A DPD1 test shows how much free chlorine is in the water, but not how much of it actually disinfects. This tool closes that gap: from the pH value, free chlorine and cyanuric acid it calculates the share of active chlorine (HOCl) and determines the right dosages for your own pool.
Why the DPD1 reading alone is not enough
The 0.3–0.6 mg/l of free chlorine adopted from the DIN standard for public swimming pools applies to a pH of 7.0, water free of cyanuric acid and chlorination that is kept constant(!). In private pools these conditions are rarely met: the pH is often around 7.3–7.4, and long-term or multi-tabs build up cyanuric acid over the course of the season. Both significantly reduce the share of active chlorine. The consequences are algae growth, cloudy water, the typical swimming pool smell and skin and eye irritation, even though the water values appear to be set perfectly.
Only the part of the free chlorine that is present in the pool water as hypochlorous acid (HOCl) has a disinfecting effect. At elevated pH and in the presence of cyanuric acid, however, the DPD1 test records the total available chlorine, not the share that is actually effective. The goal should therefore be to keep active chlorine at around 0.2–0.5 mg/l. This is exactly what a redox-based pool controller does, too. Further information
Our calculators at a glance
- Active chlorine: uses the pH value, free chlorine (DPD1), cyanuric acid and temperature to determine how much HOCl (the actual disinfectant) is really available in your pool water.
- Chlorine dosage: uses the pool volume to calculate how much chlorine is needed to raise free chlorine or active chlorine (HOCl) to your desired target value. Whether for continuous disinfection or shock chlorination.
- pH− dosage / pH+: taking the carbonate hardness (alkalinity / TA) into account, calculates the amount of the respective pH adjuster needed to bring the pH into the recommended range of about 7.0–7.4. Since carbonate hardness determines how strongly an adjuster works, the result often differs considerably from the manufacturer's information.
- Alka+ dosage (TAC): uses the pool volume to calculate how much sodium bicarbonate is needed to raise the carbonate hardness (alkalinity / TA) to a target value.
Statistics, import and export – with a free user account
With a free user account you keep a digital water log for your pool: all readings in one place, and you can see at a glance how chlorine, pH and other values develop over days and weeks.
- Save automatically: every calculation can go straight into your statistics as a reading – no need to type anything twice.
- Table view: all readings with date and time at a glance. You can add, correct, hide or delete readings and choose which values appear as columns. The app calculates active and combined chlorine automatically. If you like, it also estimates missing values from the readings before and after.
- Chart view: shows you the trend of up to four values at once, e.g. free chlorine, active chlorine, pH and cyanuric acid. This shows you when to top up and how the water reacts to a dose.
- Pool log: one card per day with your readings, your dosings, actions (e.g. backwash, top-up with amount of water, covering) and notes. Amounts calculated in the dosing calculators are added to your pool log with one click in the results. The search filters all days with a particular product, an action or a keyword in your notes, and you can see the totals for the selected period.
- Import data: if you measure with a photometer, you don't have to type in the values. Readings from the Water-I.D.® LabCom Cloud and from the Lovibond® PoolAssistant app can be imported. Each further import only adds new readings.
- Export data: download the selected period as a CSV file, e.g. to work with it in Excel or keep it on file.
- Try it first: with demo data from a typical private pool you can test the table and chart before you have your own readings.
All you need for your user account is an email address and a password. You can delete it at any time. All calculators also work without a user account – your browser then remembers your most recently entered readings or, for example, the pool volume.
The calculated values for active chlorine should be regarded as accurate to ±10 %. The greatest inaccuracy in such calculations comes from the measuring accuracy of the photometer you used to obtain the data. After each dose, the water should be measured again once it has circulated sufficiently.