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More rain does not mean a bigger tank.
This page shows you why.

Most tank calculators ask only how much water lands on your roof. That is the easy half. The size of the tank you actually need is set by something else: how long you go between useful rains. A home in a wet climate with a short dry spell needs a small tank. A home in a dry climate with a long dry season needs a much larger one, and may still never fill it. This tool works the harder half, using the dry-period method behind the current rainwater standard, and tells you honestly which situation you are in.

Units: All maths runs in your browser. Nothing you enter is stored or sent anywhere.

① Your roof and your rainfall

Catchment Roof footprint area (m²) The area the roof covers on the ground, not the sloped surface. Rain is measured on the flat, so slope does not add catchment. How much of the roof drains to the tank? A tank on one side of a pitched roof collects only that side. Picking the whole roof when only half drains will overstate what you can collect. Roof material Sets how much of the rain runs off rather than soaking in or evaporating. Average annual rainfall (mm) From any weather service for your city, or your BGMS climate zone gives a typical range.

② Your dry season

The number that decides the answer

Pick the climate that fits you. For dry and seasonal climates this tool will not guess for you, because within those zones the dry season varies too much for one number to be honest. Enter your own where asked.

Length of your dry season (days) Count the months that typically get little to no rain where you live, times about 30.

③ What you would use it for

Non-potable demand

Rainwater here is for non-potable use only: garden, toilet flushing, laundry, cleaning. Drinking, cooking, and bathing need treatment and are outside this tool.

People in the household Uses to supply from rainwater

How this tool works it out

Two forces set the answer. Supply is how much rain you can catch: roof area, the share that drains to the tank, your rainfall, and how much runs off the roof material. Demand is how much you draw each day. The tank exists to carry your demand across the gap between rains, so the dry season length is what drives its size.

Demand-limited

You collect more over the year than you use, so the only question is bridging the dry spell. The tank is sized to carry you through it, and a modest one usually does. Wetter climates land here.

Supply-limited

You cannot catch enough over the year to meet your demand. Rainwater becomes a supplement, not a sole supply. A bigger tank past a point just sits empty. Drier climates land here, which is why the dry-climate answer is often "use less", not "store more".

Before you buy or build

It is a supplement, not a guarantee

A tank is sized for a typical dry season, not a worst-case drought. Keep a mains or other backup.

The tank is only part of the system

A working setup also needs a first-flush diverter, a mesh screen against debris and mosquitoes, an overflow routed away from the foundation, and a base built for a full tank. Water weighs about one kilogram per litre, so a 5,000 L tank holds five tonnes.

Underground and buried tanks need an engineer

A buried tank is pushed inward by soil and groundwater and outward by the water inside, and those pressures do not stay balanced as it fills and empties. A half-full underground tank can carry real, uneven loads on its walls. That is a structural design question, not a sizing one. This tool gives a target volume. It cannot tell you wall thickness, bedding, or whether a site can take it.

Method and code

This estimate uses the dry-period demand approach that underpins on-site rainwater sizing, consistent in principle with the current standard BS EN 16941-1:2024, and the constant-daily-demand simplification established by Fewkes (1999). It is a planning estimate, not certification of compliance with that standard or any local code. Codes and permit rules vary, and some places restrict or require rainwater systems. Verify locally before installing.

Chapter 4 of the guide carries the full water chapter, the vernacular cistern traditions this method quietly agrees with, and the greywater options that pair with it. Read the guide (free PDF).