Pool, Spa & Water Treatment Chemistry worked example

Free Chlorine Adjustment with target free chlorine of 2.5 ppm: a worked example

This worked example runs the free chlorine adjustment numbers for a tougher week than the baseline: target free chlorine of 2.5 ppm instead of the typical 5 ppm. Calculate the free-chlorine gap between a target residual and current test after planned treatment credits.

The inputs for this scenario

  • Target free chlorine: 2.5 ppm (the input this scenario stresses; the baseline uses 5)
  • Current free chlorine reading: 2 ppm (held at the documented default)
  • Chlorine credit from scheduled dose: 0 ppm (held at the documented default)
  • Free-chlorine reserve above target: 0 ppm (held at the documented default)

Working through the calculation

  • The calculation starts from the formula this tool documents: Remaining chlorine gap = target free chlorine - current free chlorine - planned credit + reserve.
  • Remaining free-chlorine gap works out to 0.5 ppm at these inputs, and this is the headline figure for the scenario.
  • Current and planned chlorine works out to 2 value at these inputs.
  • Target free chlorine works out to 2.5 value at these inputs.
  • Utilization works out to 20 % at these inputs.

How this compares with the baseline

  • Against the tool's baseline example, where target free chlorine sits at 5 ppm and the headline result is 3 ppm, this scenario comes in 83.33% below the baseline at 0.5 ppm.
  • Use it at each service stop after a fresh DPD or FAS-DPD test, before mixing or metering the day's chlorine dose. 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

  • Remaining free-chlorine gap: 0.5 ppm (headline result)
  • Current and planned chlorine: 2 value
  • Target free chlorine: 2.5 value
  • Utilization: 20 %

Run it with your numbers

  • To rerun this with your own numbers, open the live Free Chlorine Adjustment calculator, set target free chlorine to your actual value, and adjust the remaining inputs to match your operation.

Last reviewed 2026-05-12.