Will this rain fill your tank?
Separate rain falling on the roof from water you can actually keep. Change the inputs to compare an empty barrel with one that is already partly full.
Illustrative starting values: 500 L falls on the connected roof; 400 L reaches the tank at your assumed 80% efficiency. Only 200 L fits, leaving 200 L of overflow.
How this works, and what it leaves out
One millimetre of rain on one square metre equals one litre. Collected volume = roof footprint × rainfall × your efficiency assumption. Stored volume is limited by spare tank capacity. The 80% starting value is an illustrative assumption, not a measured efficiency for your installation. Leaks, interception, first-flush diversion and rainfall intensity change delivery. This does not size the overflow pipe or establish water quality.
How to use this tool
- Enter the horizontal footprint of the roof area connected to this tank, not the total area of an unconnected roof.
- Enter rainfall for one event in millimetres and your assumed collection efficiency. The starting 80% is an example, not a measurement of your system.
- Enter tank capacity and the water already stored. Compare delivered water with the remaining space and expected overflow volume.
Ten millimetres of rain on a 50 m² roof
One millimetre on one square metre is one litre, so 50 × 10 gives 500 litres falling on the connected footprint. At an assumed 80% efficiency, 400 litres reaches the tank. An empty 200-litre barrel stores 200 litres and overflows 200 litres. If it already contains 150 litres, only 50 additional litres fits.
What the result means
Changing storage capacity does not change how much rain falls. It changes how much of the delivered runoff can be retained. This single-event model does not include water being used during the storm. For seasonal planning, rainfall timing and withdrawals matter as well as annual totals.
Questions before you start
Does a bigger tank always collect more water?
Not from the same event if the smaller tank already has enough spare capacity. A bigger tank can retain more when delivered runoff would otherwise overflow.
Can I use the result to size an overflow pipe?
No. This gives an event volume, not peak flow or pipe capacity. Rainfall intensity, roof drainage, fittings and local requirements need a separate design. It also makes no claim about water quality.
Put it in context
Use these guides alongside the tool for the choices a calculation or decision form cannot make for you.