Concentration Converter — mg/L, molarity, ppm and percent

The concentration converter above keeps three different families apart: mass concentration, molar concentration and mass fraction. Most online converters blur them together and quietly assume a density of one gram per millilitre and a molar mass you never entered. That produces confident-looking numbers that are wrong for anything other than a dilute aqueous solution of whatever compound the author had in mind.

Arb Digital publishes this converter as part of a free technical reference library. It asks for the molar mass explicitly, refuses to guess it, and states clearly which conversions are exact and which carry an assumption you need to check for your own system.

What This Concentration Converter Does

Mass concentration mode covers kilograms per cubic metre, grams and milligrams per litre, micrograms and nanograms per litre, milligrams per decilitre, grams per hundred millilitres, milligrams and micrograms per millilitre, and the imperial units still used in water treatment: pounds per cubic foot, ounces per US gallon and grains per US gallon.

Molar concentration mode covers moles per cubic metre, molar and its submultiples down to picomolar, and moles or millimoles per millilitre. Mass fraction mode covers percent, per mille, parts per million, billion and trillion, and the mass-per-mass forms such as milligrams per kilogram. The four supporting panels always show the SI unit, the common laboratory unit, a submultiple, and one related quantity computed with your molar mass.

How to Use It

1. Pick the family. If your unit has a mass over a volume it is mass concentration. If it has moles it is molar concentration. If it is a percentage or a parts-per figure it is a fraction.
2. Enter the value and choose your from and to units. The lists are filtered so you cannot accidentally convert a percentage into a molarity without going through the molar mass step.
3. Enter the molar mass of the solute in grams per mole. This is the number that links mass to moles, and without it the mass-to-molar conversion has no defined answer.
4. Read the fourth panel for the cross-family value, which shows millimolar when you are working in mass units and milligrams per litre when you are working in molar units.
5. Use Swap to confirm the inverse conversion returns your original figure.

The Formula and How It Is Calculated

Within a family, conversion is a straightforward multiplier chain: result = value × factor(from) ÷ factor(to), routed through the SI unit for that family. Mass concentration uses kilograms per cubic metre, which is numerically identical to grams per litre, a convenience worth remembering. Molar concentration uses moles per cubic metre, which is numerically identical to millimoles per litre. Mass fraction is dimensionless, with one representing the whole, so a percent is 0.01 and a part per million is 10⁻⁶.

Crossing between mass and molar concentration uses the definition of the mole: c_molar = c_mass ÷ M, where M is the molar mass in grams per mole. Working in the convenient units, grams per litre divided by grams per mole gives moles per litre directly. The mole itself is defined by fixing the Avogadro constant at exactly 6.02214076 × 10²³ per mole, as set out in the BIPM SI Brochure. Terminology for concentration quantities follows the IUPAC Compendium of Chemical Terminology, and unit style guidance comes from NIST Special Publication 811.

Why Molarity Cannot Be Guessed From mg/L

This is the point where most concentration tools fail silently. Milligrams per litre counts mass. Millimolar counts particles. Converting between them requires knowing how much mass one mole of the specific substance has, and that number is different for every compound.

Ten milligrams per litre of sodium chloride, molar mass 58.44, is 0.171 millimolar. The same ten milligrams per litre of glucose, molar mass 180.156, is only 0.0555 millimolar, roughly three times fewer particles for the same mass. For a protein with a molar mass of 66,000 grams per mole, ten milligrams per litre is 0.15 micromolar, four orders of magnitude away. Any of these could be described as "10 mg/L", and no conversion tool can resolve which molarity is meant without being told the substance. That is why this converter asks. If you know the chemical formula but not the molar mass, our molar mass converter will calculate it from standard atomic weights.

Parts Per Million Is Not Automatically Milligrams Per Litre

In dilute aqueous solutions, one part per million by mass is conventionally treated as one milligram per litre. That works because one litre of dilute water solution has a mass very close to one kilogram, so one milligram in one kilogram is one part per million. It is an approximation resting on a density assumption, not an identity.

The approximation breaks in three common situations. In concentrated brines or syrups the solution density can exceed 1.2 g/mL, making the mass-based ppm and the volume-based mg/L differ by more than 20%. In organic solvents the density may be 0.79 or 1.6 g/mL, and the two diverge immediately. And in gas-phase work, parts per million almost always means parts per million by volume, which is a mole fraction and relates to a mass concentration through the ideal gas law rather than through density alone. Because of this, mass fraction and mass concentration are kept in separate modes here, and the density needed to link them can be converted in the density converter.

Percent Solutions: w/w, w/v and v/v Are Three Different Things

A bottle labelled "5% solution" is ambiguous unless the basis is stated. Weight per weight means 5 grams of solute in 100 grams of final solution. Weight per volume means 5 grams in 100 millilitres of final solution. Volume per volume means 5 millilitres in 100 millilitres, and is used for liquid solutes such as ethanol or acids.

For water at room temperature, w/w and w/v are nearly the same because density is close to 1 g/mL. For sulfuric acid at 1.84 g/mL they differ by 84%, which is a difference no laboratory can afford to ignore. Volume per volume adds a further complication: mixing volumes are not always additive, and combining 50 mL of ethanol with 50 mL of water produces about 96 mL rather than 100 mL because the molecules pack more tightly together. This converter treats percent as a mass fraction, w/w, and leaves w/v to the mass concentration mode as grams per hundred millilitres, which is the unambiguous way to express it. Volumes convert in the volume converter and masses in the weight converter.

Molarity, Molality and Normality Are Not the Same

Three similar-sounding quantities cause persistent confusion. Molarity is moles of solute per litre of solution. Molality is moles of solute per kilogram of solvent, not solution, and is used in colligative property work precisely because it does not change with temperature. Normality is moles of reactive equivalents per litre, so a 1 M solution of sulfuric acid is 2 N because each molecule can donate two protons.

The practical consequences are real. A molar solution prepared at 20 °C is slightly less concentrated at 60 °C because the solution has expanded, while the molal equivalent is unchanged. And a titration calculation done in molarity without accounting for the equivalence factor will be off by exactly the number of equivalents per mole. This converter works in molarity, the volume-based quantity, so keep the temperature of preparation in mind for precise work and convert it in the temperature converter if needed.

Full Concentration Conversion Table

Common Mistakes to Avoid

• Converting mg/L to molarity without a molar mass — there is no universal factor, and the same mass concentration can span four orders of magnitude in molarity depending on the solute.
• Treating ppm and mg/L as identical — the equivalence relies on the solution having a density near 1 g/mL and fails for brines, syrups and organic solvents.
• Leaving a percent solution unlabelled — w/w, w/v and v/v give different amounts of solute, dramatically so for dense liquids.
• Confusing molarity with molality or normality — one is per litre of solution, one per kilogram of solvent, and one counts reactive equivalents rather than molecules.
• Assuming volumes add — mixing 50 mL of ethanol with 50 mL of water gives about 96 mL, so v/v percentages need care.

Related Free Tools From Arb Digital

Concentration work overlaps several unit families. Calculate a molar mass from a chemical formula with the molar mass converter, convert solution volumes in the volume converter, solute mass in the weight converter, and solution density in the density converter. Preparation temperature converts in the temperature converter, clinical glucose units in the blood sugar converter, and the general unit converter covers the rest.

Frequently Asked Questions

This tool converts units only. It is not clinical, laboratory, or environmental compliance guidance, and any concentration used for diagnosis, dosing, or regulatory reporting must be confirmed against the applicable method and reference range.