Pool, Spa & Water Treatment Chemistry worked example

Filter Loading Rate with system flow through filter of 33 gpm: a worked example

This worked example runs the filter loading rate numbers for a tougher week than the baseline: system flow through filter of 33 gpm instead of the typical 65 gpm. Calculate filter loading rate from system flow and effective filter area.

The inputs for this scenario

System flow through filter: 33 gpm (the input this scenario stresses; the baseline uses 65)
Effective filter area: 300 ft2 (held at the documented default)
Available filter area factor: 1 x (held at the documented default)

Working through the calculation

The calculation starts from the formula this tool documents: Filter loading rate = system flow / effective filter area / available area factor.
Ratio works out to 0.11 gpm / ft2 at these inputs, and this is the headline figure for the scenario.
Raw ratio works out to 0.11 value at these inputs.
Available area factor works out to 1 x at these inputs.
Effective filter area works out to 300 value at these inputs.

How this compares with the baseline

Against the tool's baseline example, where system flow through filter sits at 65 gpm and the headline result is 0.22 gpm / ft2, this scenario comes in 49.23% below the baseline at 0.11 gpm / ft2.
Use it when commissioning a filter, diagnosing poor clarity or short filter cycles, or verifying that flow stays within the media's rated loading range. A result at this level usually justifies acting on the stressed input before touching anything else, because every other figure in the table is downstream of it.

Results at a glance

Ratio: 0.11 gpm / ft2 (headline result)
Raw ratio: 0.11 value
Available area factor: 1 x
Effective filter area: 300 value

Run it with your numbers

To rerun this with your own numbers, open the live Filter Loading Rate calculator, set system flow through filter to your actual value, and adjust the remaining inputs to match your operation.

Last reviewed 2026-05-12.