Subscribe to our newsletter for $10 off your next order over $99!
Subscribe to our newsletter for $10 off your next order over $99!
Pool Water Testing: Free vs Total Chlorine, and Which Test Kit to Use
Quick answer: Test for free chlorine, not total chlorine — free chlorine is the active sanitiser doing the work, while total chlorine includes chlorine that's already spent. Basic test strips are fine for a quick check but don't separate free from combined chlorine reliably; a reagent-based (DPD) kit or digital photometer gives you the accurate free/total split you need to dose correctly.
What to Actually Test
Free chlorine. The active sanitiser available to kill bacteria and algae right now. This is the number that should sit in your target range — typically 1–3 ppm for most home pools.
Total chlorine. Free chlorine plus combined chlorine (chloramines) — chlorine that's already reacted with contaminants and stopped being useful. On its own, total chlorine tells you little; it only becomes useful alongside a free chlorine reading.
Combined chlorine (the gap). Subtract free from total to get combined chlorine. Above about 0.5 ppm, this is usually the source of a strong "chlorine smell" and a sign you need a shock dose, not less chlorine.
pH. Affects how effective your chlorine actually is and swimmer comfort. Most home pools target roughly 7.2–7.6.
Total alkalinity. Buffers pH against swings; low alkalinity makes pH unstable and hard to hold in range.
Stabiliser (cyanuric acid, CYA). Protects chlorine from UV breakdown, but high CYA reduces how effective a given chlorine reading actually is — worth checking periodically, not just when something seems off.
Choosing a Test Kit
| Kit type | Accuracy | Speed | Best for |
|---|---|---|---|
| Test strips | Lowest — approximate, hard to read precisely | Fastest | Quick between-test checks, not primary dosing decisions |
| Liquid reagent (DPD) kit | Good — clear colour-match free/total split | A few minutes | Most home pool owners' primary test method |
| Digital photometer | Highest — removes colour-matching guesswork | A few minutes | Owners who want precise, repeatable readings, or salt pools with a chlorinator to calibrate against |
A reagent kit that separates free and total chlorine is the practical minimum for making real dosing decisions — strips alone make it easy to miss a growing combined chlorine problem, since most strips read total chlorine only or blend the two.
How Often to Test
Two to three times a week in summer, weekly in cooler months, and always after heavy rain, a pool party, or visible cloudiness. If you run a salt chlorinator, test salt level periodically too — it isn't fixed and drops from splash-out, backwashing, and rain dilution.
Frequently Asked Questions
Should I test free chlorine or total chlorine?
Free chlorine is the number that matters for dosing and safety. Total chlorine is only useful alongside it, to calculate combined chlorine (the gap between the two). If your free chlorine is in range but total chlorine is significantly higher, you likely have a combined chlorine problem that needs a shock dose.
Are pool test strips accurate enough?
They're fine for a fast sanity check, but their imprecise colour-matching and inability to reliably separate free from total chlorine make them a poor primary tool for real dosing decisions. Use a reagent (DPD) kit or photometer for accurate readings.
Why does my total chlorine reading not match my free chlorine reading?
The difference is combined chlorine — chlorine that's already reacted with contaminants and is no longer active. A gap above about 0.5 ppm usually means it's time for a shock dose. A strong "chlorine smell" is often a sign of high combined chlorine, not too much chlorine.
Do I need a photometer, or is a reagent kit enough?
A reagent kit is enough for most home pools. A photometer is worth it if you want precise, repeatable numbers — particularly useful for calibrating a salt chlorinator's output against an accurate reading, or if you're managing a larger or commercial pool.
What is the ideal free chlorine level for a pool?
For most residential pools, free chlorine should sit between 1–3 ppm. Salt chlorinated pools often run at the lower end of this range. If your stabiliser (CYA) level is high, you may need to maintain a higher free chlorine level to achieve the same sanitising effect.