Plasmid MapHow to calculate molarity
Molarity is the amount of solute in moles divided by the final solution volume in liters. If you know the solute's mass, first divide it by the molar mass to obtain moles. The combined equation is M = m / (MW × V), where mass is in grams, molar mass is in grams per mole, and volume is in liters.
Choose the quantity you need at the top of the calculator. It can find concentration from mass and volume, the mass required for a target concentration, or the final volume corresponding to a known mass and concentration. The selected unknown disappears from the input fields. Unit conversions happen before calculation, and the result shows the equation in grams, liters and moles so you can check it.
Worked example: 100 mM sodium chloride
Using a molar mass of 58.44 g/mol, 0.5844 g of sodium chloride represents 0.01000 mol. A final solution volume of 100 mL is 0.100 L. Dividing 0.01000 mol by 0.100 L gives 0.1000 mol/L, or 100 mM. These are the example values in the calculator. You can enter the molecular-weight value yourself or choose Calculate from a chemical formula and enter NaCl.
To work in the opposite direction, select Mass for a target molarity, enter 100 mM, 100 mL and 58.44 g/mol. The result is the same 0.5844 g. For a different compound, replace the molar mass before interpreting the result.
Final volume and compound identity matter
Final solution volume includes the solute. Adding a solid to 100 mL of solvent is not the same as preparing a final volume of 100 mL. Enter the final volume used by your preparation method.
Use the molar mass of the exact chemical form on the container, including any waters of hydration. Choose a known numeric value or enter its chemical formula. The formula option uses the same CIAAW 2024 abridged atomic weights and grouped-atom parser as the molar mass calculator, which shows the full element breakdown. Calculations retain the unrounded formula mass internally.
Formula input supports parentheses, brackets and middle-dot hydrates such as CuSO4·5H2O. It does not infer a compound's identity, purity or hydrate from a name, support isotope/charge notation, or convert liquid volume into mass using density. Display rounding does not determine the precision of your measurements.
Preparing a working solution from a stock
If you already have a stock solution with a known concentration, use the dilution calculator. That calculation conserves the amount of solute and gives the stock volume needed for a final concentration. It does not require the molar mass when both concentrations use compatible units.
The definition and dilution relationship are described in OpenStax Chemistry 2e, Molarity. Calculations run locally; Copy calculation and Save calculation preserve the current inputs, equation and assumptions.