ORP (oxidation-reduction potential) is a measurement in millivolts (mV) that shows how actively pool water is being sanitised. A higher ORP means the sanitiser is working harder against contaminants. Many pool professionals aim for roughly 650–750 mV.
What ORP actually measures
Chlorine and other sanitisers work by oxidising – breaking down bacteria, organic material and other contaminants. ORP measures the water’s overall ability to do that, as an electrical potential in millivolts. It does not tell you how much chlorine you added; it tells you how effective the water is at sanitising right now.
Why ORP and chlorine are not the same thing
Two pools can hold the same chlorine level and still have very different ORP. The main reason is pH: chlorine is far more active at a pH around 7.2 than at 7.8. Cyanuric acid (stabiliser), temperature and the amount of organic load also play a role. That is why ORP is a useful second opinion next to a chlorine test.
What is a good ORP value?
- Below about 650 mV: sanitising may be too weak – check chlorine and pH.
- About 650–750 mV: a common target range for well-sanitised pool water.
- Well above 750 mV: usually more sanitiser than needed, which costs money and can irritate swimmers.
Exact targets depend on local rules, the sanitiser used and the type of pool, so always follow the guidance that applies to your installation.
Why continuous ORP monitoring helps
ORP can change within hours – after a hot day, a busy afternoon or heavy rain. A manual test once a week misses most of that. An inline monitor such as EVVO Sense measures ORP together with pH and temperature around the clock, and sends an alert when the value leaves its range.
Frequently asked questions
What does ORP stand for?
Oxidation-reduction potential. It is measured in millivolts (mV) and shows how actively the water can oxidise – and so sanitise – contaminants.
Is a higher ORP always better?
No. Too low means weak sanitising; far too high usually means more chemicals than needed. Most pools aim for a range of about 650–750 mV.
Does pH affect ORP?
Yes. Chlorine is much more active at lower pH, so the same chlorine level gives a higher ORP at pH 7.2 than at pH 7.8.
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