Dichlor and Cyanuric Acid


Jun 29, 2010
I have read that Dichor ( dichloro-s-triazinetrione dihydrate) is about 57% cyanuric acid. I have only 3700 gallons of water and started using dichlor yesterday. Last night , using the calculator, I took my pool to 3ppm FC. The PH was 7.5. Water is beautiful. I used 2.5 oz by volume to get it to the 3ppm. My CYA was 0. I tested this morning and the FC was 2ppm, PH 7.6. This evening I tested again. FC .5 PH 7.5. I added 3 oz Dichlor by volume to take it to 4 ppm. I waited a few hours and decided to test CYA for the first time. I also tested FC 5 ppm PH 7.6 CYA 0!! I don't understand why I have not built up any CYA. I am using an HTH 6 way tester. The water I put into the clear tube with the black dot on the bottom was crystal clear. I know my test kit is junk. I know I need the whiz bang TF tester but it cant be 100% wrong. How much Dichlor would I have to feed a 3700 gallon pool to get a CYA of 40?


TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
Silver Spring, MD
The CYA test won't read CYA levels below 20 (or 30) at all. It also isn't precise enough to distinguish a couple of ppm of CYA added or removed.

Dichlor adds 0.9 ppm of CYA for every 1.0 ppm of chlorine.

You can use the Pool Calculator to figure out quantities.


TFP Expert
LifeTime Supporter
In The Industry
May 20, 2007
SWSuburban Chicago, IL
It will take you between 11-14 days using the same amount of dichlor each day to reach a CYA of around 40.
The "Effects of Adding Chemicals" section of the Pool Calculator tells you how to calculate this.
2 days of dichlor isn't even enough to reach a CYA of 10... the test vial only goes to 30 so you won't show up as having any for quite a while.

Please keep all your pool testing related questions on one thread rather than start a new one. Thanks :)

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