How Big a Water Tank Do I Need? A Joburg Sizing Guide

Tank sizing is where most backup-water decisions go wrong — nearly always in the same direction: too small. The savings between tank sizes are modest; the cost of running dry on day two of an outage is the entire point of the system. Here’s how to size properly, with the arithmetic laid out.

Start with what your household actually uses

Research published in the IWA’s Water Policy journal puts average South African consumption at about 237 litres per person per day, against a world average of roughly 173 litres. Treat that as context rather than gospel — Africa Check has pointed out that national averages fold in industrial use and system losses, so they overstate what comes out of your taps. For sizing purposes, a practical planning range of 150–250 litres per person per day at normal habits works well, dropping to roughly half that under outage discipline: shorter showers, laundry postponed, the garden left alone.

The formula is then simply people × daily litres × days of coverage you want.

A family of four wanting three comfortable days: 4 × 200 × 3 = 2,400 litres minimum — which in practice means a 5,000-litre tank, because you should never size to the exact minimum.

A sizing table to start from

These are planning figures at roughly 200 litres per person per day, before irrigation. Round up to the nearest available tank size, and up again if your suburb runs long interruptions.

Household	1 day	2 days	3 days
2 people	400–600 L	800–1,200 L	1,200–1,800 L
4 people	800–1,200 L	1,600–2,400 L	2,400–3,600 L
6 people	1,200–1,800 L	2,400–3,600 L	3,600–5,400 L

Why “usable capacity” is less than the label

A 5,000-litre tank does not deliver 5,000 usable litres. The pump draw-off point sits above the tank floor, so a portion at the bottom stays put — deliberately, since that’s where sediment settles. Add the water you won’t want to use at the very end of an outage, and planning on roughly 90% of nameplate capacity is realistic. It’s a small adjustment, but it’s the difference between a tank that comfortably covers three days and one that technically covers three days.

The questions that move the number

How long do your interruptions actually run? This is suburb-specific, and it matters more than any formula. High-lying areas — Northcliff is the textbook Johannesburg case — tend to lose supply earliest and recover last. Johannesburg Water’s own figures illustrate why the system has so little slack: combined storage across the city equates to roughly 27 hours of average demand, so when a reservoir or pump station fails, the shortfall arrives quickly. Size for the long event, not the average one.

Does the tank also feed irrigation? Garden demand dwarfs household demand, and a single irrigation cycle can empty days of drinking-and-washing storage. Either size for it explicitly, or accept that the garden loses first — a decision worth making deliberately rather than discovering on day two. If irrigation is the main driver, a borehole may be the better answer than an ever-larger tank.

Whole house or essential taps? Whole-house backup drains storage faster because everything is on it. An essential-outlets system — kitchen, one bathroom — stretches the same litres considerably further, which is a legitimate way to buy more days rather than more litres.

Pool, washing machines, guests. A pool top-up, a heavy laundry day, or a full house over a long weekend all land on the same storage. None of these need their own calculation; they’re the reason you add margin rather than sizing to the exact number.

Not sure which size fits your household? Get 3 free quotes with sizing advice — free for homeowners.

What most Joburg homes actually choose

The market has largely converged on three points: 2,500 litres for townhouses and tight stands, usually on essential outlets; 5,000 litres as the mid-sized family standard; and 10,000 litres — often as twin 5,000-litre tanks — for large households, real irrigation demand, or suburbs with long interruptions.

Twin tanks deserve their own mention. They cost slightly more per litre, but they’re easier to deliver and position on terraced or tight-access stands, they spread the load on the base, and they’re redundant by design: one can be isolated for cleaning or repair while the other keeps working.

Space, base and pump — the practical constraints

A vertical 5,000-litre tank typically needs around 1.8 to 2 metres of diameter plus working room around it, and slimline formats exist precisely because that footprint doesn’t fit every property — they trade some capacity for the ability to sit tight against a wall. Whatever the shape, the tank needs a level, load-bearing base: five thousand litres weighs five tonnes, and a cracked or settling base is one of the more expensive mistakes in this category.

The pump matters to sizing too. A correctly specified pressure pump keeps flow and pressure sensible across your outlets; an undersized one turns a well-sized tank into a disappointing shower. And because pumps need electricity, ask whether an inverter-ready circuit belongs in the quote if load shedding and water outages overlap where you live — the full backup system cost guide covers what that adds.

Two mistakes worth avoiding

Sizing to the sale price. The difference between a 2,500-litre and 5,000-litre installation is small against the total project cost, and you cannot retrofit litres cheaply later. Buy the capacity once.

Forgetting the long outage. Sizing for the typical interruption is sizing for the days you’d have coped anyway. The tank earns its money on the bad week, which is exactly the scenario an under-sized system fails.

Getting it sized professionally

Reputable installers size from your actual usage, property and suburb rather than from a catalogue — and the fastest way to sanity-check their reasoning is to compare three of them. One request on WaterQuotes brings up to three vetted quotes with their sizing rationale side by side, free for homeowners and with no obligation. Compare backup system quotes.