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 calculationSee the methodology
Household

Enter totals for the whole household on a typical day, not per person — 4 showers of 5 minutes is 20 minutes. You can use decimals for anything that happens less often: 5 washing loads a week is 0.7 a day.

This sets drinking and cooking water only. Changing it does not scale the figures below — those are household totals you enter yourself, so adjust them too.

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.

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.

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

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.

Washing machine type
Roof and region

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

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 — seeHow to measure your roof catchment areafor more.

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

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.

Sun setting behind a dry, tussock-covered hillside

Plan for where you live

Rainfall is local

The same roof and household can need very different storage in different parts of New Zealand. Local rainfall changes both how much water arrives and when it arrives.

Compare rainfall by region

From everyday use to a practical answer

How the calculation works

A useful recommendation has to connect household demand with the rain your roof can actually collect. We model both sides before suggesting a tank.

Hands held beneath a stream of flowing water

Demand and catchment

Start with your household and roof

First we work out how much water you use. Then we measure the part of your roof that drains to the tank—the surface that turns rainfall into collected water.

  1. 01
    Your usage

    Daily fixtures, appliances and outdoor use become one realistic demand figure.

  2. 02
    Your roof

    Only connected roof area counts, because it sets the maximum water you can harvest.

Supply and storage

Model rainfall across the whole year

Annual rainfall alone hides dry months. The calculator follows water into and out of the tank month by month, then checks a locally relevant dry-year scenario.

  1. 03
    Your rainfall

    Regional monthly data controls how much water arrives and its seasonal pattern.

  2. 04
    The monthly balance

    Demand, inflow, storage and overflow are balanced to find the smallest suitable capacity.

Raindrops forming overlapping ripples on a water surface

Want to check the detail?

Every assumption and source is published.

See the formulas, rainfall inputs and limits behind the recommendation.

Read the methodology

A few useful answers

Common questions

Tank sizing raises practical questions. Start with these, then explore the guides for installation, water quality and backup planning.

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.

Ready when you are

Get a recommendation for your home

Enter your household use, connected roof area and region. The result updates immediately as your answers change.

Start your calculationBrowse practical guides