What this calculator does
Molar concentration (mol/L) and mass concentration (mg/L) describe the same solution in two different currencies: one counts particles, the other counts weight. Converting between them needs one extra piece of information beyond the concentration itself: the molar mass of the dissolved substance, since a mole of a heavy molecule weighs far more than a mole of a light one at the same concentration.
This is a different calculation from a basic molarity calculator, which works out mol/L from a mass and a volume of solvent. Here you already have a concentration in one unit and want it expressed in the other, which is common when comparing a lab result in mol/L against a drinking-water or environmental guideline written in mg/L, or vice versa.
The formula
To go from mol/L to mg/L, multiply the molar concentration by the molar mass (in g/mol), then by 1,000 to convert grams to milligrams. To go from mg/L to mol/L, divide by the molar mass and by 1,000. For dilute aqueous solutions, 1 mg/L is also commonly treated as approximately 1 ppm, since a litre of dilute solution weighs very close to 1,000 grams.
| Term | Meaning |
|---|---|
| mol/L | Molar concentration: moles of solute per litre of solution. |
| mg/L | Mass concentration: milligrams of solute per litre of solution. |
| Molar mass | The mass of one mole of the substance, in grams per mole, found on a periodic table or a compound’s data sheet. |
| ppm | Parts per million, approximately equal to mg/L for dilute solutions in water. |
The inputs explained
| Field | What to enter |
|---|---|
| Direction | Choose whether you are converting mol/L to mg/L, or mg/L to mol/L. |
| Concentration | The concentration value to convert, in the starting unit selected above. |
| Molar mass of the substance (g/mol) | The molar mass of the dissolved substance, in grams per mole. |
When to use it
Comparing a lab result against a guideline
A water test might report a concentration in mol/L while a regulatory limit is written in mg/L. Converting one to match the other, using the correct molar mass, is needed before they can be compared directly.
Preparing a solution to a mass target
A recipe or protocol calling for a specific mg/L concentration can be converted to mol/L to match the units used on a stock reagent bottle, or the other way round.
Working with ppm in water chemistry
Because dilute aqueous solutions have a density close to water, mg/L and ppm are used almost interchangeably in that context, so the ppm figure shown alongside the mg/L result is usually a safe like-for-like reading.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What does a range of mol/L concentrations equal in mg/L, for a molar mass of 58.44 g/mol?
Common salt concentrations converted from molar to mass concentration.
| mol/L | Converted value |
|---|---|
| 0.001 | 58.44 mg/L |
| 0.01 | 584.40 mg/L |
| 0.05 | 2,922.00 mg/L |
| 0.1 | 5,844.00 mg/L |
| 0.5 | 29,220.00 mg/L |
| 1 | 58,440.00 mg/L |
What does a range of mg/L concentrations equal in mol/L, for a molar mass of 18.02 g/mol?
Mass concentrations converted back to molar concentration.
| mg/L | Converted value |
|---|---|
| 18.02 | 0.001000 mol/L |
| 90.1 | 0.005000 mol/L |
| 180.2 | 0.010000 mol/L |
| 901 | 0.050000 mol/L |
| 1802 | 0.100000 mol/L |
| 9010 | 0.500000 mol/L |
Questions
How do I convert mol/L to mg/L?
Multiply the mol/L value by the molar mass of the substance in g/mol, then multiply by 1,000 to convert grams to milligrams. You need the molar mass; without it, the two units cannot be converted.
Is mg/L the same as ppm?
For dilute solutions in water, mg/L and ppm are close enough to be treated as equal, because a litre of dilute solution weighs very close to 1,000 grams. This approximation breaks down for concentrated solutions or non-aqueous solvents.
Why do I need the molar mass to convert these units?
Molar concentration counts particles (moles) while mass concentration counts weight, and different substances have different weights per mole. Without the molar mass there is no way to translate a particle count into a weight.
Where do I find the molar mass of a substance?
Add up the atomic masses of every atom in its chemical formula using a periodic table, or look it up on the substance’s safety data sheet, which almost always states the molar mass directly.
To work out a molar concentration from a mass of solute and a solution volume in the first place, see the molarity calculator. For diluting a stock solution to a target concentration, see the solution dilution calculator.