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Molarity calculator — solve for M, mass, volume, or moles.

Calculate molar concentration, mass of solute, solution volume, or moles using the standard molarity formula M = n/V. Supports grams, milligrams, litres, millilitres, and SI prefixed units, with a step-by-step calculation and lab preparation note.

How it worksReal-time

Inputs

Find Molarity

g/mol

Common: NaCl = 58.44 · Glucose = 180.16 · NaOH = 40.00 · HCl = 36.46

Calculation complete

Result: Find Molarity

Molarity (M)

111.012 mM
Msolved
111.012 mM

Molarity

n
27.753 mmol

Moles

m
5 g

Mass

V
250 mL

Volume

Steps

Calculation steps

  1. 1n = m / MW = 5 g / 180.16 g/mol = 27.753 mmol
  2. 2M = n / V = 27.753 mmol / 250 mL = 111.012 mM

Lab guide

Solution preparation note

To prepare 111.012 mM solution in 250 mL: weigh out 5 g and dissolve in approximately 80% of the target volume, then make up to 250 mL.

Reference

Molarity formulas

MMolarity
  • M = n / V
  • M = m / (MW × V)
nMoles
  • n = m / MW
  • n = M × V
mMass (g)
  • m = n × MW
  • m = M × V × MW
VVolume
  • V = n / M
  • V = m / (MW × M)

Field guide

Molarity: the chemist's measure of solution concentration.

What is molarity?

Molarity (M) is the most widely used measure of solution concentration in chemistry. It is defined as the number of moles of solute dissolved per litre of solution:

M = n / V

Where M is molarity in mol/L (molar), n is the number of moles of solute, and V is the total volume of the solution in litres, not the volume of solvent. This distinction matters: you dissolve the solute and then add solvent until the total solution volume reaches the target.

Combined with the mole definition (n = m / MW, wherem is mass in grams and MW is molar mass in g/mol), the full molarity formula becomes:

M = m / (MW × V)

Why moles?

Chemical reactions occur between molecules and atoms — not between grams. One mole of any substance contains Avogadro's number (6.022 × 10²³) of particles. Using moles lets chemists write balanced equations where stoichiometric ratios are whole numbers: 2 H₂ + O₂ → 2 H₂O means exactly 2 moles of hydrogen gas react with 1 mole of oxygen.

Molarity translates directly into reaction stoichiometry. If you're mixing 100 mL of 1 M HCl with 100 mL of 1 M NaOH, you have equal moles of both and the reaction is exactly stoichiometric (complete neutralisation).

How to prepare a molar solution

The standard volumetric method for preparing a solution of exact concentration:

  1. Calculate the required mass using m = M × V × MW. For 250 mL of 0.1 M NaCl (MW = 58.44 g/mol): m = 0.1 × 0.250 × 58.44 = 1.461 g.
  2. Weigh accurately on an analytical balance (typically to ±0.001 g).
  3. Dissolve in a small volume of solvent (typically 50–80% of the target volume) in a beaker, stirring until completely dissolved.
  4. Transfer to a volumetric flask of the exact target volume. Rinse the beaker 2–3 times with small amounts of solvent and add the rinsings.
  5. Make up to the mark by adding solvent carefully until the bottom of the meniscus just touches the calibration line.
  6. Mix thoroughly by inverting the stoppered flask 10–20 times.

Common concentrations and their applications

  • 1 M (1 molar): stock solutions for laboratory reagents. 1 M HCl is a common acid reagent; 1 M NaOH is a common base.
  • 100 mM (0.1 M): typical working concentrations for biochemical assays. Phosphate-buffered saline (PBS) is approximately 10 mM phosphate, 137 mM NaCl.
  • 1 mM (0.001 M): concentrations of many enzyme substrates and cofactors in biochemical experiments.
  • μM and nM: concentrations of pharmaceutical compounds, hormones, and receptor ligands. The dissociation constant (Kd) of many protein–ligand interactions falls in the nM range.

Molar mass from the periodic table

Molar mass (molecular weight, MW) is found by summing the atomic masses of all atoms in the molecular formula. Atomic masses are given in g/mol (numerically equal to the mass of one atom in atomic mass units):

  • H = 1.008, C = 12.011, N = 14.007, O = 15.999
  • Na = 22.990, Cl = 35.450, K = 39.098, Ca = 40.078
  • Glucose (C₆H₁₂O₆): 6×12.011 + 12×1.008 + 6×15.999 = 180.156 g/mol
  • NaCl: 22.990 + 35.450 = 58.440 g/mol

For complex molecules, use the molecular formula and sum systematically. Our Molecular Weight Calculator computes molar mass from any chemical formula automatically.

Dilutions and the M₁V₁ = M₂V₂ equation

When diluting a concentrated stock solution, moles of solute are conserved:

M₁ × V₁ = M₂ × V₂

To prepare 100 mL of 50 mM solution from a 1 M stock: V₁ = (M₂ × V₂) / M₁ = (0.05 × 0.1) / 1 = 0.005 L = 5 mL. Take 5 mL of the 1 M stock and make up to 100 mL. This calculation is separate from this calculator but uses the same underlying molarity concept.