Molarity calculator
Work out how much to weigh for a solution, what concentration a solution is, or convert between mg/mL and molar units.
mass = concentration × volume × molar mass
What is molarity?
Molarity is the concentration of a solution in moles of solute per litre of solution, written mol/L or M. A 1 M solution holds one mole in every litre of the finished solution. Smaller units are common in the lab: 1 M = 1,000 mM = 1,000,000 µM.
The molarity formula
Molarity = moles ÷ volume in litres, and moles = mass ÷ molar mass. Put together, the mass you need is mass (g) = molarity (M) × volume (L) × molar mass (g/mol). For 500 mL of 150 mM sodium chloride (58.44 g/mol): 0.150 × 0.500 × 58.44 = 4.383 g. The calculator rearranges the same formula to find the concentration or the volume.
Converting mg/mL to mM, and back
A concentration in mg/mL is the same number in g/L, so mM = mg/mL ÷ molar mass × 1,000. For a compound of 500 g/mol, 1 mg/mL is 2 mM. In reverse, mg/mL = mM × molar mass ÷ 1,000. Between two molar units, or two mass units, it is only a change of scale and no molar mass is needed. 1% w/v is 1 g per 100 mL, or 10 mg/mL.
Correcting for net content
A weighed powder is not always all compound. Peptides carry water and counter-ions such as TFA or acetate, and a COA may give the net peptide content as a percentage. If you enter it, the calculator works from the compound itself: it shows the powder to weigh, or the true concentration of a solution made from a weighed mass.
Results are given to four significant figures; report them to no more precision than your balance and glassware allow. The guide to molarity, mg/mL and dilutions has more worked examples.