (sq ft) changes the gallons collected
Holding every other input at the worked-example value, moving roof catchment area (sq ft) from 960 to 1,440 moves the gallons collected from 508.4 to 762.6: a spread of 254.2, or 40% of the worked-example result.
| Roof catchment area (sq ft) | Gallons collected | Liters collected |
|---|---|---|
| 960 | 508.4 | 1,924.4 |
| 1,080 | 571.9 | 2,164.9 |
| 1,200worked example | 635.5 | 2,405.5 |
| 1,320 | 699 | 2,646 |
| 1,440 | 762.6 | 2,886.6 |
Every input, tested
Which input moves the gallons collected most
Of the 3 inputs, roof catchment area (sq ft) moves the gallons collected most (127.1 across the range tested) and collection efficiency moves it least (29.9).
| Input | Tested from | To | Gallons collected at each end | Swing |
|---|---|---|---|---|
| Roof catchment area (sq ft) | 1,080 | 1,320 | 571.9 to 699 | 127.1 (20%) |
| Rainfall (inches) | 0.9 | 1.1 | 571.9 to 699 | 127.1 (20%) |
| Collection efficiency | 83.0% | 87.0% | 620.5 to 650.4 | 29.9 (4.7%) |
Two variables at once
Gallons collected by roof catchment area (sq ft) and rainfall (inches)
Across the grid the gallons collected runs from 406.7 to 915.1. Moving roof catchment area (sq ft) from 960 to 1,440 shifts it by 254.2 at the middle column, and moving rainfall (inches) from 0.8 to 1.2 shifts it by 254.2 at the middle row, so neither is the bigger lever here.
| Roof catchment area (sq ft) \ Rainfall (inches) | 0.8 | 1 | 1.2 |
|---|---|---|---|
| 960 | 406.7 | 508.4 | 610 |
| 1,080 | 457.5 | 571.9 | 686.3 |
| 1,200 | 508.4 | 635.5 | 762.6 |
| 1,320 | 559.2 | 699 | 838.8 |
| 1,440 | 610 | 762.6 | 915.1 |
The highlighted cell is the worked example: 635.5.
Step by step
The worked example, input by input
| Input | Value used | What it means |
|---|---|---|
| Roof catchment area (sq ft) | 1,200 | The footprint of the roof that drains to the gutters, not the sloped surface. |
| Rainfall (inches) | 1 | Enter the rainfall (inches) used in this calculation. |
| Collection efficiency | 85.0% | Losses to splash, first flush and overflow. 75 to 90 percent is common. |
| Gallons collected | 635.5 | |
| Liters collected | 2,405.5 | |
Inputs, definitions and assumptions
Roof catchment area (sq ft)
The footprint of the roof that drains to the gutters, not the sloped surface. The prefilled worked-example value is 1,200.
Rainfall (inches)
Enter the rainfall (inches) used in this calculation. The prefilled worked-example value is 1.
Collection efficiency
Losses to splash, first flush and overflow. 75 to 90 percent is common. The prefilled worked-example value is 85.0%.
How to use this calculator
- 1Verify the inputs. Gather roof catchment area (sq ft), rainfall (inches) and collection efficiency from your own documents; the prefilled values are examples.
- 2Save a baseline. The worked example puts the gallons collected at 635.5. Store your own version of it as Scenario A.
- 3Test one change. Start with roof catchment area (sq ft), the input with the biggest effect here: moving roof catchment area (sq ft) from 1,080 to 1,320 takes the gallons collected from 571.9 to 699, a swing of 20% of the worked-example figure.
- 4Check the extremes. At half the example roof catchment area (sq ft) (600) the gallons collected is 317.7; at double (2,400) it is 1,270.9.
People also ask
Frequently asked questions
How do you calculate rainwater harvesting?
Gallons = roof area (sq ft) × rainfall (in) × 0.623 × collection efficiency; 0.623 gallons fall on one square foot per inch of rain. Enter collection efficiency in percent (85 means 85%). At the worked-example inputs the gallons collected is 635.5.
What does the rainwater harvesting result mean?
Size a rain barrel or a cistern to the roof that fills it. At the worked-example inputs the gallons collected is 635.5. It rises with roof catchment area (sq ft), rainfall (inches) and collection efficiency.
How much does roof catchment area (sq ft) change the gallons collected?
Holding every other input at the worked-example value, moving roof catchment area (sq ft) from 960 to 1,440 moves the gallons collected from 508.4 to 762.6, a spread of 254.2.
What are the limits of this rainwater harvesting calculator?
These are material-quantity estimates. Always add a waste and overage margin, and confirm actual coverage with your specific product and supplier before purchasing. The tables on this page test roof catchment area (sq ft) only from 960 to 1,440; a value outside that range is not tabulated here.
Which input moves the gallons collected most in the rainwater harvesting calculator?
Ranked by how far each moves the gallons collected across the range tested: roof catchment area (sq ft) (127.1, 20%), rainfall (inches) (127.1, 20%) and collection efficiency (29.9, 4.7%).
If I double roof catchment area (sq ft) in the rainwater harvesting calculator, does the gallons collected double?
Doubling it from 1,200 to 2,400 takes the gallons collected from 635.5 to 1,270.9, which is 2.00 times the worked-example figure. So the result scales almost exactly in proportion. Halving it to 600 gives 317.7.
How much does rainfall (inches) matter in the rainwater harvesting calculator?
The worked example uses 1. Holding every other input at its worked-example value, moving rainfall (inches) from 0.9 to 1.1 takes the gallons collected from 571.9 to 699, a swing of 20% of the worked-example figure.
How much does collection efficiency matter in the rainwater harvesting calculator?
The worked example uses 85.0%. With the other inputs left at the worked example, moving collection efficiency from 83.0% to 87.0% takes the gallons collected from 620.5 to 650.4, a swing of 4.7% of the worked-example figure.
Which inputs change the liters collected in the rainwater harvesting calculator?
At the worked-example inputs it is 2,405.5. Roof catchment area (sq ft) takes it from 2,164.9 to 2,646, rainfall (inches) takes it from 2,164.9 to 2,646 and collection efficiency takes it from 2,348.9 to 2,462.1.
Does a bigger pilot hole ever make a screwed joint stronger?
No, the opposite: a pilot hole sized for the screw's root diameter gives the threads material to bite into as they're driven. Too large a pilot hole strips the threads' grip and weakens the joint, while too small a hole risks splitting the wood, especially near an edge.
Sources and evidence
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