THE METHOD, MADE CLEAR
Follow the volume from rain to headroom.
Raincatch estimates water volume from a rainfall depth you enter, the connected horizontal catchment area and two explicitly separate loss assumptions. Optional storage arithmetic compares that collection with free capacity you supply.
Start with an entered depth, not a forecast
The calculator does not know your location, fetch a weather report or read a rain gauge. Enter a total rainfall depth for the event you want to explore. Ten millimetres here means ten millimetres accumulated over that event. A rate such as ten millimetres per hour is different: it needs a duration before it becomes a total depth. There is no rate or duration field in this tool.
The Bureau of Meteorology’s water-storage FAQ explains the dimensional relationship: rainfall depth in millimetres multiplied by horizontal roof area in square metres gives litres. Raincatch uses that volume relationship only; it does not use a design-rainfall table, estimate storm frequency or size drainage.
Measure the horizontal connected area
Use the plan-view footprint of the catchment that drains to the collection point being modelled. Imagine looking straight down at it. Do not substitute the longer sloping roof surface. Also exclude roof sections whose drainage goes somewhere else. A house’s full footprint is not automatically the area connected to one downpipe or tank.
For a simple connected rectangular footprint, ten metres by ten metres gives 100 m². This tool accepts the area directly; it does not survey a roof, infer its slope or decide which sections are connected. The NSW Planning Portal’s catchment description also distinguishes the connected roof area measured in the horizontal plane. That definition supports the area concept; this page does not assess compliance with that program.
Gross rainfall litres = entered depth in mm × horizontal connected area in m²
As a unit check, 1 mm is 0.001 m. Over 1 m² that is 0.001 m³, which is 1 litre. Ten millimetres over 100 m² therefore represents 1,000 litres before any losses. The formula describes water over the entered area, not a guarantee that it reaches a tank.
Give each loss one place in the calculation
The capture fraction is the percentage of gross rain volume assumed to reach a separate-loss stage. At 80%, four-fifths reaches that stage and one-fifth does not. It is an entered assumption. The site does not supply a universal roof efficiency or infer losses from a roof material.
The separate event-loss field is then a fixed volume removed from the water that survived the percentage stage. It might represent a separately specified diversion or another event loss you are choosing to model. It is not a diverter-design recommendation. Your Home’s system overview distinguishes roof collection, first-flush equipment and storage as system components; Raincatch does not derive their performance.
If your chosen capture percentage already includes that same diversion or loss, enter zero in the separate loss field. Otherwise you would deduct it twice. Conversely, if you enter the fixed loss here, define your percentage to cover other capture losses before it. The tool cannot determine what a percentage from another source includes.
After-fraction volume = gross litres × capture percentage ÷ 100
Applied separate loss = smaller of entered fixed loss and after-fraction volume
Collection = after-fraction volume − applied separate loss
The capped deduction prevents negative collection. If the entered loss exceeds the available water, only the available water is deducted. The unused part is shown explicitly; it is not taken from existing water already in storage.
Worked example: the default event
10 mm over 100 m², 80% capture, then a 20 L event loss
Gross volume is 10 × 100 = 1,000 L. Under the entered 80% fraction, 800 L reaches the separate-loss stage and 200 L is not captured. Deducting the separately entered 20 L leaves 780 L of modelled collection.
With 500 L of free storage capacity supplied, the arithmetic allocates 500 L within that headroom and identifies 280 L beyond it. The full balance is 200 + 20 + 500 + 280 = 1,000 L. Each part is counted once.
These are illustrative numbers, not a forecast, measured event or recommended capture percentage, loss setting or tank size.
Worked example: a small event with a larger fixed loss
Enter 1 mm, 10 m², 80% capture and a 20 L separate loss. Gross volume is 10 L, of which 8 L survives the percentage stage. Only those 8 L can be deducted as the fixed loss. Collection is 0 L, and 12 L of the entered loss remains unused.
It would be incorrect to report negative 12 litres collected, or to remove that unused portion from the existing tank. Changing the entered loss to 3 L instead leaves 5 L of collection. The arithmetic does not decide whether a physical device actually diverts either entered volume.
Free capacity is different from total tank capacity
Optional storage comparison needs the unused space available before this event. A tank rated for 2,000 L that already contains 1,500 L would have 500 L of arithmetic headroom, assuming those quantities are the usable volumes relevant to your model. Entering 2,000 L as free capacity would describe a different starting condition.
Retained against headroom = smaller of collection and supplied free capacity
Excess over headroom = collection − retained volume
There is no sensor connection or tank-level calculation. The comparison assumes no water use, other inflow or change in available space during the event, and does not model inlet rates or rainfall timing. It cannot establish the actual overflow of a physical system. If free capacity is unknown, turn off the comparison: retained and excess stay unavailable rather than becoming zero.
Zero free capacity is a valid supplied condition: all positive collection is beyond that headroom. Exactly matching capacity produces zero excess. Zero rainfall or a zero capture fraction produces zero collection, regardless of a positive separately entered loss.
Units and exact-fit comparisons
Rainfall supports millimetres and inches. Area supports square metres and square feet. Volume supports litres, US gallons and Imperial gallons with separate labels; “gallons” alone would be ambiguous. Conversion factors are 25.4 mm per inch, 0.09290304 m² per square foot, 3.785411784 L per US gallon and 4.54609 L per Imperial gallon.
The calculator retains underlying quantities when you change units. Entered decimal values and the water-balance comparisons use decimal arithmetic before display formatting. For example, 0.1 mm over 0.1 m² at 100% capture with no separate loss gives 0.01 L. Supplied free capacity of 0.01 L fits exactly; the tool does not invent a tiny excess from binary display arithmetic.
Displayed numbers are shortened for readability. The decimal volume record and local text/CSV reports preserve the canonical millimetres, square metres and litres used for the balance. Exports distinguish zero from an unavailable storage result. They do not include a weather source because no weather is fetched.
Use the model within its scope
Invalid or blank active inputs clear usable results and disable exports. A retained blank free-capacity field is ignored while its comparison is switched off, and becomes required again when switched on. Restore example returns to the illustrative event; reloading does the same. There is no account, saved-project link, upload or persistent history.
This is volume arithmetic, not a water-quality, drinking-water, plumbing, structural, installation or flood assessment. It does not establish a suitable tank size, safe use or a system’s reliability. Keep measured information and the meanings of your loss assumptions with any exported calculation.