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Molarity Calculator

ChemistrySolutions and Concentration
Solve for

Sum of atomic masses in the formula, e.g. NaCl = 22.99 + 35.45 = 58.44.

Result (mol/L, or grams to weigh) —
Molarity (mol/L)
—
Millimolar (mmol/L)
—
Moles of solute (mol)
—
Mass of solute (g)
—
Mass concentration (g/L)
—

Molarity is moles of solute per litre of finished solution: M = (mass ÷ molar mass) ÷ volume in litres. Dissolving 5.844 g of sodium chloride (58.44 g/mol) and making it up to 1.000 L gives 0.1000 mol in one litre, a 0.1 M solution. Reverse it to find the grams to weigh for a target concentration.

About this tool

Bench chemists, biology students and anyone preparing buffers or stock solutions need to move between grams on a balance and moles per litre in a flask. This calculator works in both directions. Choose Molarity to turn a weighed mass into mol/L, or Mass to weigh to get the grams needed for a target concentration in a chosen volume. Alongside the headline number it reports millimolar concentration, the amount in moles and the mass concentration in g/L, and it writes out each step of the working. Volume can be entered in millilitres or litres. One limit: the volume is the final volume of solution, not the volume of solvent added, and the result assumes a pure, anhydrous solute unless you enter the molar mass of the hydrate you actually weigh.

How to use it

  1. Pick what to solve for

    Molarity turns a known mass into mol/L; Mass to weigh gives the grams needed for a target molarity.

  2. Enter the molar mass

    Use g/mol from the formula, including water of crystallisation if your reagent is a hydrate such as CuSO4·5H2O.

  3. Set the final volume

    Type the volume of finished solution and choose mL or L in the field's unit menu.

  4. Read the result and working

    The headline shows mol/L or grams; the stats add mmol/L, moles and g/L, and the steps show each division.

Examples

5.844 g NaCl in 1000 mL

Solve for
Molarity
Mass of solute
5.844
Molar mass of solute
58.44
Final solution volume
1000
Volume unit
mL

Result Result (mol/L, or grams to weigh): 0.1
Molarity (mol/L): 0.1
Millimolar (mmol/L): 100
Moles of solute (mol): 0.1
Mass of solute (g): 5.844
Mass concentration (g/L): 5.844

  1. Volume = 1000 mL = 1 L
  2. Moles n = 5.844 g ÷ 58.44 g/mol = 0.1 mol
  3. Molarity M = 0.1 mol ÷ 1 L = 0.1 mol/L

5.844 g of sodium chloride is one tenth of a mole, so made up to one litre it gives a 0.1 M solution, the classic teaching-lab stock.

0.5 M NaOH, 250 mL flask

Solve for
Mass to weigh
Target molarity
0.5
Molar mass of solute
40
Final solution volume
250
Volume unit
mL

Result Result (mol/L, or grams to weigh): 5
Molarity (mol/L): 0.5
Millimolar (mmol/L): 500
Moles of solute (mol): 0.125
Mass of solute (g): 5
Mass concentration (g/L): 20

  1. Volume = 250 mL = 0.25 L
  2. Moles n = 0.5 mol/L × 0.25 L = 0.125 mol
  3. Mass = 0.125 mol × 40 g/mol = 5 g

A quarter litre at 0.5 mol/L needs 0.125 mol of sodium hydroxide, which weighs 5.00 g at 40.00 g/mol.

How it is calculated

n = m ÷ MW; M = n ÷ V

m
mass of solute in grams
MW
molar mass of the solute in g/mol
n
amount of solute in moles
V
final solution volume in litres (mL ÷ 1000)
M
molarity, or amount concentration, in mol/L

Mass is converted to moles by dividing by molar mass, then moles are divided by the solution volume in litres. In Mass to weigh mode the same relation runs backwards: n = M × V, then m = n × MW. Millimolar is molarity × 1000 and mass concentration is m ÷ V in g/L. Molarity depends on temperature because volume does; for temperature-independent work use molality, which divides by kilograms of solvent instead.

When not to use it

  • Use molality instead when the solution will be heated or cooled, since molarity shifts as volume expands.
  • Do not use it for liquid solutes measured by volume without first converting to mass using density.
  • It does not cover dilution of an existing stock; that needs C1V1 = C2V2.

Common mistakes

  • Entering the volume of water added instead of the final volume after the solute dissolves.
  • Using the anhydrous molar mass while weighing a hydrated salt, which gives a weaker solution than intended.
  • Typing millilitres while the unit menu is set to litres, which makes the answer 1000 times too small.

Frequently asked questions

What is the difference between molarity and molality?

Molarity divides moles of solute by litres of solution; molality divides by kilograms of solvent. Molality does not change with temperature because mass does not, so it is used for boiling-point and freezing-point work. For dilute aqueous solutions near room temperature the two numbers are close, since a litre of dilute solution holds roughly a kilogram of water.

How do I find the molar mass of a compound?

Add the standard atomic weights of every atom in the formula. For sodium hydroxide that is 22.99 + 16.00 + 1.008 = 40.00 g/mol. For a hydrate, include the water: copper(II) sulfate pentahydrate is 159.61 + 5 × 18.02 = 249.69 g/mol. The reagent bottle label usually prints the value as well.

What does 1 M mean?

1 M, read one molar, means one mole of solute in each litre of finished solution. A 1 M glucose solution holds 180.16 g of glucose per litre. Smaller units are common in biology: 1 mM is a thousandth of that and 1 µM a millionth.

How many grams do I need for 100 mL of 1 M NaCl?

Multiply 1 mol/L by 0.100 L to get 0.100 mol, then by 58.44 g/mol: 5.844 g. Dissolve it in less than 100 mL of water, then top up to the 100 mL mark in a volumetric flask so the final volume is exact.

Why does the volume have to be the final volume?

Dissolving a solid changes the total volume, sometimes by several percent for concentrated solutions. Molarity is defined on the volume of the finished solution, so the standard practice is to dissolve the solute in part of the solvent and then fill a volumetric flask to the line.

Can I convert g/L to molarity with this?

Yes. Enter the grams in one litre as the mass, set the volume to 1 L and enter the molar mass. The mass concentration stat shows g/L directly for any other combination, so both forms are on screen at once.

Cite this tool

Using this tool in an assignment, article or report? Copy a citation, or a link for your website.

APA

Tools Directory Editorial Team. (2026, October 17). Molarity Calculator. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/chemistry/solutions-and-concentration/molarity-calculator/

MLA

Tools Directory Editorial Team. “Molarity Calculator.” Tools Directory, 17 Oct. 2026, https://tools.milunir.com/chemistry/solutions-and-concentration/molarity-calculator/. Accessed 27 Oct. 2026.

Chicago

Tools Directory Editorial Team. “Molarity Calculator.” Tools Directory. Last modified October 17, 2026. Accessed October 27, 2026. https://tools.milunir.com/chemistry/solutions-and-concentration/molarity-calculator/.

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