Independent NZ calculator

What size water tank do you need?

Most tank calculators multiply your daily water use by a number of dry days and stop there. That ignores the two things that actually decide whether a rainwater system works: how much roof drains to your tank, and how much rain falls where you live.

This one models both, month by month, and tells you plainly when your roof cannot collect enough — because in that situation no tank size fixes it.

Start your calculation See the methodology

Household

Use a typical day. You can use decimals for activities that happen less often.

People in household

Toilet flushes (flush)

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.

Baths (bath)

Shower time (minute)

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.

Tap use (hand washing, teeth, food prep) (minute)

This uses the midpoint of BRANZ’s 6, 8 or 12 L/min aerated-tap rates. Unaerated taps can use 18–28 L/min.

Washing dishes by hand (wash)

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.

Dishwasher (load)

Washing machine loads (load)

Washing machine type Washing machine — traditional top loader Washing machine — front loader

Garden hose (minute)

Other water use (litres per day)

Roof and region

These two details determine how much water the tank can collect.

Roof area plumbed to tank (m²)

Not sure?

For a simple rectangular house, length × width of the footprint in metres is close enough. Only count the roof that actually drains to the tank — see How to measure your roof catchment area for more.

Rain falls vertically, so a steep roof catches the same as a flat one over the same ground footprint.

Region

Live result · updates as you change the form

Your result

Complete your roof and region

We’ll show the recommended capacity and how low the tank may get.

How it works

Your usage. You enter how much water your household uses each day.
Your roof. The area of roof that actually drains to the tank sets how much rain you can catch.
Your rainfall. Local monthly rainfall for your region drives how much water arrives, month by month.
A monthly balance. The result comes from balancing what you use against what your roof collects over the year — not a rule of thumb.

The calculator and the methodology page carry the full detail.

Common questions

What size water tank do I need?

It depends on three things: how much water your household uses, how much roof area drains to the tank, and how much rain falls where you live. A tank only stores what the roof catches, so a bigger tank cannot make up for a roof that is too small or rainfall that is too low. The calculator on this site works all three out together.

What happens if my tank runs dry?

You buy carted water, which is why the size matters. The calculator flags when your roof and local rainfall cannot supply your usage at all, because in that situation no tank size fixes the problem and you need to either use less water, drain more roof area to the tank, or budget for carting.

Not sure where to start? Browse the guides for practical next steps.

What size water tank do you need?

Complete your roof and region

Your water use

Your roof can't collect enough for this level of use.

You need more than one tank.

How it works

Common questions