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Water

Tank Sizing: How Much Storage Do You Need?

A practical guide to calculating water storage requirements by matching supply from your roof to demand from your garden, household, and dry-season needs.

By Arborpedia Team·November 10, 2025

Calculating Demand

Before choosing a tank, you need to know how much water you actually use. This is the demand side of the equation, and it is where most people make their first mistake: they guess, and they guess low. A systematic accounting of daily, weekly, and seasonal water use reveals numbers that are often two to three times what people assume.

Garden irrigation is typically the largest non-potable demand. A vegetable garden in warm weather needs roughly 4 to 6 litres per square metre per day. A 50-square-metre kitchen garden therefore requires 200 to 300 litres daily during summer, or about 6,000 to 9,000 litres per month. Fruit trees need 40 to 80 litres each per week depending on species and size. A modest orchard of 10 established trees adds 400 to 800 litres per week. Lawns, if you choose to water them, consume 4 to 8 litres per square metre per week. Add it all up and a moderately sized property with a productive garden, a small orchard, and some ornamental plantings can easily require 15,000 to 25,000 litres per month during the growing season.

Household non-potable use adds another layer if you plan to connect harvested rainwater to toilets and laundry. Toilet flushing accounts for roughly 30 to 50 litres per person per day; a family of four uses 120 to 200 litres daily. A front-loading washing machine uses 50 to 70 litres per load. Over a month, household non-potable use for a family of four totals approximately 5,000 to 8,000 litres. The critical period for sizing purposes is the longest stretch you expect to rely on stored water without significant rainfall input, typically the dry season in seasonal climates or a prolonged summer drought in temperate regions.

Calculating Supply

The supply side depends on your roof catchment area and local rainfall. The core formula is roof area in square metres multiplied by monthly rainfall in millimetres multiplied by a runoff coefficient (typically 0.80 to 0.85 for metal roofs after accounting for gutter losses and first-flush diversion). This gives you monthly litres of supply.

What matters for tank sizing is not the annual total but the monthly pattern. A location receiving 800 millimetres of annual rainfall spread evenly across 12 months has a very different storage requirement than one receiving the same 800 millimetres concentrated in a 4-month wet season. In the first case, a modest tank refills continually and a few thousand litres of storage may suffice. In the second, the tank must hold enough water at the end of the wet season to bridge an 8-month dry period, requiring storage of tens of thousands of litres.

Build a monthly water budget on paper or in a spreadsheet. For each month, enter the expected supply (roof area times monthly rainfall times coefficient) and the expected demand. The running balance, starting from a full tank at the beginning of the dry season, reveals the maximum cumulative deficit. That deficit is your minimum tank size. A 100-square-metre roof in a climate with a 5-month dry season, where demand runs at 8,000 litres per month and supply drops to 1,000 litres per month, produces a cumulative deficit of 35,000 litres. That is the storage volume needed to make it through the dry season without running out.

Matching Supply to Demand

The monthly water budget often reveals a mismatch. Either your roof cannot supply enough water to meet demand even with unlimited storage, or your demand is so modest relative to supply that a small tank will do. Recognising which situation you are in saves money and frustration.

If supply exceeds demand on an annual basis, the tank size is determined by the longest gap between filling and emptying. A roof that delivers 60,000 litres annually against a demand of 30,000 litres has plenty of water in total, but if 50,000 of those litres arrive in 4 months and demand is spread over 12, you still need a large tank to bridge the gap. Conversely, if supply and demand are both spread evenly through the year, a tank that holds 2 to 4 weeks of demand may be sufficient because it refills regularly.

If demand exceeds supply, no amount of storage will fix the deficit. You need to either reduce demand, through more efficient irrigation like drip systems, mulching, or wicking beds, or increase supply by connecting more roof area, adding surface catchment, or supplementing with municipal or bore water. The water budget makes this analysis explicit. Rather than buying the biggest tank you can afford and hoping for the best, you can identify exactly where the bottleneck is and address it directly.

Redundancy, Cost, and Practical Choices

Once you know your target volume, practical considerations shape the final decision. A single large tank is the cheapest option per litre stored. A 10,000-litre polyethylene tank typically costs 30 to 50 percent less per litre than two 5,000-litre tanks. However, multiple smaller tanks offer redundancy: if one develops a leak or needs cleaning, the others continue to supply water. They also fit into spaces where a single large tank would not, and they can be placed at different elevations and locations to serve different parts of the property by gravity.

The cost per litre of stored water drops dramatically with scale. A 1,000-litre tank might cost 0.50 to 1.00 per litre of capacity. A 10,000-litre tank drops to 0.10 to 0.20 per litre. A 25,000-litre tank or underground concrete cistern can fall below 0.10 per litre. For large-volume storage, ferrocement tanks built on site are often the most economical option in regions where labour is affordable, achieving costs below 0.05 per litre.

Start with a tank that meets your minimum calculated requirement, then plan for future expansion. Most tank systems can be extended by adding another tank and connecting it to the first via a pipe at the base (for equalised water levels) or via overflow (for cascading fill). Buying a 5,000-litre tank now with a plan to add a second one next year is often wiser than buying a 10,000-litre tank that strains the budget and delays other improvements, like building the soil organic matter that reduces irrigation demand in the first place.

See Also

tank sizingwater storagecalculationplanning