How pool filter size is worked out

Turnover Rate & Flow Requirements

The turnover rate defines how many times the entire pool volume passes through the filter per day. Australian guidance commonly uses 1 complete turnover every 4 to 6 hours for residential pools, which is about 4 to 6 turnovers per day. Required flow rate (L/min) = Pool Volume (L) ÷ Turnover Period (min) This flow rate must be achievable within the filter's rated capacity without exceeding maximum design velocity through the media bed.

Filter Media Hydraulics

Different filter media types have fundamentally different hydraulic characteristics: Sand filters: Operate at design flow rates of 40 to 50 m³/m²/h through the media bed. Filtration occurs in the top 150 to 200mm of the sand bed where particles are trapped in interstitial spaces. Effective particle removal down to 20 to 40 microns. Cartridge filters: Pleated polyester media provides large surface area in compact housings. Design flow rate of 1.5 to 2.5 m³/m²/h of media surface area. Removes particles down to 10 to 20 microns. D.E. (Diatomaceous Earth) filters: Finest filtration at 2 to 5 microns. Operates at 2.5 to 5.0 m³/m²/h through the septum-supported D.E. cake.

• Sand: 40 to 50 m³/m²/h design velocity, 20 to 40 micron removal
• Cartridge: 1.5 to 2.5 m³/m²/h, 10 to 20 micron removal
• Glass media: Similar hydraulics to sand but 5 to 15 micron removal
• D.E.: 2.5 to 5.0 m³/m²/h, 2 to 5 micron removal (finest available)

Pressure Drop & System Curves

As a filter loads with captured debris, the pressure differential across the media increases. Our model calculates the clean-filter pressure drop using the Ergun equation (for granular media) or manufacturer-specific curves (for cartridge/DE), then estimates the operating pressure range throughout a typical filter cycle. The point where the pump curve and system curve meet gives the likely flow rate. That matters because an undersized filter can reduce flow and increase pressure. An undersized filter creates excessive backpressure, reducing pump flow and increasing energy consumption.

Sizing Recommendation Logic

Our algorithm selects the minimum filter size that satisfies all constraints simultaneously: 1. Adequate filtration area for the pump's maximum flow rate 2. Design velocity within manufacturer specifications 3. Sufficient dirt-holding capacity for the expected debris load 4. Clean-filter pressure drop compatible with available pump head 5. Backwash flow rate achievable with existing plumbing Where the system allows it, choosing a filter above the calculated minimum can reduce pressure and lengthen cleaning intervals.