Parts per million
Solute density
lb/cu ft
Milligrams per liter
Solute molar mass
Molar concentration

If you have ever wondered how to convert from PPM to mg/L, this calculator is the tool for you! It will help you convert between both of these ratios for the solution of any substance in water. It also allows you to find the molar concentration of the solution. If you are not sure what PPM or mg/L means, it's high time you found out!

What are PPM and mg/L?

PPM and mg/L are two different measures of substance concentration.

  • PPM, or parts per million, is the number of parts of a substance in a million parts of the solution or mixture. For example, let's assume you want to measure the salinity of water. PPM is the number of salt parts per a million parts of the entire solution, both water and salt.
  • mg/L, or milligrams per liter, is a measure of concentration. It states how many milligrams of the substance can be found in a liter of the solution or mixture.

Converting between PPM and mg/L

The relation between PPM and mg/L is dependent on the solute density. Imagine that you add 10 grams of a substance to a liter of water. If the substance is dense, like oil, it will have a smaller volume - and consequently, the solution will have a smaller PPM ratio. For substances of low density, for example alcohol, the PPM ratio will be much higher, even though the mg/L ratio stays constant.

For converting PPM to mg/L, follow these steps:

  1. Choose the solute - is the substance diluted with water, acetone or something else? Let's choose oil with the density equal to 920 kg/m³.
  2. Determine the PPM value for your solution. Let's say you have created a solution with 1230 PPM of oil.
  3. Use the following formula to find the mg/L ratio:

milligrams per liter = PPM * density / 1000

  1. In this case,

milligrams per liter = 1230 * 920 / 1000 = 1131.6 mg/L

It means that 1230 PPM is equivalent to 1131.6 mg/L of oil in water.

Special case: water

As you may know, the density of water is equal to 1000 kg/m³. It means that every cubic meter of water weighs 1000 kg. Let's recalculate the units:

1000 kg/m³ = 1,000,000 g/m³ = 1,000,000,000 mg/m³ = 1,000,000 mg/dm³ = 1,000,000 mg/L

That means that every liter of water contains exactly a million milligrams of water. That means that if the substance dissolved in water has a density that is equal to or almost equal to the density of water, you can assume that 1 PPM = 1 mg/L.

Remember that this equality is valid only for very specific conditions - pure water at a standard pressure and temperature.

Calculating the molar concentration

If you already know the mg/L ratio of your solution, you can also use this PPM to mg/L converter to calculate the molarity. This parameter describes the number of moles in a liter of the solution and is expressed in molars (1 M = mol/L).

To find the molarity, you need to know an additional parameter - the molar mass of your solute (substance dissolved in water). It is expressed in grams per mole. Use the following formula:

molarity = milligrams per liter / (molar mass * 1000)

For example, oil has a molar mass equal to 900 g/mol. We can use the previously calculated mg/L ration to determine the molarity:

molarity = 1131.6 / (900 * 1000) = 0.00126 M

Bogna Haponiuk.

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