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Molarity calculator

Q: How do I convert a concentration in mg/mL to molarity?

Divide by the molar mass: mg/mL is the same as g/L, so M = (g/L) / (g/mol). For example, 8.77 mg/mL NaCl is 8.77 g/L ÷ 58.44 g/mol = 0.150 M (150 mM). In the calculator, enter the mass in mg and a volume of 1 mL and solve for concentration.

Q: How do you calculate molarity?

Molarity is moles of solute divided by litres of solution: M = n / V, and n = mass / molar mass. So M = mass / (molar mass × volume).

To make a solution of a given molarity, weigh mass (g) = concentration (mol/L) × volume (L) × molar mass (g/mol). Enter any two of mass, concentration and volume plus the molar mass, and the calculator gives the third, with units from kilograms to nanograms and molar to picomolar.

What it does: Calculates the mass to weigh for a solution of given molarity and volume, or the molarity or volume from a mass, with units from kg to ng and M to pM.
Formula: $\begin{aligned} m &= C \times V \times M \\ &\quad (\mathrm{g} = \mathrm{mol/L} \times \mathrm{L} \times \mathrm{g/mol}) \\ C &= m \div (V \times M) \\ V &= m \div (C \times M) \end{aligned}$
Example: For 500 mL of 150 mM NaCl (M = 58.44 g/mol): m = 0.150 mol/L × 0.500 L × 58.44 g/mol = 4.383 g, made up to 500 mL.

Last updated 8 November 2026

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At a glance

Molarity calculator at a glance
Formula	$m = C \times V \times M$
Solves for	Mass, concentration or volume
Units	g, mg, µg; M, mM, µM, nM; L, mL, µL
Output	The answer and a plain instruction for the bench
Runs in	Your web browser; nothing is uploaded
Price	Free, no account needed

What it does

How to use the molarity calculator

Choose what to calculateMass to weigh, the resulting concentration, or the final volume.
Enter the molar massPick a reagent or type the formula weight from the bottle (it includes any water of hydration).
Enter the known valuesType the other two values with their units; the result updates as you type.

How it works

$\begin{aligned} m &= C \times V \times M \\ C &= \frac{m}{V \times M} \\ V &= \frac{m}{C \times M} \end{aligned}$

where $m$ is the mass in grams, $C$ the molar concentration in mol/L (M), $V$ the volume in litres and $M$ the molar mass in g/mol. All inputs are converted to these base units first.

Worked example

To make 500 mL of 150 mM NaCl $(M = 58.44\ \mathrm{g/mol})$ :

$\begin{aligned} m &= 0.150\ \mathrm{mol/L} \times 0.500\ \mathrm{L} \\ &\quad {}\times 58.44\ \mathrm{g/mol} \\ &= \mathbf{4.383\ g} \end{aligned}$

Dissolve in about 400 mL of water and make up to 500 mL.

Frequently asked questions

How do I convert a concentration in mg/mL to molarity?

Divide by the molar mass: mg/mL is the same as g/L, so $\mathrm{M} = (\mathrm{g/L}) / (\mathrm{g/mol})$ . For example, 8.77 mg/mL NaCl is $8.77\ \mathrm{g/L} \div 58.44\ \mathrm{g/mol} = 0.150\ \mathrm{M}$ (150 mM). In the calculator, enter the mass in mg and a volume of 1 mL and solve for concentration.

How do you calculate molarity?

Molarity is moles of solute divided by litres of solution: $M = n / V$ , and $n = \text{mass} / \text{molar mass}$ . So $M = \text{mass} / (\text{molar mass} \times \text{volume})$ .

What is the difference between molarity and molality?

Molarity is moles per litre of solution; molality is moles per kilogram of solvent. Molality does not change with temperature, molarity does slightly.

Should I use the molar mass of the hydrate?

Yes, if you weigh the hydrate. MgCl₂·6H₂O is 203.30 g/mol, while anhydrous MgCl₂ is 95.21 g/mol; use the formula weight printed on your bottle.

Why make up to volume instead of adding solvent to the volume?

The solute takes up space, so dissolving in the full volume of solvent gives slightly more than the target volume and a lower concentration.

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