SettingsSettings for this calculationUS
The horizontal (footprint) area of roof draining to the collection point.
Use the flat footprint area of the roof section feeding the gutter/downspout you're collecting from, not the sloped surface area.
The rainfall depth for the event you're estimating.
1 in (25 mm) of rain is a common moderate rainfall benchmark — check a specific forecast or historical average for your area.
Estimated water collected
673 gallons
This assumes 100% collection efficiency — actual yield is somewhat lower due to evaporation, first-flush diversion (recommended to skip the initial dirty runoff), gutter overflow in heavy rain, and roof surface losses.
- Roof catchment area
- 1,080 sq ft
They open the calculator with your figures already in it
Rain Barrel and Rainwater Collection Calculator: 673 gallons — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Standard rainwater harvesting formula: gallons collected = roof catchment area (sq ft) x rainfall (inches) x 0.623
Inputs used
- Roof Catchment Area
- 1080 sq ft
- Rainfall Amount
- 1 in
Intermediate steps
- Roof catchment area
- 1,080 sq ft
Confidence note: This assumes 100% collection efficiency — actual yield is somewhat lower due to evaporation, first-flush diversion (recommended to skip the initial dirty runoff), gutter overflow in heavy rain, and roof surface losses.
What this calculation does not cover
- The figure is gross catchment. No runoff or yield coefficient is applied, nothing is subtracted for the depth the roof surface and gutter absorb before runoff even starts, and evaporation and any volume sent to a first-flush diverter are not deducted. The water that actually reaches the barrel is meaningfully less than this.
- The calculation does not know your storage volume. It reports what falls on the roof, not what you keep — once the barrel is full the rest leaves through the overflow, so for anything beyond a small event this number is larger than the water you end up with.
- Rainfall is entered as a total depth with no duration, so intensity never enters the arithmetic. Whether the gutter, downspout and diverter can carry the peak rate is a separate sizing problem, and water that overshoots an undersized eaves gutter is lost before the barrel sees it.
- This is a single event, not a supply. There is no seasonal distribution and no dry-spell interval behind it, so it cannot size a cistern or tell you whether the store will still hold water when you need to draw on it.
- This is not a water-quality or compliance assessment. It says nothing about what the roof covering leaches into the runoff, nothing about treatment for any use beyond irrigation, and nothing about local rules — some jurisdictions restrict or require registration of rainwater capture, and any tie-in to household plumbing brings backflow prevention and cross-connection requirements with it. Confirm both locally.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Standard rainwater harvesting formula: gallons collected = roof catchment area (sq ft) x rainfall (inches) x 0.623
Cite this page
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