Skip to content

Rain Event and Tank Capture Calculator

Model one rain event with catchment area, runoff assumptions, first-flush diversion and available tank space. Inspect retained inflow and overflow.

Use this result well

Inputs that matter
Rain and storage units, Horizontal catchment area, Rainfall depth for this event, Runoff routed to collection (%), and 3 more
Output to expect
New water retained
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
Was this tool helpful?

Related Tools

Tools you might need next

Container Watering Frequency Calculator

Estimate how often to water container plants based on pot size and climate. Free container watering frequency calculator for garden irrigation.

Gardening Water
Irrigation Flow, Runtime and Catch-Test Calculator

Calculate drip emitter delivery, measured emitter flow, sprinkler precipitation and sampled uniformity. Convert a chosen water amount into runtime.

Gardening Water
Crop ET and Irrigation Water Budget Calculator

Estimate crop ET from entered ETo and Kc, then compare a separate effective-rain and efficiency budget in metric or US units.

Gardening Water

Reference & details

How it works

Calculate theoretical rain volume

Metric square meters multiplied by millimeters gives liters. In US units, square feet × inches × 144/231 gives US liquid gallons.

Gross rain volume = horizontal catchment area × event rainfall depth

Account for routing and diversion

Apply your documented runoff percentage, then remove the entered first-flush volume up to the amount actually routed.

Tank inflow = max(0, gross volume × routed percentage ÷ 100 − first flush)

Apply the free storage limit

Free space is usable capacity less initial stored water. New retained water is capped by that space; remaining inflow is overflow.

Retained = min(inflow, capacity − initial volume); overflow = inflow − retained

Updated: September 2026

Example Scenarios

100 m² × 10 mm produces 1,000 liters theoretically. At 80% routed runoff and 20 liters first flush, inflow is 780 liters. A 500-liter tank starting at 100 liters retains 400 and overflows 380.

When initial water equals usable capacity, all new inflow becomes overflow and newly retained water is zero.

Zero rainfall produces zero new inflow and leaves the initial tank water unchanged.

Common Mistakes to Avoid

Using sloped roof surface area as catchment area

Use the horizontal area actually routed to this tank.

Treating all runoff as stored water

Inspect diversion and available capacity; overflow is reported separately.

Reading a volume result as design approval

Resolve support, access, overflow routing, local rules and water quality separately.

FAQ

Yes. A full tank has no free space, and a zero-rain event adds no water. Initial stored water cannot exceed usable capacity.

No. Diverting initial runoff does not establish potability or suitable water quality for an intended use.

One event does not describe annual rainfall timing, demand or storage behavior. Annual yield and supply reliability need a time-series model.

No. It reinterprets the numbers. Convert every area, depth and storage input to the selected system before calculating.

About Rain Event and Tank Capture Calculator

Use horizontal catchment area and one observed or assumed rain event. Select metric or US units, enter routed runoff, first-flush volume, usable tank capacity and initial stored water. The model assumes no water use during the event. It does not establish water quality, annual supply or a safe installation.