Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Gallons per storm
635
Gallons per year
24147
55-gal barrels to hold one storm
12
Weeks of garden watering
10.6
Average gallons per month
2012
Water bill savings per year
$178
Psst — share this and help Rex grow
One click, a permanent link with your numbers baked in.
How to use this
- 1Enter roof catchment area (sq ft).
- 2Enter rain per storm (in).
- 3Enter annual rainfall (in).
- 4Enter collection efficiency (%).
- 5Enter garden use per week (gal).
- 6Read your gallons per storm on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
One inch of rain on 1,000 sq ft of roof yields about 623 gallons before losses. Collection efficiency of 85% is a realistic planning number.
Worked example
Using the values the calculator loads with:
Inputs
- Roof catchment area: 1200 sq ft
- Rain per storm: 1 in
- Annual rainfall: 38 in
- Collection efficiency: 85 %
- Garden use per week: 60 gal
Results
- Gallons per storm: 635
- Gallons per year: 24,147
- 55-gal barrels to hold one storm: 12
- Weeks of garden watering: 10.6
- Average gallons per month: 2,012
- Water bill savings per year: $178
What each field means
Inputs
- Roof catchment area (sq ft)
- The roof catchment area used in the calculation, measured in sq ft. Starts at 1200 sq ft so you have a working example on load.
- Rain per storm (in)
- The rain per storm used in the calculation, measured in in. Starts at 1 in so you have a working example on load.
- Annual rainfall (in)
- The annual rainfall used in the calculation, measured in in. Starts at 38 in so you have a working example on load.
- Collection efficiency (%)
- The collection efficiency used in the calculation, measured in %. Starts at 85 % so you have a working example on load. Accepted range: 30–100 %.
- Garden use per week (gal)
- The garden use per week used in the calculation, measured in gal. Starts at 60 gal so you have a working example on load.
Results
- Gallons per storm
- Returned as a decimal number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Gallons per year
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- 55-gal barrels to hold one storm
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Weeks of garden watering
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Average gallons per month
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Water bill savings per year
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the rainwater collection & barrel sizing calculator work?
One inch of rain on 1,000 sq ft of roof yields about 623 gallons before losses. Collection efficiency of 85% is a realistic planning number. The underlying maths is: Gallons = area × inches × 0.623 × efficiency.
What do I need to enter?
5 values: roof catchment area, rain per storm, annual rainfall, collection efficiency, and garden use per week. Each field starts with a sensible default, so you can change one number at a time and watch the result move.
What does the gallons per storm result mean?
It is returned as a decimal number and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds.
Is this calculator free, and do I need an account?
Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser.
How accurate is the result?
It applies the standard formula exactly, so the arithmetic is precise. Estimates assume standard, average conditions — local rules, pricing, and materials vary.
Who is this tool for?
It's built for anyone doing everyday planning maths — a quick estimate that would otherwise need four browser tabs and a calculator app.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Rainwater Harvest Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/rain-barrel
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/rain-barrel" target="_blank" rel="noopener">Rainwater Harvest Calculator — RevenueLab</a> (2026).</p>
Source: [Rainwater Harvest Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/rain-barrel) (2026).