Pool Filter Size Calculator Australia - What Size Do I Need?

Wondering what size filter you need for your swimming pool? Use this calculator to quickly work out what size filter is best for your pool.

Works with sand, glass, cartridge and DE filters.

Pool Filter Size Calculator

This is how many hours it takes for your pump/filter to circulate and filter the entire contents of your pool. Generally recommended as 4 hours for indoor pools and 6 hours for outdoor pools.

How Do I Size A Pool Filter In Australia?

To size a pool filter, you need the following information:

• Pool volume
• Desired turnover rate
• Preferred filter type
• Pump flow rate (if you have an existing pump)

Turnover Rate

The pool volume is pretty straightforward, but what is the desired turnover rate?

This is essentially a measure of how long it will take your filter to process an amount of water equivalent to one full volume of your pool. The smaller this number is, the larger your filter needs to be (as it needs to process the same amount of water in a shorter time).

The turnover rate is generally recommended as 4 hours for indoor pools and 6 hours for outdoor pools. A lower turnover rate is more conservative and will give you more spare capacity in your filter (useful if you have problems in your pool down the road).

Filter Type

There are three main filter types to choose from in Australia:

• Sand/glass media filters
• Cartridge filters
• Diatomaceous Earth (DE) filters

There are pros and cons to each of these filter types, which we won't discuss here. Hopefully by now you've selected your filter type!

Calculations

Now that you know the volume, turnover rate, your filter type, and maybe your pump flow rate too, you can calculate the necessary filter size.

Step 1 - Calculate Filter Flow Rate

To work out the necessary filter flow rate, divide the pool volume in litres by the turnover rate in minutes.

Filter Flow Rate = Pool Volume / Turnover Rate

For a 60,000 L pool, the calculation would look like this:

Filter Flow Rate = 60,000 L / (6 hrs x 60 mins/hr)
Filter Flow Rate = 60,000 L / 360 mins
Filter Flow Rate = 167 LPM (litres per minute)

In this example, we would need the filter to have a maximum flow rate of more than 167 LPM. Remember, the filter doesn't set the flow rate, the pump does, so we just need to ensure that the filter has the capacity to handle this flow.

Step 2 - Add Spare Capacity

It's often recommended to oversize your filter, so that you have plenty of spare capacity and can go longer between backwashing/cleaning.

For this reason, it's often worth adding an extra 10-20% on top of the previously calculated flow rate as a buffer.

Filter Flow Rate = 167 LPM x 1.2 = 200 LPM

Step 3 - Check Against Pump Flow Rate

After this calculation, the next step is to check whether our pump flow rate exceeds this filter flow rate.

If our pump had a typical flow rate of 220 LPM, selecting a filter with a maximum flow rate of 200 LPM would not be good.

In this case, the pump would force too much water through the filter, damaging it and causing channelling through the filter media, which reduces filter efficiency.

In this case, the pump flow rate is higher than our calculated filter flow rate based on volume/turnover, so we must set the filter maximum flow rate greater than this pump flow rate.

Note: If you haven't sized your pump yet, check out this pool pump size calculator.

Step 4 - Consider Filter Minimum Backwash Flow Rate

Finally, if we are sizing a sand/glass filter, we must consider the minimum backwash flow rate.

Not all filters specify this, but this is the minimum flow rate that the pump must put out to adequately backwash the sand or glass filter media. If the pump is too weak, it won't be able to backwash the filter properly.

Again, note your pump flow rate and when selecting a filter, ensure that the minimum backwash flow rate is less than your pump flow rate.

What Size Sand Filter For A 50,000 Litre Pool?

Based on the calculations outlined above, here's how to size a sand filter for a 50,000L pool. We'll use the standard 6 hour turnover rate for outdoor residential pools.

Filter Flow Rate = Volume / Turnover Time
Filter Flow Rate = 50,000 L / (6 hrs x 60 mins/hr)
Filter Flow Rate = 50,000 L / 360 mins
Filter Flow Rate = 139 LPM

Add on a 20% capacity safety margin.

Filter Flow Rate = 139 LPM x 1.2 = 167 LPM

In this case, you would need to choose a sand filter with a maximum flow rate of more than 167 LPM (litres per minute).

As it is a sand filter, you would need to also consider the minimum backwash flow rate, making sure that it is larger than your pump flow rate (if known).

What Size Cartridge Filter Do I Need?

The calculations for cartridge filters are the same as with sand and glass filters, you just don't need to consider the minimum backwash flow rate, as cartridge filters don't need backwashing.

Let's size a cartridge filter for a 80,000 L pool.

Filter Flow Rate = Volume / Turnover Time
Filter Flow Rate = 80,000 L / (6 hrs x 60 mins/hr)
Filter Flow Rate = 80,000 L / 360 mins
Filter Flow Rate = 222 LPM

Add on a 20% capacity safety margin.

Filter Flow Rate = 139 LPM x 1.2 = 267 LPM

For this pool, you would need a cartridge filter with a maximum flow rate of more than 267 LPM.

Summary

Have any questions about this pool filter size calculator or how it works? Found any errors or just have general feedback? Let me know over at the contact page!

If you need to size a heat pump, check out this pool heat pump size calculator!