Water usage factors
The calculator estimates your household's daily water demand from a set of usage factors. Each factor is a single figure (in litres) that the calculator multiplies by the number of times that activity happens per day. The table below lists every factor the calculator uses, the figure it applies, the published range where one exists, and the source it was taken from.
This table is generated directly from the same data the calculator reads, so the figures shown here are exactly the figures the calculator uses.
| Activity | Unit | Litres used | Published range | Source |
|---|---|---|---|---|
| Toilet flushesCount a full flush as one. Modern dual-flush toilets use 6/3 L or 4.5/3 L; older cisterns can use up to 12 L. | flush | 6 | 3–12 | BRANZ |
| Baths | bath | 180 | Point value | LEARNZ |
| Shower timeWatercare reports about 12 L/min for an average shower and 5–9 L/min for a low-flow showerhead. Measure yours for a closer estimate. | minute | 12 | 5–12 | Watercare |
| Tap use (hand washing, teeth, food prep)This uses the midpoint of BRANZ’s 6, 8 or 12 L/min aerated-tap rates. Unaerated taps can use 18–28 L/min. | minute | 9 | 6–12 | BRANZ |
| Washing dishes by handThis is a full sink-fill wash with rinsing under a running tap. If you fill a basin and rinse in it your figure is much lower — enter tap minutes instead. | wash | 100 | 33–440 | CHOICE (Australia) |
| Dishwasher | load | 13 | Point value | CHOICE (Australia) |
| Washing machine — traditional top loader | load | 117 | Point value | Consumer NZ |
| Washing machine — front loader | load | 59 | Point value | Consumer NZ |
| Drinking and cookingA rounded planning allowance for drinking and cooking. Adult fluid needs vary; the cited guidance gives 2.1–2.6 L/day from drinks alone. | person per day | 3 | Point value | New Zealand Ministry of Health |
| Garden hose | minute | 15 | Point value | Watercare |
Working assumptions
Beyond the per-activity figures, the calculator applies a small number of fixed constants. These are stated here with their values and the reason for each. Where a constant does not yet have a verified citable source, that is said plainly rather than implied.
- Verified source
Collection allowance
80%The fraction of roof rainfall collected. BRANZ allows 10–20% collection losses through spillage and evaporation; we use the higher loss allowance, giving 80% collection. This is a design allowance, not measured efficiency for every roof.
RUNOFF_COEFFICIENTRead source: Bulletin 664: Residential rainwater systems, section 3.0.3 (August 2021)
Tank size rounding
The model rounds its result up to the next representative capacity: 500, 1,000, 2,000, 3,000, 4,000, 5,000, 9,000, 10,000, 13,500, 15,000, 20,000, 25,000, 30,000 litres. This is a market ladder, not a claim that every supplier stocks every size. Confirm the selected product’s nominal and usable capacity before purchase.
The ladder was checked against current New Zealand ranges from:
The balance model
The calculator simulates a year, month by month, to find the smallest tank that keeps water available throughout. For each month it works out three things:
- Inflow. The amount of rain that lands on your roof and makes it into the tank. One millimetre of rain over one square metre is exactly one litre of water, so the inflow is your roof area in square metres multiplied by that month's rainfall in millimetres, multiplied by the runoff coefficient.
- Demand. The amount of water your household uses that month, which is your daily demand in litres multiplied by the number of days in the month.
- Level. The tank's water level at the end of the month is the level at the start, plus inflow, minus demand. This level is clamped between zero and the tank's nominal capacity.
The model then checks whether the tank ever runs dry over the year. The smallest tank that avoids running dry is the recommended size.
The twelve months are repeated until the tank ends the year at the level it started, so the opening level is the one the system settles at rather than an assumption we picked. Without this, a tank that begins full is credited with that first fill as though it were free water, which flatters any result where the roof does not quite keep up.
When the roof cannot cover demand on its own
If a year's rainfall on your roof adds up to less than a year's demand, no tank of any size can supply the household by itself, and some water has to come from another source. That is a normal result in drier districts, and it is not a reason to stop at "not enough" — the calculator still sizes a tank, because how much you have to bring in depends on how much of the wet season you can hold onto.
How much capacity is worth depends on how far short the roof falls. Where the roof covers most of the demand, a larger tank carries wet-season rain into the dry months and measurably cuts what you need to bring in. Where it falls a long way short, the tank spends most of the year empty and every size performs about the same; the calculator says so rather than implying a bigger tank would help. In both cases it reports the volume you would still need each year, and the roof area that would close the gap entirely.
That top-up volume is rainwater arithmetic. It is not a recommendation about storage for carted water, which depends on what your carrier delivers in one load — a figure this site does not publish, because we have no sourced national figure for it.
Limitations
Simulations use nominal tank capacity. Actual usable water depends on the tank shape, outlet height, pump intake and sediment clearance. BRANZ Bulletin 664 requires a low-level outlet at least 300 mm above the base, but that does not translate into one percentage for every tank. Confirm the usable volume and connection details for the product and installation before purchase.
The model is a useful estimate, but it has limitations that are worth stating plainly.
- Monthly timestep. The model works in whole-month steps. A month that balances on average can still contain a multi-week dry spell that the model cannot see inside that month. This is why a minimum-reserve floor exists underneath the balance model, so the recommended tank is not sized to the absolute edge of the monthly average.
- Average-year rainfall. The model uses average-year rainfall, but real years vary. Each region therefore supplies a factor derived from its annual 10th-percentile rainfall divided by its annual mean. The calculator scales all twelve monthly normals by that factor, so this remains a sensitivity scenario rather than a historical year.
Roof area: footprint, not slope
The roof area you enter should be the footprint of the roof — the area it covers when viewed from directly above — not the sloped surface area. The balance model already accounts for the fact that a sloped roof collects the same amount of rain as its horizontal footprint, so entering the sloped area would overestimate your collection.