Rainwater Harvesting Storage Sizing Calculator
Accurately size your rainwater harvesting system by calculating collection potential, usage, and recommended tank capacity.
Last reviewed: June 2026Collection Surface
Rainfall
Usage
System
System Summary
Monthly Water Balance
Monthly Water Balance Table
| Month | Rainfall (in) | Collection (gal) | Usage (gal) | Surplus/Deficit | Tank Level | Status |
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How the Rainwater Calculator sizes storage
This calculator models a roof-fed rainwater system from the variables that change the result most: collection area, annual rainfall, monthly rainfall pattern, roof runoff coefficient, first-flush loss, filtration loss, daily demand, seasonal demand, reserve days, and any existing tank capacity. It converts rainfall to gallons with the standard estimate of 0.623 gallons per square foot per inch of rain, then reduces the raw collection by the selected roof coefficient and system losses.
The most useful result is not just annual collection. A home can collect plenty of water over a full year and still run dry during a long dry season. That is why the calculator runs a month-by-month balance. Each month adds collected water, subtracts expected use, checks whether the tank overflows, and records whether the tank runs empty. The reliability percentage tells you how often the modeled tank can meet demand under the selected rainfall pattern.
Choosing realistic inputs
- Roof area: Use the catchment area that drains to the tank, not the whole house footprint if only part of the roof is connected.
- Runoff coefficient: Metal roofs usually collect more usable water than rough, flat, or planted roofs.
- First flush: A diverter discards the first runoff from each rain event, reducing stored volume but improving water quality for non-potable uses.
- Daily usage: Separate irrigation, toilet, laundry, and whole-house assumptions. A garden-only system needs a very different tank than a year-round indoor system.
- Monthly pattern: Choose the profile that best matches your climate or enter custom rainfall if you have local data.
Use cases and limits
Rainwater harvesting is often practical for irrigation, landscape resilience, toilet flushing, laundry, equipment washdown, or emergency non-potable storage where local code allows it. Do not assume harvested roof water is safe to drink. Potable use usually requires approved roof materials, screened collection, sediment control, filtration, disinfection, periodic testing, backflow protection, and compliance with local plumbing rules.
Before buying a tank, also plan the overflow path, tank base, mosquito screening, freeze protection, pump location, and maintenance access. A correctly sized tank that overflows against a foundation or cannot be cleaned is still a poor installation.