Molarity Calculator

How to use this calculator

There are 4 inputs.

Mass of Solute is the mass of the substance you’re dissolving, in grams. If you weighed out 58.44 g of NaCl on a balance, enter 58.44. This is the mass of the solute only, not the final solution.

Molar Mass of Solute is the molar mass of that substance in g/mol. For NaCl it’s 58.44 g/mol. For glucose (C₆H₁₂O₆) it’s 180.16 g/mol. Look this up from a periodic table or use the reference table below. Don’t confuse this with the mass of your sample. They can be equal by coincidence (as in the default example), but they’re different quantities.

Volume of Solution is the total volume of the final solution, not the volume of solvent you added. Enter the number and select your unit from the dropdown.

Volume Unit lets you choose between Litres (L), Millilitres (mL), and other common options. The calculator converts to litres automatically before dividing.

Click Calculate and you get 3 outputs:

• Molarity (M): moles of solute per litre of solution. This is the main result.
• Moles of Solute: how many moles are dissolved. Intermediate step, shown explicitly so you can use it elsewhere.
• Concentration (g/L): mass of solute per litre of solution. Useful when you need to describe concentration by mass rather than by moles.

What problem this calculator solves

Lab protocols run on molar concentrations. Reaction rates, buffer recipes, titration calculations, enzyme assays: all of them specify concentration in mol/L. To actually make the solution, you need to work backward from a target molarity to a mass you can weigh.

The calculation isn’t difficult, but it has 2 steps (grams to moles, then moles to molarity) and a unit conversion hidden inside the volume step. In a busy lab, that’s exactly where transcription errors land. You enter 250 mL but forget to convert to 0.250 L, and your molarity is off by a factor of 1000.

The calculator handles the unit conversion automatically and shows intermediate steps, so you can verify each part of the chain before trusting the final number.

The concept explained simply

Imagine you’re making lemonade. The “concentration” of lemon is how much juice you put in per litre of drink. More juice, same total volume: higher concentration. Same juice, more water: lower concentration.

Molarity is that same idea, but instead of measuring juice by volume, you measure solute by moles. Moles capture the actual number of particles dissolved, which is what determines most of the chemistry: how acidic the solution is, how it behaves in a reaction, what its osmotic pressure does.

Two solutions can have the same mass of solute but completely different molarities if the solutes have different molar masses. 58.44 g of NaCl and 58.44 g of glucose dissolved in 1 L give you 1.000 M and 0.324 M respectively. Same mass, very different particle counts.

Molarity measures how crowded the solute particles are in a given volume of solution. It's a particle density, counted in moles rather than individual molecules.

The formula explained

M is molarity in mol/L, n is moles of solute, V is volume of solution in litres.

Since moles aren’t usually your starting point, you first calculate n from mass:

m is mass in grams, M_r is molar mass in g/mol.

Combining them:

This is what the calculator evaluates. Mass divided by molar mass gives moles. Moles divided by volume gives molarity. The two steps collapse into one expression.

The mass concentration output uses:

That’s just grams of solute divided by litres of solution, no moles involved. It answers “how many grams per litre” rather than “how many moles per litre.”

Reference table: molar masses for common lab solutes

Substance	Formula	Molar mass (g/mol)
Sodium chloride	NaCl	58.443
Hydrochloric acid	HCl	36.461
Sodium hydroxide	NaOH	39.997
Sulfuric acid	H₂SO₄	98.079
Acetic acid	CH₃COOH	60.052
Glucose	C₆H₁₂O₆	180.156
Sucrose	C₁₂H₂₂O₁₁	342.297
Potassium chloride	KCl	74.551
Calcium chloride	CaCl₂	110.983
Sodium bicarbonate	NaHCO₃	84.007
Ethanol	C₂H₅OH	46.068
Ammonium sulfate	(NH₄)₂SO₄	132.140

Real-world examples

The buffer preparation

A biochemistry lab needs 500 mL of 0.1 M phosphate buffer. They’re using sodium phosphate dibasic (Na₂HPO₄, molar mass = 141.96 g/mol). How much do they weigh out?

The acid dilution check

A student dissolves 3.65 g of HCl (molar mass = 36.46 g/mol) in water and makes up to 200 mL total volume. What’s the molarity?

The stock solution dilution

A lab has a 5 M NaCl stock solution and needs to prepare 100 mL of 0.9% saline. Saline is 0.154 M NaCl. How many mL of stock do they need?

Common mistakes people make

Entering volume of solvent instead of volume of solution. If you add 250 mL of water to a solid, the final solution volume is more than 250 mL. It might be 253 mL. Molarity is defined using the total solution volume. Always transfer to a volumetric flask and fill to the mark, then use that volume.

Forgetting to convert mL to L. Molarity is mol per litre. If your volume is 250 mL and you enter 250 without switching the unit dropdown to mL, the calculator will treat it as 250 L and give you a molarity 1000 times smaller than reality. The volume unit selector exists for exactly this reason.

Using the wrong molar mass for hydrated salts. CuSO₄ has a molar mass of 159.61 g/mol, but CuSO₄·5H₂O (the form commonly sold in labs) is 249.68 g/mol. If you weigh out the blue hydrated crystals and use 159.61, your molarity is wrong by a factor of 249.68/159.61 = 1.56.

Treating molarity and molality as the same thing. Molarity (mol/L) uses volume of solution as the denominator. Molality (mol/kg) uses mass of solvent. They’re equal only in very dilute aqueous solutions where the solvent density is close to 1 kg/L. For anything concentrated, they diverge. This calculator gives molarity.

Rounding molar mass too aggressively. Using 18 for water instead of 18.015 introduces a 0.08% error. For a 1 M solution made to 4 significant figures, that matters. Use at least 4 significant figures in the molar mass input.

Molarity vs. other concentration units

Molarity is the most common concentration unit in solution chemistry, but it’s not the only one. Knowing where molarity works well and where other units make more sense saves a lot of confusion.

Molality (mol/kg solvent) doesn’t change with temperature because it’s based on mass, not volume. Liquids expand when heated, so a 1 M solution at 20°C is slightly less than 1 M at 80°C. For colligative properties (boiling point elevation, freezing point depression) and anything involving temperature changes, molality is the better unit.

Mass percent (% w/w) is just grams of solute per 100 g of solution. Simpler to prepare without a volumetric flask. Used heavily in industrial and food chemistry where volume precision is less critical than mass precision.

Parts per million (ppm) is the go-to for very dilute solutions like trace contaminants in water. 1 ppm in aqueous solution is approximately 1 mg/L, which is about 0.000001 M for a solute with molar mass of 1000 g/mol. Reporting a concentration of 0.00000034 M is awkward. Reporting 0.34 ppm is not.

Normality (N) was common in older analytical chemistry. It’s like molarity but counts equivalents rather than moles. For H₂SO₄ in a neutralization, 1 mole provides 2 moles of H⁺, so a 1 M solution is 2 N. Most modern chemistry has moved away from normality, but you’ll still encounter it in older protocols.

Molarity is the right unit when the reaction happens in solution at a controlled temperature and you need to track moles directly. For everything else, check which concentration unit the formula or protocol actually requires.

What to do with the result

For reaction stoichiometry in solution: multiply molarity by volume (in litres) to recover moles. Moles × molar ratio from the balanced equation = moles of product or other reactant. Then convert to grams or back to a volume of solution as needed.

For titration calculations: at the equivalence point, moles of titrant = moles of analyte (adjusted for stoichiometry). M_titrant × V_titrant = M_analyte × V_analyte for a 1:1 reaction. Plug in 3 knowns, solve for the 4th.

For dilution: use C₁V₁ = C₂V₂. Your calculated molarity is C₁. Set the target concentration as C₂ and the target volume as V₂. Solve for V₁, which is how much of your stock solution to take.

For pH calculations: for strong acids, molarity equals hydrogen ion concentration directly. A 0.01 M HCl solution has [H⁺] = 0.01 mol/L, giving pH = 2. For weak acids, you’ll need the Ka and an equilibrium calculation, but molarity is still the starting point.

The bottom line

Molarity is what connects a mass on a balance to a reaction in a flask. Once you have it, everything else in solution chemistry follows: stoichiometry, dilution, titration, pH.

The formula is two steps compressed into one. Grams to moles via molar mass, then moles per litre. The calculator runs both steps and shows you the intermediate moles so you can catch any errors before they carry forward.

The one thing to get right every time: volume of solution, not volume of solvent, in the correct unit. Everything else is arithmetic.