How to Calculate Molarity From Grams and Volume
A clean lab-style walkthrough of molarity: grams to moles, milliliters to liters, concentration units, and common mistakes.
By CalciTools Editorial ·
Molarity is one of the first chemistry formulas that feels easy until the units get mixed. The calculation itself is short: moles divided by liters. Most mistakes happen one step earlier, when grams have not been converted to moles or milliliters have not been converted to liters.
The molarity formula
If you are starting with grams, you need one more formula first.
Example: 5.84 g NaCl in 1.00 L solution
- Find molar mass: NaCl is about 58.44 g/mol.
- Convert grams to moles: 5.84 / 58.44 = 0.0999 mol.
- Use final volume in liters: 1.00 L.
- Calculate molarity: 0.0999 mol / 1.00 L = 0.0999 M.
Rounded to sensible lab precision, that is about 0.100 M NaCl.
What if the volume is in milliliters?
Convert milliliters to liters before dividing. A 250 mL solution is 0.250 L, not 250 L. This one unit slip changes the answer by a factor of 1,000.
| Molar unit | Meaning |
|---|---|
| M | moles per liter |
| mM | millimoles per liter, or M x 1,000 |
| uM | micromoles per liter, or M x 1,000,000 |
| nM | nanomoles per liter, or M x 1,000,000,000 |
Common mistakes to check before recording the answer
- Using solvent volume instead of final solution volume.
- Forgetting to convert mL to L.
- Using molecular weight in mg/mmol without adjusting units.
- Reporting too many significant figures.
- Confusing molarity with molality, which uses kilograms of solvent.
Try it with your numbers
Enter mass, molar mass, and solution volume to convert across M, mM, uM, and nM.
Open the Chemical Solution Molarity Calculator →