As pool service guys, we use the term parts per million several times a day when checking the chemical parameters of a swimming pool. It’s the unit of measure that we use for the amount of chlorine, alkalinity, salt, TDS, cyanuric acid, and so on.
So what exactly is PPM? It’s a ratio; such as 1 percent is a ratio of one in a hundred. PPM is a ratio of one in a million. We use this ratio to tell us the amount of a chemical in the pool compared to the amount of water we have in the pool. It’s a weight ratio: one pound of chemical per 1 million pounds of water.
However, we do not use pounds to measure water; we use gallons. One gallon of water weighs about 8.33 pounds, so 1 million pounds of water is 120,000 gallons. In the pool industry, one part per million represents one pound of chemical per 120,000 gallons of water.
It just so happens that one gallon of chlorine, 12% trade grade (pool grade bleach, the stuff that you buy at a distributor) contains one pound of chlorine. So if you poured it in a 120,000-gallon pool, you just put in 1 PPM. In a 60,000-gallon pool that’s 2 PPM; a 30,000-gallon pool, 4 PPM and so on. The general formula is: pounds of chemical x 120,000 / gallons in the pool
The same math applies to the addition of salt and cyanuric acid. For example: a 40-pound bag of salt in a 120,000-gallon pool is 40 PPM, in a 15,000-gallon pool it is 320 PPM.
When adding other chemical compounds, such as dicchlor, trichlor, bicarb and such, you must factor in the active ingredient of the compound. Here’s a link to a general formula for the commonly used pool chemicals.