Understanding your result
Additional stored water is limited by the empty space in the tank. Overflow is collectable water that arrives after that space is filled. The result assumes no water is used during the event and does not simulate a daily or yearly storage balance.
If the tank’s capacity is unknown, use the Tank Volume Calculator with its internal dimensions before entering usable storage here.
The formula
One millimeter over one square meter equals one liter. Collection efficiency first reduces the roof rainfall volume; the separate first-flush input is then deducted once. Stored addition is the lesser of collectable volume and unused tank capacity. Final storage includes water already present.
Worked example
A 100 m² roof receiving 20 mm rain gets 2,000 L. At 80% efficiency and a 10 L first flush, 1,590 L remain. A 1,000 L tank already holding 200 L accepts 800 L and overflows by 790 L, finishing full.
How to use this calculator
- Measure the horizontal roof area actually connected to the tank.
- Enter rainfall for one event, collection efficiency, and first-flush liters.
- Enter capacity and current storage; compare retained water with overflow.
Use plan area rather than sloped roof surface
Rainfall depth is measured vertically, so use the roof’s horizontal projection. Count only sections whose gutters feed the tank. A steep roof has more surface area than its plan area; entering the larger sloped area overstates collection under this model.
Keep loss assumptions separate
Collection efficiency represents losses before the first-flush amount specified here. If an efficiency estimate already includes first flush, set the separate first-flush input to zero to avoid counting it twice. These assumptions can change substantially between surfaces and system conditions; the example values are not site recommendations.
A tank size needs a sequence of rainfall and demand
A single storm calculation shows overflow but cannot establish the best annual tank size. Repeated rainfall, dry periods, and water use change the starting storage for each event. Rainwater quality and permissible uses also depend on catchment materials, maintenance, treatment, and local requirements, none of which is determined by volume.
Assumptions & limitations
What this calculation assumes
- There is no tank drawdown during the modeled event.
What to keep in mind
- Does not retrieve local rainfall, assess water quality, or size roof drainage hydraulics.
Common questions
Can I use total monthly rain?
You can calculate potential volume, but the storage and overflow result treats all rain as one event with no use between storms. That is not a monthly water balance.
What if first flush exceeds available runoff?
Collectable water is zero rather than negative. Existing tank water remains unchanged.