Understanding Water Tank Capacity: Why Your Tank Might Hold Less Water Than Expected

When shopping for a rainwater tank, you’ll notice tank capacities listed like “4000 litres” or “10,000 litres”. But here’s something many homeowners (and some tradies) don’t realise: your tank won’t actually hold quite that much water once it’s installed and ready to use.

This might seem like a small detail, but it can be crucial when you’re trying to meet council requirements – especially for Stormwater Detention Systems.

What Does “Nominal Capacity” Really Mean?

The capacity listed on water tanks is refer to as the “Nominal Volume.” This represents the tank’s total internal space before the overflow fitting determines your maximum water level.

Here’s the key point: your tank can only fill up to where the overflow is positioned, not to the very top of the tank. The overflow fitting – typically located on the top corrugation of the tank wall – creates a “dead space” above it that can never hold water.

While other fittings like inlet strainers and outlet valves are also installed, these have minimal impact on your tank’s usable capacity. It’s really the overflow position that makes the difference.

Why This Matters for Council Requirements

Here’s where things get particularly important for homeowners: Council Stormwater Detention Requirements.

Many councils across Australia now require new developments and renovations to include On-Site Stormwater Detention (OSD). They might specify you need “minimum 5000 litres of detention capacity” to manage stormwater runoff from your property.

If you purchase a “5000L” tank thinking it meets this requirement exactly, you might find yourself in hot water with council inspections. Some councils are quite strict about this – they want to see the actual detention capacity up to the overflow level, not just the nominal tank size.

Real-World Example – 5500 Litre Re-use & Detention Tank

Let’s say your council requires you to install a water tank for:

• 4500 litres of stormwater detention, and
• 1000 litres rainwater reuse (for toilets, laundry, etc).

You plan on ordering a 5500L Slimline Tank (1050mm W x 2700mm L x 2020mm H) based on the plans provided to you by your engineer – sounds like the perfect size, right?

Unfortunately now. Firstly, while the diagram shows a Slimline Tank (capsule shaped tank), the capacity of a Slimline Tank in those dimensions is actually only 5240L. Rather, the engineer probably meant a 5500 litre Modline Tank (1050mm W x 2700mm L x 2020mm H).

But with a 100mm overflow at the top of the tank, the “dead space” at the top of the tank would be 276L (1050mm W x 2700mm L x 100mm H), meaning that the actual total working volume for this tank size would be approx. 5224L. So, after allowing for 1000L rainwater re-use, the available volume available for stormwater detention would be approx. 4224L – potentially falling short of council requirements.

The space above the overflow can represent approx. 2-10% of unusable volume (empty space), depending on the tank’s dimensions, overflow size and overflow placement (sometimes it is installed slightly lower than top of tank due to corrugations). The most common overflow size is 100mm which generally creates a 2-5% capacity loss.

Rather, for this project the tank would need to be made slightly taller, longer or wider to increase capacity. E.g. any of the following sizes could work:

• 5984L Modline Tank (1050mm W x 2700mm L x 2170mm H)
  • Total Working Volume 5708L / Empty Space 276L
  • Detention 4600L / OSD Overflow Height 450mm (base to centre)
  • Rainwater Reuse 1103L
    
• 5995L Modline Tank (1050mm W x 2900mm L x 2020mm H)
  • Total Working Volume 5698L / Empty Space 276L
  • Detention 4511L / OSD Overflow Height 450mm (base to centre)
  • Rainwater Reuse 1187L
    
• 5995L Modline Tank (1150mm W x 2700mm L x 2020mm H)
  • Total Working Volume 5698L / Empty Space 276L
  • Detention 4511L / OSD Overflow Height 450mm (base to centre)
  • Rainwater Reuse 1187L

Different Councils, Different Rules

Another think to consider is that now all local councils handle this issue the same way:

Stricter councils may:

• Require engineering certificates showing actual usable capacity up to overflow level
• Specify that overflow positioning must be accounted for in detention calculations
• Reject installations that don’t meet the exact detention volume required

More flexible councils might:

• Accept nominal tank capacity as sufficient
• Allow small variations (within 5-10%)
• Focus more on overall stormwater management rather than precise volumes

It’s always worth checking with your local council before purchasing to understand their specific requirements.

How to Avoid Problems

1. Size Up to Account for Overflow Position

If your council requires exactly 5000L of detention, consider purchasing a 5500L or 6000L tank. This gives you a comfortable buffer to account for the unusable space above the overflow.

2. Ask About Actual Detention Volume

At Tank Shop, we can tell you:

• Expected actual capacity up to overflow level for all our tanks
• What size tank you’ll need to achieve specific detention volumes
• How different tank models compare for detention applications
• Alternative overflow configurations might maximise detention volume

3. Get Professional Advice

For Stormwater Detention Systems, we recommend consulting with:

• A plumber experienced in rainwater systems
• A stormwater engineer
• Our technical team at Tank Shop

4. Documentation

Keep all specifications and capacity calculations for council submissions. Some councils may request detailed drawings showing how you’ll achieve the required detention volume up to the overflow level.

Understanding Your Tank’s Actual Capacity

Understanding how overflow positioning affects usable tank capacity isn’t just technical detail – it can save you from costly mistakes and council compliance issues.

When purchasing a rainwater tank, especially for stormwater detention:

• Always ask about actual capacity up to overflow level
• Don’t assume nominal capacity equals detention capacity
• Size up if you’re close to meeting minimum requirements
• Work with experienced retailers who understand overflow positioning impacts

When evaluating a tank for detention purposes, remember:

• The overflow fitting determines your maximum water level
• Space above the overflow cannot contribute to detention capacity
• This “dead space” is typically 2-5% of total tank volume

At Tank Shop, we stock quality Australian tanks from trusted manufacturers that will serve you well for decades. We’ll help you get the sizing right from the start to ensure you meet all requirements and avoid headaches down the track.

Looking for expert advice on tank sizing for your property? Contact Tank Shop today – we’ll help you choose the right tank to meet both your needs and council requirements, taking overflow positioning into account.