1. If you have measured or you know the TDS value of the water in your pool, enter the value in the field Current TDS

 

2. If not, the calculator estimates TDS by of the age of water. Enter "0" in the field Current TDS and then enter age of water in the field Water age

 

The TDS affects the ISL, but you only need to test for the water TDS if you have a salt pool or the water is very old. Otherwise, enter the approximate age of the water and don't worry.