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Rainwater Harvesting Storage Capacity Estimator

This estimator sizes a practical rainwater storage target from roof catchment, design rainfall, runoff efficiency, and intended demand coverage. It is useful when comparing tank sizes for homes, small commercial sites, gardens, or other non-potable applications where both supply and demand matter. The result shows the rainwater volume available from the selected rainfall event and the amount of storage needed to cover a chosen number of demand days. The recommended storage target is the smaller of those two values, because a tank larger than the available event yield cannot be filled by that event alone, while a tank smaller than the demand target may run out sooner. Real projects should also consider seasonal rainfall, overflow, first-flush diversion, site constraints, and local water-quality requirements.

Inputs

m²
mm
%
L/day
days
Result
—
Recommended storage capacity
Event harvest potential—
Demand-based storage—
Limiting factor—

1. Enter the catchment area
Use the horizontal roof area that actually drains to the rainwater system.

2. Set a design rainfall depth
Enter the rainfall depth for the event or planning period you want to evaluate.

3. Allow for collection losses
Enter the percentage of rainfall expected to reach storage after runoff and system losses.

4. Enter daily demand
Use the combined daily demand for the fixtures or uses supplied by rainwater.

5. Choose coverage days
Set how many demand days you want the tank to bridge, then review the limiting factor.

Harvested volume (L) = Roof area (m²) × Rainfall (mm) × Collection efficiency Demand storage (L) = Daily demand (L/day) × Coverage days Recommended storage (L) = min(Harvested volume, Demand storage)

One millimeter of rain on one square meter equals one liter before losses. Collection efficiency is entered as a percentage and converted to a decimal.

What the result means

The result is a screening-level tank capacity that balances the selected event yield with the desired demand buffer.

A final tank size may be larger or smaller after seasonal water-balance modeling, dead storage, overflow allowance, and local design requirements are included.

Given
Roof area = 180 m²; rainfall = 50 mm; collection efficiency = 85%; daily demand = 450 L/day; coverage = 5 days.

Calculation
Event yield = 180 × 50 × 0.85 = 7,650 L.
Demand storage = 450 × 5 = 2,250 L.
Recommended storage = min(7,650, 2,250) = 2,250 L.

Result
2,250 L

For this selected event, demand coverage—not rainfall yield—sets the storage target.

Why is the recommended tank smaller than the harvested volume?

The estimator limits storage to the demand buffer when the selected rainfall event produces more water than you expect to use during the chosen coverage period. Additional capacity can still be useful for seasonal storage, but that requires a longer-term water-balance analysis.

Should roof area be measured on the slope?

Use the horizontal projected catchment area rather than the sloped surface area when rainfall is reported as a vertical depth. Include only roof sections that actually drain to the collection system.

What collection efficiency should I use?

Use a project-specific value that reflects roof material, first-flush diversion, filter losses, splash, leakage, and other collection losses. The calculator does not prescribe a universal efficiency.

Can this size a potable rainwater system?

It can estimate storage volume, but potable systems require additional treatment, monitoring, plumbing, and regulatory design considerations. Storage sizing alone does not establish water safety.

How does this differ from the water savings estimator?

Storage capacity focuses on tank volume for a selected yield and demand buffer. Water savings estimates how much mains or purchased water could be displaced over a period.