by poolman » Thu May 15, 2014 11:32 pm
It's a lot simpler than that.
You have 4 parts per million. That means that if your pool held 1,000,000 gallons, it would contain 4 gallons of chlorine at 4ppm.
If it held one-tenth as much - 100,000 gallons - it would contain 0.4 gallons of chlorine.
If it held just one-tenth of that - 10,000 gallons, which is what you have - it would contain 0.04 gallons of chlorine.