How to convert mg/mL and µg/mL to molarity
By Remetric Labs · Last reviewed October 26, 2026
Run the numbers in the Molarity Calculator.
Try the tool →Why you need molecular weight
mg/mL is mass per volume; molarity (mol/L) is particles per volume. Bridging them requires the mass per particle - the molecular weight in g/mol. Without it the conversion is undefined, which is why the Molarity Calculator asks for it explicitly.
The formula
Molarity (M) = (mass concentration in g/L) ÷ (molecular weight in g/mol). Convert first: 1 mg/mL = 1 g/L; 1 µg/mL = 0.001 g/L = 1 mg/L. Units cancel: (g/L) ÷ (g/mol) = mol/L.
Worked examples
NaCl (58.44 g/mol) at 5 mg/mL: 5 g/L ÷ 58.44 g/mol = 0.0856 mol/L = 85.6 mM. A 100 µg/mL protein at 50,000 g/mol: 0.1 g/L ÷ 50,000 = 2 × 10⁻⁶ M = 2 µM. Reverse check: 10 mM glucose (180.16 g/mol) = 0.01 × 180.16 = 1.8016 g/L = 1.80 mg/mL.
Limits
Use the correct species weight (anhydrous vs hydrate salts differ), and remember w/v % (1% = 10 mg/mL) before converting. Purity, pH-dependent speciation, and non-ideal solutions shift effective concentration - the arithmetic is exact, the chemistry needs judgment. For diluting the resulting stock see dilution ratios.