This molarity calculator solves the core solution-chemistry relationship M = moles ÷ liters in whatever direction you need: find molar concentration from a mass, find how much solute to weigh out, or find the volume to make a target concentration. A separate dilution mode uses M1V1 = M2V2 so you can enter any three values and solve the fourth.
Molarity (symbol M, units mol/L) is the most common way chemists express concentration because it ties directly to the number of reacting particles. Whether you are preparing a buffer, standardizing a titrant, or diluting a stock solution, these two formulas cover the arithmetic. Enter your values and the calculator returns both the answer and the moles involved.
The Molarity Formula: M = mol/L
Molarity is defined as moles of solute divided by liters of solution:
M = moles ÷ volume (L)
And moles convert to mass through molar mass (MW):
moles = mass (g) ÷ molar mass (g/mol)
Combining them, this moles-to-molarity conversion becomes M = (mass ÷ MW) ÷ volume. Rearranging lets you solve for any variable:
- Mass needed = molarity × volume × MW
- Volume = (mass ÷ MW) ÷ molarity
Molar mass comes from the periodic table — sum the atomic masses of every atom in the formula. Sodium chloride (NaCl) is 22.99 + 35.45 = 58.44 g/mol; glucose (C6H12O6) is 180.16 g/mol.
The Dilution Equation: M1V1 = M2V2
Diluting a stock solution conserves the number of moles, so concentration times volume before equals concentration times volume after:
M1 × V1 = M2 × V2
Here M1 and V1 are the concentration and volume of the concentrated stock, and M2 and V2 are the diluted final concentration and volume. A dilution calculator rearranges this to find whatever is unknown — most often V1, the amount of stock to pipette. The solvent (usually water) you add equals V2 − V1. Keep both volumes in the same unit; this tool uses liters throughout, so 250 mL is entered as 0.25 L.
Example: Preparing 0.5 M NaCl
How much salt makes 200 mL (0.2 L) of 0.5 M NaCl solution?
- moles = molarity × volume = 0.5 × 0.2 = 0.1 mol
- mass = moles × MW = 0.1 × 58.44 = 5.844 g
Weigh 5.844 g of NaCl and dissolve it to a final volume of 0.2 L. To check the reverse, dissolving 5.844 g in 0.2 L gives (5.844 ÷ 58.44) ÷ 0.2 = 0.1 ÷ 0.2 = 0.5 M. For a dilution, making 1 L of 0.5 M from a 2 M stock needs V1 = (0.5 × 1) ÷ 2 = 0.25 L of stock topped up with 0.75 L of water.
Frequently Asked Questions
What is the molarity formula?
Molarity equals moles of solute divided by liters of solution: M = mol ÷ L. Since moles = mass ÷ molar mass, you can also write M = (grams ÷ g/mol) ÷ liters. The unit is mol/L, written M, so a 0.5 M solution contains 0.5 moles of solute per liter.
How do you convert moles to molarity?
Divide the number of moles of solute by the volume of solution in liters. For example, 0.1 mol dissolved in 0.2 L gives 0.1 ÷ 0.2 = 0.5 mol/L, or 0.5 M. Make sure the volume is the total solution volume, not just the solvent added.
How do you calculate a dilution?
Use M1V1 = M2V2, where M1 and V1 are the stock concentration and volume and M2 and V2 are the final concentration and volume. To find how much stock to use, rearrange to V1 = (M2 × V2) ÷ M1, then add solvent until you reach V2.
How much solute do I need for a given molarity?
Multiply molarity by volume in liters to get moles, then multiply by molar mass to get grams: mass = M × V × MW. To make 1 L of 1 M glucose (MW 180.16), you need 1 × 1 × 180.16 = 180.16 g dissolved to a final volume of 1 L.
What is the difference between molarity and molality?
Molarity is moles of solute per liter of solution and depends slightly on temperature because volume expands with heat. Molality is moles of solute per kilogram of solvent and is temperature-independent. Molarity (M) is by far the more common lab unit; molality (m) is used in colligative-property calculations.