Molarity Calculator

Common Molecular Weights Reference

Click any row to copy the molar mass into the Moles calculator above.

What is Molarity?

Molarity is the most common way to express the concentration of a solution in chemistry. It is defined as the number of moles of solute dissolved in exactly one litre of solution, and is given the symbol M (or mol/L). The official SI unit is mol dm⁻³, which is numerically identical to mol/L.

The molarity formula is straightforward:

A 1 M (one molar) solution contains exactly one mole of solute per litre. A 0.5 M solution contains half a mole per litre, and so on.

Millimolar (mM) vs Molar (M) — What is mM?

The millimolar unit (mM) is used when concentrations are too small to conveniently express in full molar units. The relationship is simple:

• 1 M = 1,000 mM = 1,000,000 µM = 1,000,000,000 nM
• 1 mM = 0.001 mol/L (one millimole per litre)
• 1 µM = 0.000001 mol/L (one micromole per litre)

Millimolar is especially common in biochemistry, pharmacology, and medicine. Blood glucose is often reported in millimolar: a normal fasting blood glucose level is approximately 4–6 mM in the UK (note that US labs often use mg/dL instead). Drug IC50 values are routinely expressed in µM or nM.

How to Calculate Moles from Mass (mol cal)

Before you can find molarity, you need to know the number of moles. The mole calculation formula is:

The molar mass is the mass of one mole of a substance, numerically equal to the relative molecular mass in grams. You find it by summing the atomic masses of all atoms in the formula from the periodic table.

Worked example: 5.85 g of NaCl in 500 mL

Dilution Calculations: C₁V₁ = C₂V₂

When you dilute a solution, the number of moles of solute stays constant—only the volume changes. This gives the dilution equation:

Where: C₁ = initial concentration, V₁ = volume of stock solution taken, C₂ = final (target) concentration, V₂ = final total volume.

Dilution example: Making 100 mL of 0.1 M HCl from 2 M stock

Take 5 mL of 2 M HCl and make up to 100 mL with distilled water. This is called a 1-in-20 dilution (dilution factor = 20).

You can use any consistent units for C₁ and C₂ (M, mM, µM) as long as they match. For volumes, keep them consistent too (mL or L).

Molarity vs Molality: What is the Difference?

Both are measures of concentration, but they use different denominators:

For most GCSE and A-Level chemistry work, and for laboratory preparation of solutions, molarity (mol/L) is the standard. Molality appears mainly in physical chemistry and colligative property calculations.

GCSE and A-Level Chemistry Context

Molarity appears throughout GCSE and A-Level chemistry, particularly in:

• Titration calculations: moles of acid = molarity × volume (in litres). Knowing moles lets you use molar ratios from the balanced equation to find the unknown concentration.
• Making standard solutions: calculating exactly how many grams of solid to dissolve in a volumetric flask to prepare a solution of known molarity.
• Gas law and stoichiometry problems: converting between mass, moles and volume of reactants and products.
• Equilibrium constant expressions: Kₑ expressions use molar concentrations (mol/L) raised to their stoichiometric powers.

At A-Level, students are expected to fluently convert between mass, moles, concentration and volume, and to handle serial dilutions in practical work. This molarity calculator supports all those conversions in one place.

Lab Practical Examples

Preparing 250 mL of 0.5 M glucose solution: Molar mass of glucose (C₆H₁₂O₆) = 180.16 g/mol. Moles needed = 0.5 × 0.25 = 0.125 mol. Mass = 0.125 × 180.16 = 22.52 g. Dissolve in about 200 mL of water in a 250 mL volumetric flask, then make up to the mark.

Preparing 1 L of 1 M NaOH: Moles = 1. Mass = 1 × 40.00 = 40.00 g. Dissolve in water and make up to 1 L. NaOH dissolves exothermically, so allow to cool before making up to volume.

Serial dilution: Starting from 10 mM stock, perform a 1:10 dilution to give 1 mM, then another 1:10 to give 0.1 mM (100 µM). Each step: take 1 mL of stock + 9 mL water = 10 mL of solution at 10-fold lower concentration.

Frequently Asked Questions

What is mM in chemistry?

mM stands for millimolar, equal to 0.001 mol/L (one millimole per litre). It is widely used in biochemistry and medicine because many biological concentrations—such as ion concentrations in blood serum—are far smaller than 1 mol/L. For example, normal serum potassium is about 3.5–5.0 mM.

How do I calculate molarity step by step?

1. Find the molar mass of the solute (sum of atomic masses from the periodic table). 2. Calculate moles: n = mass ÷ molar mass. 3. Convert volume to litres if needed (divide mL by 1000). 4. Apply: Molarity = n ÷ V(L). 5. Convert to mM or µM if needed (× 1000 or × 1,000,000).

What is the difference between molarity and molality?

Molarity uses the volume of the solution (L) as the denominator, so it changes with temperature. Molality uses the mass of the solvent (kg) and is temperature-independent. For most lab work, use molarity. For boiling point elevation and freezing point depression calculations, use molality.

How do you do a dilution calculation?

Use C₁V₁ = C₂V₂. Identify which value is unknown, then rearrange to solve. Always keep concentration units consistent (both in M or both in mM) and volume units consistent (both in mL or both in L).

What is the mol cal formula?

moles = mass (g) ÷ molar mass (g/mol). The molar mass of any element equals its relative atomic mass in grams; for compounds, add the atomic masses of all constituent atoms.

How do I convert M to mM or µM?

Multiply by 1,000 to convert M to mM. Multiply by 1,000,000 to convert M to µM. For example, 0.005 M = 5 mM = 5,000 µM. Divide by 1,000 to go from mM back to M.