Rainwater Tank Overflow Calculator

Method: catchment, diversion, storage, then demand

One millimeter of rainfall over one square meter is one liter. Roof runoff in liters = plan area × rainfall × efficiency / 100. Inflow = max(0, runoff - fixed diversion). Overflow = max(0, initial storage + inflow - capacity). Stored water after the event is capped at capacity. Only then is demand drawn: supplied = min(demand, stored water); unmet demand is the difference.

This order matters. Water lost through overflow cannot be recovered to meet later demand. Subtracting demand before capping storage would incorrectly assume the withdrawal creates space during the storm. The model deliberately assumes no consumption during rainfall and no pump inflow limit.

Worked example: a 20 mm storm

A 100 m² catchment at an assumed 80% efficiency produces 1,600 L. A 100 L diversion leaves 1,500 L for the tank. A 2,000 L tank starting with 1,000 L spills 500 L and finishes the storm full. The subsequent 600 L demand is supplied completely, leaving 1,400 L. Increasing capacity to 3,000 L would retain all inflow and leave 1,900 L after the same demand.

Using the outputs to compare tanks

Change capacity while keeping the storm and starting water constant to isolate storage effects. Compare both a nearly empty and nearly full starting tank: a large annual collection estimate can hide repeated spills. For a second storm, use the previous remaining water as its starting storage. This manual event chain is not a historical rainfall simulation or a reliability percentage.

Measure only connected catchment area. Enter usable capacity after dead storage rather than a marketing label if the pump cannot withdraw the bottom water. Avoid counting gutter losses twice: efficiency represents proportional loss before the separately entered fixed diversion. A diversion greater than available runoff gives zero inflow, not negative stored water.

Assumptions, limits, and responsible use

Inputs support 0.1–100,000 m², 0–2,000 mm, 0–100% efficiency, 1–10,000,000 L usable capacity, and the labeled volume limits. Results round to two decimals; calculations retain precision. This is dimensional arithmetic, not an empirical rainfall forecast. Actual efficiency varies with roof material, storm intensity, blockages, and plumbing.

The calculator does not size gutters, overflow pipes, foundations, or drainage. It does not assess potable-water treatment or local rules. Never use predicted overflow volume as an overflow pipe flow rate: duration and peak intensity are missing. Consult local water and building guidance for installation; use measured rainfall records for longer-term reliability analysis.

Related planning

Use the drip irrigation runtime calculator to estimate an irrigation withdrawal, or the water usage cost calculator for household demand costs. Neither models tank overflow.

Planning estimate. Last updated: 2026-10-09.