Molarity is the standard way chemists describe how concentrated a solution is. It tells you how many moles of a dissolved substance (the solute) are present in each litre of solution, and it is usually written as mol/L or simply M. Our free molarity calculator works in both directions: enter an amount in moles and a volume to get the concentration, or enter a mass in grams together with the molar mass to find out how many moles you have and what concentration they will make.
Whether you are preparing a buffer in the lab, checking homework for a chemistry class, or scaling a recipe for a practical experiment, an accurate molarity calculation saves time and prevents wasted reagents. Type in your numbers, choose the volume unit, and the result appears straight away with a short preparation note you can follow at the bench.
Unlike a general-purpose calculator, this tool is built specifically for concentration work. It accepts volumes in litres or millilitres, it checks that every value makes sense before it calculates, and it shows the moles and volume used so you can double-check your working. If you need wider chemistry help, browse our chemistry calculators hub or use the scientific calculator for general maths.
Molarity Calculator
Work out the concentration of a solution in moles per litre (M). Choose how you want to calculate it, enter your values, and get an instant result.
Result
How to Use the Molarity Calculator
- Choose a calculation mode: “From moles and volume“ if you know the amount of solute in moles, or “From mass and molar mass“ if you weighed the solute in grams.
- For the moles mode, enter the amount of solute in moles (for example, 0.5).
- For the mass mode, enter the mass in grams and the molar mass in g/mol (you can look molar masses up in the table below).
- Enter the final volume of the solution and pick the unit, litres (L) or millilitres (mL).
- Click Calculate to see the molarity, the moles used, the volume in litres, and a preparation note describing how to make the solution.
- Click Reset to clear the form and start a new calculation.
The Molarity Formula
Molarity (M) = moles of solute ÷ volume of solution in litres
moles of solute = mass in grams ÷ molar mass in g/mol
To convert a volume in millilitres to litres, divide by 1,000 (for example, 500 mL = 0.5 L). The calculator handles this conversion for you when you select the mL unit.
Rearranging the formula is just as useful. If you know the molarity you want and the volume you need, the moles required are molarity × volume in litres. To find the mass to weigh out, multiply those moles by the molar mass.
Worked Example
Suppose you dissolve 58.44 g of sodium chloride (NaCl) in water and make the solution up to exactly 1 litre. The molar mass of NaCl is 58.44 g/mol, so the amount of solute is 58.44 ÷ 58.44 = 1 mol. Molarity = 1 mol ÷ 1 L = 1 M. Using the calculator: choose the mass and molar mass mode, enter mass 58.44, molar mass 58.44, volume 1 L, and the result is 1.0000 M.
A second quick check: 0.5 mol of solute in 2 L of solution gives 0.5 ÷ 2 = 0.25 M. Enter moles 0.5 and volume 2 L in the first mode and the calculator returns 0.2500 M.
Common Lab Concentrations
| Solute | Molar mass (g/mol) | Typical target | Mass for 1 L |
|---|---|---|---|
| Sodium chloride (NaCl) | 58.44 | 0.9% saline is about 0.154 M | 9.0 g for 0.154 M |
| Glucose (C6H12O6) | 180.16 | 0.1 M | 18.02 g |
Rules of thumb for common targets with NaCl (58.44 g/mol): for 1 L of solution you need about 5.84 g for 0.1 M, 29.22 g for 0.5 M, 58.44 g for 1 M, and 116.88 g for 2 M. With glucose (180.16 g/mol), 1 L at 0.1 M needs 18.02 g, 0.5 M needs 90.08 g, 1 M needs 180.16 g, and 2 M needs 360.32 g.
Tips for Accurate Solutions
- Always make the solution up to the final volume in a volumetric flask rather than adding a full litre of water, because dissolving a solute changes the total volume slightly.
- Check whether your solute is a hydrate (for example, CuSO4·5H2O). Hydrates have a higher molar mass than the anhydrous form, and using the wrong value is the most common source of error.
- Molarity changes a little with temperature because liquids expand and contract, so record the temperature for very precise work.
- Weigh solids on a calibrated balance and tare the container so you measure only the solute.
Related Tools
For proportional reasoning practice, try the percentage calculator. If you work with peptides, the peptide calculator estimates molecular weight from a sequence. For the underlying theory, the LibreTexts chapter on concentration is a reliable reference: chem.libretexts.org.
Frequently Asked Questions
What is molarity in simple terms?
Molarity is the number of moles of a substance dissolved in one litre of solution. A 1 M solution contains exactly one mole of solute per litre, so molarity is a concentration: the higher the molarity, the more concentrated the solution.
How do I convert mL to L in a molarity calculation?
Divide the millilitres by 1,000. For example, 250 mL is 0.25 L. In this calculator you can simply select the mL unit and the conversion is done automatically before the molarity is worked out.
What if I only know the mass?
Use the second mode of the calculator. Enter the mass in grams and the molar mass of the substance in g/mol. The tool converts the mass to moles by dividing, then divides by the volume in litres to give the molarity.
Is molarity the same as molality?
No. Molarity uses the volume of the whole solution in litres, while molality uses the mass of the solvent in kilograms. Molality does not change with temperature, but molarity is far more common in everyday lab work because volumes are easier to measure.
What does 0.25 M mean?
It means each litre of the solution contains 0.25 moles of the solute. Two litres of a 0.25 M solution therefore contain 0.5 moles in total, and 500 mL contains 0.125 moles.