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Roof storage calculator

Rain Barrel Size Calculator

Estimate how much runoff reaches a roof-fed barrel or cistern, compare it with your installed storage, and see how much water would overflow in the selected storm.

Calculate storage vs overflow

Use the horizontal roof area connected to these barrels.

Why one inch of rain can fill a barrel quickly

Rainfall volume scales with roof area. A small barrel connected to a large roof section can fill early in a storm, so overflow routing is part of the design rather than an edge case.

Wrivel uses the horizontal projected roof area feeding the storage and an explicit runoff coefficient. The result is a planning volume, not a peak-flow calculation. Gutter, downspout, inlet and overflow capacity still need separate checks.

Use the connected roof area, not the whole roof by default

If one downspout feeds one barrel, estimate the roof plan area that drains to that gutter branch. If multiple downspouts feed a linked tank system, add the contributing plan areas. Do not automatically use the total roof unless the whole roof actually reaches that storage.

What the overflow number means

The calculator assumes storage is empty at the start of the selected storm. Captured water is limited by installed capacity; the remainder is shown as overflow. Real systems can capture less if tanks are partly full, first-flush devices discard water, gutters bypass flow or inlet capacity is limited.

Always plan the overflow: once storage fills, water still needs a safe path away from the foundation and neighboring property. Do not route overflow to a location that simply recreates the original pooling problem.

Rain barrel vs larger cistern

A barrel is useful for modest storage and garden reuse, but the storm volume may show that multiple small barrels capture only a fraction of a roof event. A larger cistern can increase storage, while still requiring a structurally appropriate base, screened inlet, overflow and local compliance where applicable.

Use storage and drainage as separate questions

You want to reduce roof runoff reaching one wet spotUse this calculator for storage, then the downspout drainage guide for the overflow/outlet route.
You want underground infiltrationUse the field infiltration worksheet and dry-well sizing screen; above-ground barrel math does not prove subsurface suitability.
You only need total storm volumeUse the simpler runoff volume calculator.
Several surfaces and outlets interactUse the full property planner.

Formula used

estimated roof runoff = rainfall depth × horizontal roof area × runoff coefficient

In US customary units, Wrivel converts one inch over one square foot using 0.6233766 US gallons. Metric inputs are converted consistently and displayed in liters.

Planning limits

The calculator does not size gutters, downspouts, inlet screens, first-flush devices, overflow piping or structural support. It also does not determine whether harvested rainwater may be used for a particular purpose under local rules.