# Molarity calculator

> Calculate the mass to weigh for a molar solution, or the molarity or volume from a mass. Units from kg to ng and M to pM, with common lab reagents. Free.

URL: https://www.labcanvas.com/tools/molarity-calculator

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.

## Quick answer

- **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:** m = C × V × M (g = mol/L × L × g/mol); C = m ÷ (V × M); V = m ÷ (C × M).
- **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.

Free, in the browser, no account needed. Updated 2026-11-08.

## Features

- Solve for mass, concentration or volume
- Molar mass presets for common reagents
- Unit conversion (g, mg, µg; M, mM, µM, nM; L, mL, µL)
- Plain-language instruction for the bench

## How to use it

1. **Choose what to calculate**: Mass to weigh, the resulting concentration, or the final volume.
2. **Enter the molar mass**: Pick a reagent or type the formula weight from the bottle (it includes any water of hydration).
3. **Enter the known values**: Type the other two values with their units; the result updates as you type.

## 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 |

## 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.

## 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.