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 flushes Count 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 time Watercare 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 hand This 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 cooking A 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.
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.