mg/mL to Molarity Calculator

How do you convert mg/mL to molarity?

Divide the mass concentration in mg/mL by the molecular weight in g/mol to obtain molarity in mol/L.

The relationship is simple because mg/mL and g/L have the same numerical value. A solution that contains 10 mg/mL also contains 10 g/L. Dividing 10 g/L by a molecular weight of 180.16 g/mol gives about 0.0555 mol/L, or 55.5 mM.

This conversion is especially useful when a supplier reports solubility or stock concentration in mg/mL but an assay protocol specifies the working concentration in mM or µM.

Why molecular weight changes the molarity of the same mg/mL concentration

At the same mass concentration, a lower-molecular-weight compound contains more molecules and therefore has a higher molarity than a higher-molecular-weight compound.

For example, 1 mg/mL of a 100 g/mol compound is 10 mM, while 1 mg/mL of a 500 g/mol compound is only 2 mM. The mass concentration is identical, but each mole of the heavier compound contains five times more mass.

This is why you cannot convert mg/mL to mM without molecular weight. A concentration such as 5 mg/mL does not have one universal molar value; the answer depends on what chemical is dissolved.

Which molecular weight should you use for salts, hydrates, and solvates?

Use the molecular weight that matches the exact chemical form represented by your weighed mass or stated stock concentration unless your protocol explicitly defines concentration as parent-compound equivalents.

A free base, hydrochloride salt, sodium salt, hydrate, and solvate can all have different formula weights. If the bottle contains a salt but you divide by the molecular weight of the free compound, the calculated molarity of the material actually weighed will be wrong.

Batch-specific water content or solvation can also matter for some reagents. Supplier tools and certificates of analysis may list a batch-specific molecular weight or correction. Follow that information when accurate stock preparation is important.

• Read the full reagent name, not only the parent molecule name.
• Check the molecular weight on the vial or certificate of analysis.
• Distinguish chemical-form molarity from active-moiety or parent-compound equivalents when a protocol makes that distinction.
• Account for purity or potency separately if the protocol requires an effective rather than nominal concentration.

Using mg/mL to mM conversion for stock solutions

Once mg/mL has been converted to mM, that molar concentration can be used as C1 in standard dilution calculations such as C1V1 = C2V2.

Suppose a compound stock is 25 mg/mL and its molecular weight is 250 g/mol. The stock is 0.100 M, or 100 mM. If you need a 10 µM working solution, you can then calculate the required dilution from a 100 mM stock.

Large dilution factors are often more accurate when prepared through an intermediate stock rather than by pipetting a very small volume directly into the final assay. Use your laboratory's validated dilution practice and pipette range.

Common mg/mL to molarity conversion mistakes

Most errors come from incorrect molecular weight, unit confusion, or forgetting that salts and hydrates may have a different formula weight from the parent compound.

• Do not multiply by 1,000 before dividing by molecular weight when your starting unit is already mg/mL; the mg/mL to g/L numerical conversion is 1:1.
• Do not enter molecular weight in kDa or mg/mmol without converting it to g/mol first.
• Do not use the parent drug or free-base molecular weight when the weighed material is a salt unless your method explicitly uses parent equivalents.
• Do not confuse mM with µM: 1 mM equals 1,000 µM.
• Do not infer solubility from molarity alone; a mathematically possible concentration may exceed the compound's physical solubility in the chosen solvent.
• For experimental stocks, confirm solvent compatibility, stability, and storage conditions in the reagent documentation.