The metric system is based on multiples of 10, so it’s easy to convert from parts per million (ppm) to milligrams per litre (mg/L) or millilitres per litre (mL/L).
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.
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 litre (mg/L). One milligram is one-millionth of a litre. The rest of the world already knows this because the metric system is standard and based on multiples
Why is PPM important?
The term ppm (parts per million) is a concentration unit that indicates the number of parts of a given substance mixed into one million parts of whatever it’s mixed into.
In pool chemistry, PPM in water (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.
To help visualize what PPM is, think about coffee. For example, if you have 3 tablespoons of sugar in your coffee and it makes 5 cups at 8 oz each, you have 24 tablespoons total or 24 x 4 = 96 tablespoons of coffee with sugar dissolved into it. Now suppose you have 2 tablespoons of sugar in your coffee and it makes 5 cups at 8 oz each again: now we have only 12 tablespoons total dissolved into our 5 cups of coffee! So we say that there are 2 tbsp/5gal or 0.4 tbsp/gal or 40ppm dissolved sugar in our 5 gallons (19 litres) of coffee!
In pool terms: If you add 1 gallon (3.8 litres)
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. We’ll explain in a moment.
You may be thinking, “Wait, sometimes I recommend 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 App LSI calculator and textbook chemistry ranges to work with. Just remember to prioritize LSI first and range chemistry second.
PPM to PPB
Phosphate can also be measured in Parts Per Million (PPM). One PPM is one-thousandth of one percent by weight or volume. If your pond has 1 PPM total phosphate, then there are 1000 ppb total phosphate. Another exception is phosphate, which is usually measured in Parts Per Billion (PPB). One PPB is one-thousandth of one PPM in water RO. 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
The standard unit of measurement in swimming pool chemistry is parts per million (PPM). PPM’s metric equivalent is milligrams per litre (mg/L). One milligram is 1/1000th of a millilitre, and one millilitre is 1/1000th of a Litre. A thousand times a thousand equals one million, hence the term parts per million.
The usual units used in chemistry are milligrams per kilogram (mg/kg), or grams per kilogram (g/kg) but these are not used in swimming pool chemistry because they would be difficult to measure accurately with most common laboratory equipment.