PPM Calculator: ppm to mg/L, Percent, ppb and Molarity

Parts per million and percent are the same kind of number, a ratio, counted out of a million in one case and out of a hundred in the other. The calculator opens on ppm conversions: one ppm value in, and mg/L, percent, ppb and, if you give a molar mass, molarity out. Two more modes turn a percentage into a weighing or measuring recipe.

How to use the calculator

The panel, "PPM, percent and molarity converter", opens on the first option under "Type": ppm conversions.

1. Type the concentration in the box marked ppm (mg/L). Large values can include thousands separators, such as 20,000.
2. Leave the molar mass (MW) box filled in if you want molarity, or clear it if you only need mg/L, percent and ppb. It opens at 58.44 g/mol, sodium chloride.
3. Read the results: mg/L, percent, ppb and the molarity in the most readable unit. The formula line underneath spells out each conversion.

The two percent modes

• % w/v, for a solid dissolved in a liquid. Enter the percentage and the final volume in mL, plus a molar mass if you want the molar concentration as well. The result reads "Dissolve ... g and bring to ... mL".
• % v/v, for one liquid diluted in another. Enter the percentage and the final volume. The result reads "Measure ... mL of the liquid and make up to ... mL".

The two percent modes share the same boxes, so your numbers carry over when you switch between them.

The formulas

Worked example with the default values

The calculator opens with 100 ppm and a molar mass of 58.44 g/mol (sodium chloride).

1. mg/L: 100 ppm in water is about 100 mg/L.
2. Percent: 100 ÷ 10,000 = 0.01 percent.
3. ppb: 100 × 1,000 = 100,000 ppb.
4. Molarity: 100 mg/L is 0.1 g/L, and 0.1 ÷ 58.44 = 0.001711 mol/L, or 1.711 mM.

The tool shows "100 ppm ≈ 100 mg/L (dilute water solutions)", then "= 0.01 % = 100000 ppb" and "= 1.711 mM for MW 58.44 g/mol".

The percent modes with their defaults

Switch to % w/v and the boxes hold 0.9 percent, 500 mL and 58.44 g/mol. Grams: 0.9 ÷ 100 × 500 = 4.5 g. Molarity: 0.9 × 10 = 9 g/L, and 9 ÷ 58.44 = 0.154 mol/L. The tool shows "Dissolve 4.5 g and bring to 500 mL" and "Equals 154 mM for MW 58.44 g/mol". That is normal saline, the same 154 mM the osmolarity calculator starts from. The % v/v mode reuses 0.9 percent and 500 mL to give "Measure 4.5 mL of the liquid and make up to 500 mL". A more familiar case: 70 percent v/v ethanol in a 500 mL bottle is 350 mL of ethanol made up to 500 mL with water.

Going the other way

For the reverse direction, run the same factors backwards:

When ppm equals mg/L, and when it does not

Strictly, ppm by mass is milligrams per kilogram. A liter of water weighs about a kilogram, so for drinking water, culture media and dilute standards, mg/L and ppm are interchangeable, which is why the tool marks that line with ≈. The shortcut fails when the liquid is far from 1 g/mL. Ethanol is about 0.79 g/mL, so 1 mg/L in ethanol is roughly 1.27 ppm by mass. For gases, ppm means parts by volume or by moles (carbon dioxide in air is roughly 420 ppm), which is not mg/L at all, so this tool does not apply.

Common mistakes

• Dividing ppm by 1,000 to get percent. That gives parts per thousand. Divide by 10,000.
• Mixing up % w/v and % w/w. 1% w/v is 1 g per 100 mL of solution; 1% w/w is 1 g per 100 g of solution.
• Adding the solvent instead of making up to volume. Ethanol and water shrink when mixed, so 70 mL of ethanol plus 30 mL of water gives less than 100 mL.
• Using the wrong molar mass for an ion. A water report giving 100 ppm of sodium means sodium ions, molar mass 22.99, not sodium chloride. For a formula you need the weight of, use the molar mass calculator.

FAQ

How do you convert ppm to mg/L?

For water and other dilute aqueous solutions, 1 ppm equals 1 mg/L, so the number stays the same. This works because a liter of water weighs about one kilogram. For liquids much lighter or heavier than water, divide mg/L by the density in g/mL to get ppm by mass.

How do you convert ppm to percent?

Divide the ppm value by 10,000. For example, 2,500 ppm is 0.25 percent. To go from percent to ppm, multiply by 10,000.

How many ppb are in 1 ppm?

One ppm equals 1,000 ppb. To convert ppm to ppb, multiply by 1,000, so 0.05 ppm is 50 ppb.

How do you convert ppm to molarity?

Treat ppm as mg/L, divide by 1,000 to get g/L, then divide by the molar mass. For 100 ppm sodium chloride: 0.1 g/L ÷ 58.44 g/mol = 0.001711 mol/L, or 1.711 mM. The calculator's ppm conversions mode shows this line whenever the molar mass box is filled in.

What is the difference between % w/v and % v/v?

Percent w/v is grams of solute per 100 mL of solution, used for solids such as salts and sugars. Percent v/v is milliliters of a liquid per 100 mL of solution, used for liquids such as ethanol or glycerol.