Molarity Calculator
Calculate the molarity of a solution, the mass of solute required, or the volume of solvent needed. Enter any two of the three quantities (plus molecular weight) to solve for the third.
Calculator
Solve for
Enter values and calculate to see the result
Result
c = m / (Mw × V)
Molarity (c) equals mass (m) divided by the product of molecular weight (Mw) and volume (V). Rearrange to solve for any unknown.
About this calculation
What this is
Molarity is the amount of solute per litre of solution (M = mol/L). Combined with the mole–mass relationship (n = mass ÷ molecular weight), it lets you convert between what you weigh on a balance and the concentration your protocol needs.
Why it is used
Buffers, media, and reagent stocks are specified in molar units, but solutes are dispensed by mass — every solution preparation is a molarity calculation in one direction or another.
Why this calculator exists
The three quantities (molarity, mass, volume) plus molecular weight and unit prefixes make a small web of conversions. This calculator solves for whichever one you need and keeps the units explicit at every field.
Key assumptions
- The molecular weight is for the form you are actually weighing (anhydrous vs hydrated salts differ).
- The entered volume is the final solution volume, not the solvent volume added.
- The solute fully dissolves and behaves ideally at the target concentration.
Limitations
- At high concentrations, solution volume is not strictly additive and molarity deviates from the ideal calculation.
- It does not adjust for hydrate water or counterions — enter the correct molecular weight for your bottle.
- Temperature-dependent volume changes (significant above ~1 M or outside room temperature) are not modelled.
What this calculates
- Molarity
- Molar concentration of the solution.
- Mass of solute
- Mass of solute required.
- Volume
- Volume of solution.
- Moles of solute mol
- Number of moles of solute in the solution.
Frequently asked questions
How do I use the molarity calculator?
First choose what to solve for — molarity, mass of solute, or volume — under "Solve for". Enter the molecular weight of your solute in g/mol, then fill in any two of the remaining three fields (mass, molarity, volume) with their units. The calculator computes the third quantity and also reports the number of moles of solute.
What is the molarity formula this calculator uses?
Molarity is moles of solute per litre of solution (M = mol/L), and moles come from mass divided by molecular weight (n = mass / MW). Combining them gives M = mass / (MW x volume in L), which the tool rearranges depending on your mode: mass = M x MW x volume, or volume = mass / (MW x M). For example, 58.44 g NaCl (MW 58.44) made up to 1 L gives 1 M.
Which mass, concentration, and volume units are supported?
Mass can be entered in g, mg, or µg; molarity in M, mM, µM, or nM; volume in L, mL, or µL. Each field has its own unit selector, and the calculator reconciles the prefixes internally, so you can weigh in milligrams and target a micromolar concentration without manual conversion.
Why does the calculator always ask for the molecular weight?
Any conversion between mass and molar amount runs through the moles bridge n = mass / MW, so molecular weight is required in every mode. Enter the value for the chemical form you are actually weighing — anhydrous and hydrated salts differ, and the calculator does not adjust for hydrate water or counterions. Using the wrong bottle form is the most common source of concentrated or under-concentrated buffers.
Is the volume the solvent I add or the final solution volume?
The volume field is the final volume of the solution, not the amount of solvent to add. In practice you dissolve the solute in less solvent and then top up to the target volume, since the dissolved solute itself takes up space. The calculator also assumes the solute fully dissolves and behaves ideally — at high concentrations, real volumes deviate from the calculation.
For research and educational use only. Not for clinical or diagnostic decisions. Always verify calculations independently before use in critical applications.