Parts per Million to Molarity: Formula and Table

Short answer: for a dilute aqueous solution, 1 ppm is about 1 mg/L, so molarity (mol/L) = ppm ÷ (1000 × molar mass in g/mol). The reverse is ppm ≈ molarity × molar mass × 1000. The shortcut is a derived relationship, and it holds only when the solution density is close to 1.0 g/cm³.

Formula: parts per million to molarity

Parts per million (ppm) is a dimensionless ratio of 10⁻⁶. Written as a mass fraction, 1 ppm means 1 mg of solute per 1 kg of mixture. For a dilute aqueous solution the mass density is approximately 1.0 g/cm³, so 1 kg of solution occupies about 1 L and the mass fraction in ppm is approximately a mass concentration in mg/L (IUPAC Green Book, 3rd ed., section 3.9.1). The conversion below combines that approximation with the definition of amount concentration (mol/L).

ppm to mol/L: c (mol/L) = ppm ÷ (1000 × M), where M is the molar mass in g/mol.
ppm to mmol/L: c (mmol/L) = ppm ÷ M.
mol/L to ppm: ppm ≈ c (mol/L) × M × 1000 (equivalently, mg/L = mmol/L × M).

If you need the molar mass first, see how to calculate molar mass. For moles to molarity, see mol to molarity.

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

The Green Book states that dilute solutions are often measured or calibrated to a known mass concentration in mg/dm³ (the same as mg/L), and that this unit is then preferred to ppm. The shortcut therefore needs three conditions.

The ppm value is a mass fraction (w/w). Parts per million is language-dependent and ambiguous: the Green Book table of such abbreviations gives the example of a volume fraction of helium of 20 ppm, so the same label can mean a mass, volume or amount ratio. In gas measurements the label can refer to a volume or mole ratio instead.
The solution is dilute and aqueous. The 1 ppm = 1 mg/L step relies on a density near 1.0 g/cm³. For concentrated, dense or non-aqueous solutions, convert using the actual density: mg/L = ppm × density (g/mL).
The label says what it measures. If a report says only “ppm” with no basis, ask whether it is mg/L, mg/kg or a mole or volume ratio before converting.

The derived relationship above follows from the definitions; it is not a quotation from the standards.

Lookup table: ppm (mg/L) to mmol/L

Each cell is ppm ÷ M, which gives mmol/L for a dilute aqueous solution. The columns use round molar masses so that the table applies to any compound with that molar mass; find your compound’s molar mass and scale (the relationship is linear). Values are rounded to three significant figures. Divide by 1000 for mol/L. The table is plain arithmetic from the formula above.

ppm (mg/L)	M = 50 g/mol (mmol/L)	M = 100 g/mol (mmol/L)	M = 200 g/mol (mmol/L)	M = 500 g/mol (mmol/L)
1	0.02	0.01	0.005	0.002
5	0.1	0.05	0.025	0.01
10	0.2	0.1	0.05	0.02
50	1	0.5	0.25	0.1
100	2	1	0.5	0.2
500	10	5	2.5	1
1000	20	10	5	2

Worked examples

Example 1: convert parts per million to molarity

Illustrative case: a dilute aqueous standard of a compound with a molar mass of 250 g/mol is labelled 25 ppm (taken as 25 mg/L).

Divide by 1000 to get g/L: 25 mg/L ÷ 1000 = 0.025 g/L.
Divide by the molar mass: 0.025 g/L ÷ 250 g/mol = 1.0 × 10⁻⁴ mol/L.
Result: 0.10 mmol/L, or 100 µmol/L.

Example 2: convert molarity to parts per million

Illustrative case: a dilute aqueous solution of a compound with a molar mass of 180 g/mol has a concentration of 2.0 mmol/L (0.0020 mol/L).

Multiply by the molar mass: 0.0020 mol/L × 180 g/mol = 0.36 g/L.
Convert to mg/L: 0.36 g/L × 1000 = 360 mg/L.
Result: about 360 ppm (mass basis, density near 1.0 g/cm³).

The compounds in both examples are generic teaching values, not real reagents.

Compute it in Excel, R or Python

Put the ppm value in A2 and the molar mass (g/mol) in B2.

Excel: =A2/(1000*B2) gives mol/L; =A2/B2 gives mmol/L. Reverse, with molarity in C2: =C2*B2*1000.
R: ppm_to_molar = function(ppm, mm) ppm / (1000 * mm), then ppm_to_molar(25, 250) returns 1e-04.
Python: def ppm_to_molar(ppm, mm): return ppm / (1000 * mm), then ppm_to_molar(25, 250) returns 0.0001. Reverse: def molar_to_ppm(c, mm): return c * mm * 1000.

All three assume a dilute aqueous solution. For other matrices, multiply ppm by the density in g/mL first.

How to calculate parts per million

Parts per million is a ratio multiplied by 10⁶. For a mass fraction, ppm = (mass of solute ÷ mass of mixture) × 10⁶. The NIST Guide to the SI defines mass fraction as the mass of the component divided by the mass of the mixture. For example, 5 mg of solute in 1 kg of mixture is 5 × 10⁻⁶, or 5 ppm. The same quantity is written 5 mg/kg. For the percent form of the same idea, see mass percent of a solution.

Is molarity moles per liter?

Yes. Molarity is amount of substance (moles) of solute per litre of solution, in mol/L (the same as mol/dm³). Both NIST and IUPAC note that the word “molarity” and the symbol M are older usage: the NIST Guide to the SI calls them obsolete, and the Green Book says the term should be avoided because of possible confusion with molality. The preferred quantity name is amount-of-substance concentration (amount concentration). On a lab label, write the unit explicitly, such as 0.10 mmol/L. To step between millimolar and molar, see millimolarity to molarity.

Reporting checklist from the standards

NIST states that the terms part per million, part per billion and part per trillion are language-dependent and not acceptable for use with the SI, and its unit checklist says ppm, ppb and ppt are not used to express the values of quantities; write the ratio explicitly instead (NIST’s own example uses µA/A rather than ppm).
The Green Book suggests abandoning such abbreviations because of the ambiguities involved, and gives mg/g as the replacement for a mass fraction and a known mass concentration in mg/L for dilute solutions.
Write the quantity and the unit: “mass concentration 25 mg/L” or “amount concentration 0.10 mmol/L”, not a bare “25 ppm”.

Safety note: read the SDS

Concentration conversions do not replace the safety data sheet. Before preparing or diluting any chemical standard, read its SDS for hazards, handling and disposal; see how to read a safety data sheet. A low ppm or mmol/L value does not make a substance safe.

FAQ

How do I convert parts per million to molarity?

For a dilute aqueous solution, divide the ppm value (mg/L) by 1000 times the molar mass in g/mol. Example: 25 ppm of a 250 g/mol compound is 1.0 × 10⁻⁴ mol/L.

How do I convert molarity to parts per million?

Multiply mol/L by the molar mass in g/mol and by 1000 to get mg/L, which is about ppm in dilute water. Example: 0.0020 mol/L of a 180 g/mol compound is about 360 ppm.

How do I calculate parts per million?

Divide the mass of solute by the mass of the mixture and multiply by 10⁶. 5 mg in 1 kg is 5 ppm.

Is molarity moles per liter?

Yes, mol/L. The preferred name today is amount-of-substance concentration. Related: moles vs molarity.

Sources

NIST, Guide to the SI, Chapter 7 (section 7.10.3) and Chapter 8 (section 8.6), nist.gov, checked 2026-10-29.
NIST, SI unit rules and style conventions checklist, physics.nist.gov, checked 2026-10-29.
IUPAC, Quantities, Units and Symbols in Physical Chemistry (Green Book), 3rd ed., 2012, sections 3.9.1 and 3.10.1, iupac.org, checked 2026-10-29.

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