What does PPM mean?

PPM stands for Parts Per Million. Those of us in the pool business often take acronyms for granted, as though everyone knows them. This article will explain PPM and why it matters.

Covered in this article:

Parts Per Million Definition

Parts per million? A million of what? Parts of what? PPM is the US standard unit of measurement in water chemistry. It tells us the density of a given substance dissolved in water. Examples include free chlorine, calcium hardness, and total alkalinity.
 
1 PPM means that substance is one-millionth of the total amount of water. It seems like a tiny amount–and it is–but water does not always need a lot of a given chemical. For example, just 1.0 ppm of free chlorine can be enough to keep a pool safe and disinfected. It may not be enough to keep the pool algae-free, but that's another topic for another article.
 
Where did the million come from? To find out, we did some research which led us to the metric system. PPM is the equivalent of milligrams per liter (mg/L). One milligram is one-millionth of a liter. The rest of the world already knows this because the metric system is standard and based on multiples of 10. For us Americans, we need to take the time to convert. Thankfully it's simple (unlike ºFahrenheit to ºCelsius).
1 cubic meter (m3) = 1,000 liters (L)
1 liter (L) = 1,000 milliliters (mL)
Then we transition from liquids into solids, because most chemicals are dissolved into water, not liquids themselves.
1 milliliter (mL) = 1,000 milligrams (mg)
1 liter (L) = 1,000,000 milligrams (mg)
So one milligram (mg) is one-millionth of one liter (L). That means mg/L is the metric equal to PPM.
1 mg/L = 1 PPM

You can check our math using free online conversion calculators like this.

Why is PPM important?

Any standard unit of measurement is important to understand. In pool chemistry, PPM (or its metric equivalent mg/L) is that standard. It tells us the amount–expressed as concentration (or density) in the water–of a given substance. Or, in the case of Total Alkalinity (TA), the sum of several substances.
 
Traditional water chemistry ranges use PPM for almost everything except pH, which has its own logarithmic scale:
 
pH scale, pH, pH chart

Ideal chemistry targets

Textbooks give ranges for ideal chemistry–measured mostly in PPM. Sources differ, but in general, those ranges look something like the chart below. These are from various textbooks and not Orenda's opinions. We'll explain in a moment.

  Acceptable Ideal
Total Alkalinity (TA) 80 - 120 ppm 90 - 110 ppm
Calcium Hardness (CH) 200 - 400 ppm 250 - 400 ppm
Cyanuric Acid (CYA) 30 - 100 ppm 30 - 50 ppm
Total Dissolved Solids (TDS) 1500 ppm over tap < 1500 ppm over tap
Salt (if applicable) 3000 - 3600 ppm 3000 - 3400 ppm
Free Chlorine (FC or FAC) 1 - 4 ppm 1 - 4 ppm
Total Chlorine (TC or TAC) 1 - 4 ppm 1 - 4 ppm
Combined Chlorine (CC) < 0.2 ppm < 0.2 ppm
Phosphate < 1500 ppb < 500 ppb

Chemistry ranges shown were compiled from multiple textbooks and online resources.

You may be thinking, "Wait, Orenda sometimes recommends chemistry outside of those ranges." You are correct; sometimes, we do. That's because the Langelier Saturation Index (LSI) reigns supreme. So, rather than focusing on ideal chemistry ranges, focus on the LSI. And for that, we have the Orenda App LSI calculator and textbook chemistry ranges to work with. Just remember to prioritize LSI first and range chemistry second.

PPM to PPB

Another exception is phosphate, which is usually measured in Parts Per Billion (PPB). One PPB is one-thousandth of one PPM. So phosphate concentration is hopefully below 1 PPM, which would be 1,000 PPB. So to convert PPM to PPB, multiply by 1,000, and vice versa.
1 PPM = 1,000 PPB
1 mg/L = 1,000 µg/L

Of course, if your phosphate level exceeds 1,000 ppb, you could use PPM. But generally, PPB is easier to use because phosphate levels should be kept below 0.5 ppm (or 500 ppb)

Conclusion

The standard unit of measurement in swimming pool chemistry is parts per million (PPM). PPM's metric equivalent is milligrams per liter (mg/L). One milligram is 1/1000th of a milliliter, and one milliliter is 1/1000th of a Liter. A thousand times a thousand equals one million, hence the term parts per million.

If you want to learn more about water chemistry and take control of your swimming pool, try our free online educational program Orenda Academy™.

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