Molarity Calculator: Mass, Volume & Molecular Weight

This molarity calculator solves the equation $M = \frac{m}{MW \times V}$ for any of its four variables: molarity (M), mass (m), molecular weight (MW), or volume (V). Select which variable to calculate, fill in the other three, and the result appears instantly. A built-in compound search lets you look up common reagents. NaCl, HCl, NaOH, glucose, and hundreds more, to auto-fill the molecular weight field. The tool accepts a wide range of units: concentration in M, mM, µM, nM, mol/m³, g/L, mg/L, ppm, or ppb; mass in grams, milligrams, kilograms, or pounds; and volume in liters, milliliters, cubic meters, gallons, and more. All conversions are handled internally, so you can work in whatever units your protocol uses. Results are suitable for solution preparation, dilution planning, and stoichiometry.

The core equation is $M = \frac{m}{MW \times V}$, where M is molarity in mol/L, m is solute mass in grams, MW is molecular weight in g/mol, and V is solution volume in liters. Rearranging gives three additional forms: $m = M \times MW \times V$ for mass, $V = \frac{m}{M \times MW}$ for volume, and $MW = \frac{m}{M \times V}$ for molecular weight. As a worked example, suppose you dissolve 5.844 g of NaCl (MW = 58.44 g/mol) in enough water to make 1.000 L of solution. Molarity = 5.844 / (58.44 × 1.000) = 0.1000 M. If you instead need to know how much NaCl to weigh for a 0.5 M solution in 250 mL, rearrange: m = 0.5 × 58.44 × 0.250 = 7.305 g. The calculator performs this algebra for you, select the unknown variable and enter the rest.

Start by choosing what to solve for. Molarity, Mass, Volume, or Molecular Weight, using the Calculate For selector. Next, search the compound dropdown to auto-fill molecular weight (e.g., type "NaCl" and select sodium chloride, MW 58.44 g/mol), or enter a custom MW manually. Fill in the remaining known values in any supported unit: the calculator converts to base SI internally. For example, to find the molarity of a solution made from 11.69 g of NaCl in 500 mL of water, select Molarity, enter MW 58.44, mass 11.69 g, and volume 500 mL. The result, 0.2 M, appears instantly. You can switch concentration units (M, mM, µM, ppm, g/L) and the displayed result updates without recalculating. Use the output directly for dilution planning, reagent preparation, or as input for stoichiometric calculations.

The defining equation is $M = \frac{m}{MW \times V}$. You need mass $m$ in grams, molecular weight $MW$ in g/mol, and volume $V$ in liters. Rearranging: mass $m = M \times MW \times V$; volume $V = \frac{m}{M \times MW}$; molecular weight $MW = \frac{m}{M \times V}$. Toggle "Calculate For" to solve for whichever variable you don't know.

Molarity counts moles per liter of the whole mixture; molality counts moles per kilogram of solvent. Add more solute and the total volume grows, molarity drops a bit. Molality, based on solvent mass, doesn't care. For boiling-point elevation and freezing-point depression, textbooks use molality for that reason. In dilute aqueous work, the two are close enough that many lab protocols don't bother distinguishing.