How big should your rain tank be? Our free calculator works it out for your spot in Canada. It runs a tank through every day of your local rain record, with your roof and your water use, and shows how each size would have done.
Our Rainwater Collection Calculation page shows how much water a year of rain puts off your roof. That is your supply. It can’t tell you the tank size, because rain doesn’t fall on the days you use water. A dry August can empty a tank that looks generous on paper.
Roof area × a year of rain = litres a year. Good for the big picture. Silent about dry spells.
Every day of every year on record: rain in, water used, overflow when the tank is full, and every day it runs dry. That is what sizes a tank.
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Tap the map where your tank will go. The calculator finds the Environment and Climate Change Canada weather stations around you and blends their daily records by distance.
It needs at least 30 years of daily history to size a tank. Where there isn’t enough, it tells you so instead of guessing.
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Your roof’s footprint (a length × width helper does the maths), what it’s made of, and what the water is for: the garden, toilets and laundry, or drinking and bathing.
Then the litres a day you use indoors and outdoors. The form shows what that adds up to over a year.
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The smallest tank that meets the target: your use covered on at least 95% of days across the whole record, and holding up in the driest years. Beside it: how many days it would have run empty, and how much mains water it replaces.
When the right size is two tanks plumbed together, it says so. If even the largest size falls short, it says that plainly too.
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Every tank size it tested, not just the pick, so you can see where a bigger tank stops paying. The driest year is replayed day by day, and a month-by-month view shows when you would still need another source.
Save it as a PDF with your own notes, or share the link with your installer.
Daily records from Environment and Climate Change Canada stations near you, decades deep.
Sizes a supplier delivers to your area, multi-tank setups included, priced as ranges with delivery. Where none delivers, it says the sizes are indicative.
Too little weather history? It refuses to size rather than invent an answer, and a missing day is never counted as a dry one.
Month by month: when the tank covers you, and when you would top up from another source.
A report link carries no address and no coordinates, only distance and bearing to the weather stations.
Choose the months each use applies, size against one station’s record, or test every size up to cistern scale.
For installers and the curious, the Advanced page opens the settings most people never need:
Rainwater for drinking, cooking or bathing needs filtration and disinfection, and your local health authority has to approve the system. Rainwater for toilets and laundry needs plumbing that is separated and labelled so it cannot cross-connect with your drinking supply. The calculator sizes storage; it does not test or certify water quality.
Results are estimates from past weather, and future years will differ. Where a supplier delivers to your area, prices are shown as ranges that include delivery; a firm quote depends on your site, access and installation.
Cleanflo Water Technologies designs and installs complete rainwater harvesting systems. Click the logo for a free quote.
