Practical guide
Rainwater Harvesting Formula, Roof Area and First Flush
Rainwater potential, water admitted to storage and demand actually served are different quantities. Keep the inputs and event timing explicit when comparing a collection system.
Updated 16 October 2026
Reviewed by the 99tools team. Editorial and corrections policy.
What is rainwater harvesting?
Rainwater harvesting collects rainfall runoff for storage and later use. A rooftop system connects a suitable roof catchment to gutters, downpipes, debris screening, a storage tank and an appropriate outlet or pump arrangement. Treatment depends on the intended use. The purpose is to make use of water that would otherwise run off the site, where a suitable system can collect it. Collection is intermittent and depends on rain; it is not an unlimited supply or a guarantee of independence from municipal water.
Rainwater harvesting metric formula
One millimetre of rain over one square metre of horizontal area is one litre before losses. Gross roof rainfall volume in litres therefore equals connected horizontal roof-plan area in m² multiplied by rainfall in mm. Apply the entered pre-diversion collection percentage, then subtract the actual first-flush diversion for that event, never below zero. The tool shows gross volume, other collection losses, diverted litres and water reaching storage separately. It does not assume that every roof has the same collection efficiency.
Which roof area should I enter?
Use only the horizontal plan area that actually drains into this system. A pitched roof has a larger sloping surface than its horizontal projection, but that larger surface must not be multiplied by vertical rainfall depth. Do not count both roof slopes twice or include disconnected downpipes. Roof extensions, shared gutters and bypassed sections need their own catchment checks. If you know only roof-plane dimensions, establish the connected plan area before using this calculator. The gutter tool can assist quantity planning but does not approve hydraulic capacity.
Collection efficiency and first-flush devices
The pre-diversion collection percentage represents the portion remaining after losses other than the explicit first flush entered here. Do not count the same loss in both inputs. A first-flush device diverts initial runoff; the actual amount depends on its chamber, reset condition and connected arrangement. Enter the litres diverted for each separate event represented by a row. If the percentage already includes all first-flush losses, enter zero separate diversion. No universal diversion volume or automatically recommended percentage is imposed by this calculator.
Worked rainwater collection example
For an illustrative connected roof-plan area of 120 m² and 50 mm rainfall, gross rainfall volume is 6,000 litres. With an entered 85% pre-diversion collection percentage, 5,100 litres remain before explicit diversion. An entered 100-litre first flush leaves 5,000 litres reaching storage. If a 5,000-litre operational tank already contains 1,000 litres, it admits 4,000 litres and overflows 1,000 litres before withdrawal. If the following entered demand is 600 litres and no protected reserve applies, end storage is 4,400 litres. These are arithmetic assumptions, not product specifications or weather predictions.
Rainwater harvesting parts, pipes and fittings
A practical system may include gutters, downpipes, leaf screening, a first-flush device, suitable tank inlet, screened overflow, access provisions and a tap or pump, with treatment matched to use. Select compatible parts and actual pipe sizes for the installation instead of treating a quantity estimate as a flow design. Tank support, overflow routing and connections also need appropriate specification. This independent tool does not provide a universal shopping list, certify a kit or claim that one pipe diameter suits every roof.
Use the related calculators
Apply the explanation above with your own figures. Open a calculator below, check every input and review the result assumptions before making a decision.
Frequently asked questions
What is the rainwater harvesting formula?
Gross litres = connected horizontal roof-plan area in m² × rainfall in mm. Apply the entered collection percentage and subtract explicit first-flush litres without double-counting losses.
Does one millimetre of rain equal one litre?
It equals one litre over one square metre before collection losses. Rainfall depth alone is not a total volume.
Should I enter sloping roof area?
Enter connected horizontal roof-plan area for vertical rainfall, rather than the larger sloped surface.
How does the tank starting level affect results?
Water already stored reduces available space. The model fills that space, overflows excess inflow, then serves the following demand.
What does a first-flush device do?
It diverts initial runoff. Enter the actual event diversion and avoid counting the same loss in the collection percentage.