Concentration Converter (mol/L, g/L, ppm, ppb)

Understanding the concentration converter — concept and principle

Concentration describes how much of a substance is dissolved in a given volume of solution. Chemistry uses moles per liter, water testing reports mg/L or ppm, and medicine and food labels use mmol/L or percent. This converter takes one of these values together with the molar mass of the solute and shows the same concentration as g/L, mg/L, ppm, ppb, mol/L, mmol/L and % w/v.

Grams per litre become molecules per litre once you divide by the molar mass.

How it is calculated

Everything is first converted to grams per liter (g/L):

from mol/L: g/L = mol/L × M, where M is the molar mass in g/mol
from mg/L or ppm: g/L = value ÷ 1,000
from ppb (µg/L): g/L = value ÷ 1,000,000

Then: mg/L = g/L × 1,000; ppm ≈ mg/L; ppb ≈ g/L × 10⁶; mol/L = g/L ÷ M; mmol/L = mol/L × 1,000; % w/v = g/L ÷ 10 (grams per 100 mL).

The principle

A mole is a fixed number of particles (6.02214076 × 10²³), and the molar mass tells you how many grams one mole weighs. Dividing a mass concentration by the molar mass therefore turns "grams per liter" into "particles per liter", which is what controls chemical reactions and osmotic effects. The ppm and ppb shortcuts rely on the fact that one liter of dilute aqueous solution weighs close to 1 kg, so 1 mg per liter is about 1 mg per kg, one part per million by mass.

Worked example

Default: 250 mg/L of sodium chloride, M = 58.44 g/mol.

To g/L: 250 ÷ 1,000 = 0.25 g/L.
mol/L: 0.25 ÷ 58.44 = 0.0042779 mol/L.
mmol/L: 0.0042779 × 1,000 = 4.2779 mmol/L (shown as 4.277891855).
ppm ≈ 250, ppb ≈ 250,000, % w/v = 0.25 ÷ 10 = 0.025%.

How to read the result

For context: physiological saline is 0.9% w/v, which is 9 g/L or about 154 mmol/L of NaCl; seawater holds roughly 35 g of salts per kg. Drinking-water limits are usually written in mg/L or µg/L, so ppm and ppb are the convenient units there. Note that mmol/L of a salt counts formula units; NaCl releases two ions, so its osmolar concentration is about twice its molar concentration.

Common mistakes

Using the molar mass of the wrong form, for example anhydrous instead of a hydrate salt.
Applying ppm ≈ mg/L to concentrated solutions, oils or solvents whose density is far from 1 kg/L.
Confusing % w/v (grams per 100 mL) with % w/w (grams per 100 g) or % v/v.
Mixing up ppm by mass with ppm by volume used for gases in air.

Keep in mind: The ppm and ppb equivalences assume a dilute water solution near room temperature. For precise laboratory, clinical or regulatory work, use mass fractions with the measured density and follow the method your standard specifies.

FAQ

Frequently asked questions

Is ppm the same as mg/L?

For dilute water solutions approximately yes, because 1 L of such a solution weighs about 1 kg.

What is 0.9% saline in mol/L?

9 g/L ÷ 58.44 g/mol = 0.154 mol/L.

Where do I find the molar mass?

Add the atomic weights of the formula's atoms from a periodic table, or check the reagent label or a safety data sheet.

Sources: BIPM, The International System of Units (SI Brochure), 9th edition · IUPAC, Quantities, Units and Symbols in Physical Chemistry (Green Book)

Checked 8 Nov 2026. Factors from the SI Brochure (BIPM, 9th ed.) and NIST Special Publication 811 (2008), Appendix B.