Pillow tank capacity: how to size it properly

Usable or nominal capacity: don’t confuse them

When a manufacturer advertises a 30 m³ tank, that figure is the total volume: the tank at full charge. The volume you can actually use is always somewhat less.

Two factors eat into the available capacity. First, dead volume: the pump strainer never reaches the lowest point, so a few centimetres of unusable water always remain at the bottom.

In practice: to be sure of 30 m³ usable, choose a nominal capacity of 32 to 33 m³. That margin avoids the classic disappointment of “I thought I had 30 m³”.

Pillow tank capacity: the calculation method

The capacity you need depends on the use. For rainwater harvesting, the basic formula is simple:

Collectable volume (litres) = roof area (m²) × rainfall (mm/year) × 0.8

The 0.8 coefficient covers collection efficiency: some water evaporates, runs past the gutter or is lost in the first flush off the roof. Rainfall varies widely from region to region.

Region	Average rainfall	Annual collection for 120 m²
South East and East Anglia	550 – 700 mm	≈ 53–67 m³
Midlands	700 – 850 mm	≈ 67–82 m³
South West and Wales	900 – 1,400 mm	≈ 86–134 m³
Lake District and Scottish Highlands	1,500 – 3,000 mm	≈ 144–288 m³

Beware the trap: annual collection is not the capacity to install. A tank smooths demand between rainfalls; it does not store a whole year. Size it at 5 to 8% of annual collection — three to four weeks of autonomy.

For agricultural irrigation the reasoning is different: you start from the crop’s needs. Reckon on 3,000 to 5,000 m³ per hectare per season depending on species and climate, and size the tank as a buffer between two supplies.

What capacity to choose for your use

The table below gives realistic orders of magnitude observed in the field. They are starting points: precise sizing takes account of your rainfall and your actual uses.

Use	Typical capacity	Note
Vegetable plot / garden watering	1 à 5 m³	Enough for seasonal use without a lawn
Domestic rainwater harvesting	5 à 10 m³	WCs, washing machine, outdoors
Livestock watering	10 à 30 m³	Depending on herd size and distance to mains
Market garden irrigation (≤ 1 ha)	50 à 150 m³	Buffer against summer restrictions
Arable crop irrigation	200 à 1,000 m³+	Winter storage for summer
Liquid nitrogen fertilisers	10 à 50 m³	Compatible PVC fabric, bunding recommended
Fire-water reserve	30 à 120 m³	Volume set by the authority — see the dedicated section

Storing effluents and liquid fertilisers deserves particular care: the high-tenacity PVC fabric must be approved for the liquid stored, and secondary containment is often required depending on the site.

For rainwater harvesting in a crawl space, volume is not the only constraint: the height available beneath the floor also limits what capacity you can install.

Fire-water reserve: capacity is set by regulation

When the tank serves as a fire-water reserve, you no longer choose the volume: the competent authority sets it, based on the site’s risk, the built area and the resources available locally.

In the UK the applicable framework is BS 9990 for fire-fighting water supplies, alongside the LPC Rules / BS EN 12845 where sprinklers are served. Required volumes and flows are determined case by case: confirm them with your Fire and Rescue Service before ordering the tank.

To be accepted, the reserve must meet the local framework’s requirements: material, flanges, draw-off arrangement, signage and accessibility of the draw-off area. Flocea supplies the tank’s technical documentation for the submission to the authority.

The process is straightforward: you submit the data sheet and compliance documentation to the relevant fire safety service, which approves the design and then inspects the installation at handover. Flocea sizes your reserve free of charge — just get in touch.

Cost per m³: real value for money by volume

The cost per m³ stored falls sharply as volume rises. A small tank is expensive per cubic metre; a large one spreads the fixed manufacturing cost.

Capacity	Indicative price excl. VAT (supply)	Cost per m³
5 m³	900 – 1,300 €	≈ 220 €/m³
10 m³	1,500 – 2,000 €	≈ 175 €/m³
30 m³	3,500 – 4,500 €	≈ 130 €/m³
50 m³	5,000 – 6,500 €	≈ 115 €/m³
100 m³	9,000 – 12,000 €	≈ 105 €/m³
200 m³	16,000 – 22,000 €	≈ 95 €/m³

These prices are excluding VAT and installation. Fitting typically adds 15 to 30% of the tank price for a simple installation, more where earthworks are substantial.

For rainwater harvesting, it pays to reduce consumption first and identify which uses can genuinely be substituted, before sizing the project.

One large tank or several?

Above 100 m³ the question arises: one large tank or several units linked together? A single tank is the cheapest per cubic metre and the quickest to install.

Criterion	Single tank	Multiple tanks
Cost per m³	Lower	Higher (+10 to 20%)
Ground	A single flat area	Adapts to slopes
Maintenance	Storage 100% out of service	Tanks isolated one by one
Scalability	Fixed	Further units can be added

That modularity suits a growing operation: install 100 m³ now, add another tank next year as the acreage expands.

Installation constraints by capacity

The larger the capacity, the more the installation needs framing. A 5 m³ tank goes down in half a day on a slab or a bed of compacted sand. A large reserve requires a more thorough base and suitable access.

A few rules not to overlook. UV protection is built into the high-tenacity PVC fabric (typical resistance −30 °C to +70 °C). Siting it in a crawl space or in shade extends its life: 15 to 20 years against 10 to 12 in permanent full sun. Also respect distances from buildings and boundaries — particularly for a fire-water tank, whose draw-off area must stay clear and accessible.

Frequently asked questions

What pillow tank capacity should you choose for a house?

For a detached house, a 5 to 10 m³ tank covers watering, washing and supplying the WCs. Below 5 m³ the reserve runs dry quickly in summer.

How do you calculate the tank volume you need?

For rainwater: roof area (m²) × rainfall (mm/year) × 0.8 gives annual collection in litres. The tank is then sized at 5 to 8% of that figure — three to four weeks of autonomy.

What capacity is required for a fire-water reserve?

The volume is not yours to choose: the fire safety authority sets it, based on the site’s risk and the framework applicable in your area. Check with them before ordering — compliance is assessed on the drawings, before installation.

What does a pillow tank cost by capacity?

As a guide, excluding installation: €900 to €1,300 excl. VAT for 5 m³, €1,500 to €2,000 for 10 m³, €3,500 to €4,500 for 30 m³ and €9,000 to €12,000 for 100 m³. Cost per m³ falls sharply with volume. Fitting typically adds 15 to 30% of the tank price.

Is a cheaper imported pillow tank worth it?

For firefighting use, no: without compliance with the applicable framework, the authority will reject the tank and insurance will not cover it. For other uses, a low-grade imported sheet ages badly under UV. A tank with a 5-year warranty and certified material remains the safe long-term choice.

Sizing your tank: the essentials

Getting a pillow tank’s capacity right means thinking in usable volume, calculating your real need rather than copying a neighbour, and factoring in regulation from the outset — especially for a fire-water reserve. The right volume covers your use without paying for empty space or running dry during a restriction.

Precise sizing takes account of your rainfall, your ground and your local constraints. That is exactly what a manufacturer cutting and building to measure can work out with you, quotation and capacity study in hand.