Rainwater Collection Calculator
Size rainwater harvesting tanks and estimate annual collection based on roof area, rainfall, and usage patterns.
About this calculator
This calculator keeps its four outputs cleanly separated by which inputs feed them. Recommended Tank Size is Daily Usage multiplied by a fixed 21-day buffer -- it never references Roof Area, Annual Rainfall, or the First Flush Diverter setting at all, so tank sizing is purely a function of how much water you plan to draw each day. Net Annual Collection runs the opposite way: it multiplies Roof Area by Annual Rainfall (using the standard 0.623 gallons-per-square-foot-per-inch conversion and an 85% collection efficiency), then subtracts First Flush Diverter losses assumed over roughly 60 rain events per year -- Daily Usage plays no part in that collection total.
Roof Area and Annual Rainfall multiply straight into that formula together, so nudging either one by the same fraction swings gross collection by that identical fraction; neither input structurally outranks the other. Pump Size, meanwhile, is set purely from Roof Area at a rate of 1 GPM per 500 square feet served, with a 1 GPM floor, and ignores rainfall, usage, and first-flush settings entirely.
Inputs
Results
Net Annual Collection
92,319 gal
Recommended Tank Size
4,200 gal
≈ 11 hot tubs
How to Use This Calculator
- Enter Roof Area (sq ft), Annual Rainfall (in), and Daily Usage (gal).
- Set First Flush Diverter (gal).
- Review Net Annual Collection (gal) and Recommended Tank Size (gal).
- Use Days of Supply (days) and Pump Size (GPM) to inform your decision.
- Use the chart to visualize the results and explore different scenarios by adjusting inputs.
How the result changes with Roof Area (sq ft)
| Roof Area (sq ft) | Net Annual Collection | Recommended Tank Size |
|---|---|---|
| 2,500 | 44,660 gal | 4,200 gal |
| 3,750 | 68,489 gal | 4,200 gal |
| 7,500 | 139,979 gal | 4,200 gal |
| 12,500 | 235,297 gal | 4,200 gal |
What each input means
- Roof Area (sq ft)
- Total roof catchment area in square feet
- Annual Rainfall (in)
- Average annual rainfall in inches for your location
- Daily Usage (gal)
- Expected daily non-potable water usage (irrigation, toilets, cooling)
- First Flush Diverter (gal)
- Gallons diverted per rain event to remove roof contaminants
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersRoof Area (sq ft) = 5000, Annual Rainfall (in) = 36, Daily Usage (gal) = 200, First Flush Diverter (gal) = 50 = 4 input(s) provided
- Calculate Net Annual Collection92319 = 92319
- Calculate Recommended Tank SizeRecommended Tank Size4200 = 4200
- Calculate Days of SupplyDays of Supply462 = 462
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why doesn't Roof Area change my Recommended Tank Size?
Recommended Tank Size is calculated purely from Daily Usage (gal) multiplied by a fixed 21-day buffer. Roof Area, Annual Rainfall, and First Flush Diverter all affect how much water you collect, but none of them enter the tank-sizing formula.
Does Roof Area or Annual Rainfall matter more for Net Annual Collection?
Neither dominates -- Net Annual Collection multiplies Roof Area by Annual Rainfall directly, so an equal percentage increase in either input raises gross collection by that same percentage before the First Flush Diverter loss is subtracted.
Why does raising First Flush Diverter lower my Net Annual Collection?
The calculator assumes about 60 rain events per year and subtracts First Flush Diverter gallons for each one from gross collection. A higher per-event diversion volume removes more total gallons over the year, which is the tradeoff for keeping more roof contaminants out of the tank.
Why is Pump Size based only on Roof Area?
Pump Size uses a simplified rule of 1 GPM of pump capacity per 500 square feet of roof served, with a 1 GPM minimum. Daily Usage, Annual Rainfall, and First Flush Diverter do not factor into this sizing rule at all.
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