Rainwater Tank Calculator (South Africa)

There is no single answer, because it depends on three things the usual rules of thumb ignore: how much roof you can gutter into the tank, how much water you actually use, and crucially when your town's rain falls relative to when you need it. A 5 000 litre tank on a 120 square metre roof covers roughly 84% of a typical garden's watering in Johannesburg and about 62% of the same garden in Cape Town. The calculator above works your own figure out rather than guessing.

Johannesburg is a summer rainfall region and Cape Town is a winter rainfall region. Gardens drink most in summer, so in Johannesburg the rain arrives exactly when you want it and a modest tank is topped up constantly. In Cape Town the rain falls in winter, when the garden barely needs it, so the tank has to carry water across four or five dry months to be useful. Carrying water is what costs capacity, which is why a Cape Town garden needs a far bigger tank for the same result.

More than almost anything else you can change. Here is the cleanest way to see it, with no comparison between towns involved. Take one town, one roof and one 5 000 litre tank, and compare steady year round indoor use against summer peaked garden use at the same peak day figure. The garden pattern uses about 35% less water over a year, so on volume alone it should always score better. In Johannesburg it scores 27 percentage points better, exactly as you would expect. In Cape Town it scores less than one point better: the 35% saving buys you virtually nothing, because the water arrives six months before you need it and a tank that size cannot hold it that long. That gap between 27 points and one point is timing, and nothing else.

One millimetre of rain on one square metre of roof is one litre of water, before losses. So a 120 square metre roof in a 600 mm a year town sees about 72 000 litres land on it. You will not get all of it: some evaporates off a warm roof, some overshoots the gutter in a downpour, and some is lost to first flush. The calculator applies a runoff coefficient for your roof material, 0.90 for metal down to 0.75 for flat concrete, which is the standard way of allowing for this.

It depends far more on your water tariff than on your rainfall. Municipal water is billed in stepped blocks, so the water you stop buying comes off your top block, not at an average rate. On Johannesburg's 2026/27 residential tariff a household using 12 kilolitres a month saves about R27 a kilolitre, while one using 35 kilolitres saves about R74. The same tank pays back nearly three times faster for the heavy user. Enter your actual monthly usage in the calculator to see your own figure rather than an average.

In Johannesburg, on a conventional meter, no. This surprises people and it is worth checking on your own bill. Johannesburg charges residential sanitation as a flat monthly amount banded by erf size, from R397.85 for an erf up to 300 square metres to R1 688.12 above 2 000 square metres, and that charge does not move whatever your water consumption does. The fixed water demand management levy, R107.74 a month, does not move either. Only the water blocks fall. Households on a prepayment meter are different: their sanitation is charged by volume, so for them saving water does cut both bills.

No, and the law says so explicitly rather than by implication. Schedule 1 of the National Water Act 36 of 1998 lists the water uses you may make without a licence or registration, and item (1)(c) is, in full, to "store and use run-off water from a roof". Reasonable domestic use and small gardening not for commercial purposes are on the same list. You do not need permission and you do not need to register. If you have read that a strict reading of the National Water Act makes rainwater harvesting illegal, which does circulate online, that is simply wrong: the Act names it as a permitted use.

Because of the very next clause, and it is the piece that reconciles the two camps arguing about this. Schedule 1(2) says an entitlement under the Schedule "does not override any other law, ordinance, bylaw or regulation, and is subject to any limitation or prohibition thereunder". So the National Water Act permits you to store and use your roof runoff, and your municipality can still regulate how you install it. In practice that bites on plumbing rather than on collection: connecting a tank to the taps inside your house raises cross-connection and backflow rules, and the City of Cape Town for instance publishes formal guidelines for alternative water installations. Filling a tank from a downpipe and running a hosepipe off it is not the part anyone regulates.

No. Nothing obliges an existing household to install one, and Schedule 1 of the National Water Act permits rainwater harvesting rather than requiring it. Building and plumbing requirements for new construction are set by your municipality and by the national building regulations, so if you are building rather than retrofitting, ask your local building control what applies before you plan the roof drainage.

It is a device that sends the first few litres of each rainfall event to waste, taking the dust, bird droppings and leaf litter off the roof with it, before the cleaner water behind it reaches the tank. It is a water quality device rather than a water quantity one, and it does cost you some yield. Because it removes a fixed volume per rainfall event, the annual cost depends on how many rainy days your town gets rather than how much rain falls, which is why the figure the calculator quotes differs by town. For garden use many people skip it; if the water is going anywhere near a tap indoors, fit one.

Because in a dry town it can be true, and it is more useful to say so than to quote a fantasy tank size. In Upington, on around 150 mm of rain a year, a 100 square metre roof simply cannot catch enough water to cover a typical garden however much storage you put under it, so the tool reports the ceiling your roof can reach instead. The honest options at that point are to gutter more roof into the tank, cut demand, or treat the tank as a useful top-up rather than a supply.

Ten years of daily rainfall, 2015 to 2024, from Open-Meteo's Historical Weather API, whose default blends ERA5-Land, a roughly 11 km resolution model, with the newer and sharper ECMWF IFS HRES model from 2017 onwards. That resolution is why a wet mountain suburb can come out differently from a dry town 20 km away, detail a coarser 25 km reanalysis such as plain ERA5 would blur together. It is still a gridded estimate rather than a reading at your address, so your own street can still differ from the town figure. Where you know better, the advanced section has a rainfall override that feeds straight through the maths, and it also shows you how far that coarser ERA5 grid disagrees with the default for your town, which is a fair measure of how much to trust it.

Because most quoted South African rainfall normals are long-term averages weighted towards the twentieth century, and the last decade has been considerably drier over much of the interior. Running the same dataset over 1991 to 2000 reproduces the familiar published figures closely, Pretoria at 681 mm against a commonly quoted 674, Bloemfontein at 574 against 582. Run it over 2015 to 2024 and Johannesburg is down 27%, Bloemfontein 25% and Upington 29%. We deliberately use the recent decade. Sizing a tank on 1970s rainfall would leave you short in the climate you are actually buying for.

JoJo is the brand name most South Africans reach for, much as people say Hoover for a vacuum cleaner, but the sizes are effectively standard across manufacturers, so the answer does not depend on whose tank you buy. The calculator's size list, 750, 1 000, 2 500, 5 000, 10 000, 20 000 and 30 000 litres, matches what JoJo, Rototank, Eco Tanks and the rest all sell, and it recommends the smallest of those that meets your target rather than a made-up number. Worth knowing before you choose: South African tank prices jump sharply above 5 000 litres, so a 10 000 litre tank often costs about three times a 5 000 litre one rather than twice, which is why the payback column tends to worsen quickly past that point.

The overflow runs out and is lost, which is exactly what the overflow figure in the results is telling you. It is worth looking at, because it is the honest argument for a bigger tank: if a large share of what your roof catches is spilling, extra capacity will actually capture something. If the overflow is small, a bigger tank has nothing left to catch and you are just buying plastic. Plumb the overflow somewhere sensible, into the stormwater or well away from foundations, rather than letting it pool against the house.

For watering a garden, no. For anything you drink, wash in or cook with, yes, and treat it as a real question rather than a formality. Roof runoff picks up dust, bird droppings and whatever has been growing in the gutter, so an indoor supply normally wants a first-flush diverter, filtration and disinfection, plus a properly sealed and light-proof tank so nothing breeds in it. This calculator sizes tanks by volume and does not model water quality at all. If the water is going anywhere near a tap indoors, get a plumber who does alternative water installations to specify the treatment.

They solve different problems and the honest answer is often neither on its own. A rainwater tank costs little, needs no licence, and delivers exactly when it has been raining, which is precisely when your garden needs it least. A borehole costs tens of thousands to drill with no guarantee of water, and then costs electricity every time you pump, but it does not care what the weather did last week. In a summer rainfall town a tank covers the growing season well and a borehole is the drought insurance; in a winter rainfall town a tank alone struggles to bridge to summer, which is what the sizing above will show you. If you are weighing the running cost of a borehole, our Borehole Pump Electricity Cost Calculator prices the pumping against your actual Eskom tariff.

Look at the tank size table rather than only the headline. Reliability rises steeply at first and then flattens, so there is usually a knee beyond which each extra thousand litres buys very little. Note also that South African tank prices jump sharply above 5 000 litres, a 10 000 litre tank often costs three times a 5 000 litre one rather than twice, so the payback column tends to get worse quickly past that point. If your reason for a tank is surviving municipal outages rather than saving money, size it on the days of supply you want instead and ignore the payback figure.