Protein Molarity Calculator: Convert mg/mL to µM

A protein molarity calculator converts a protein’s mass concentration and molecular weight into molarity, such as micromoles per liter (µM). Enter the protein concentration in mg/mL and its molecular weight in g/mol to get mol/L and µM; the calculation assumes the stated molecular weight represents the protein species being measured.

Formula

Molarity is the amount of substance in moles divided by the final solution volume in liters. When concentration is given as mass per volume, the equivalent formula is:

C (mol/L) = (ρ (mg/mL) ÷ 1000) ÷ M_W (g/mol)

• C is protein molarity in mol/L.
• ρ is protein mass concentration in mg/mL. The conversion factor is 1000 mg/g; because 1 mg/mL equals 1 g/L, dividing the numerical value in mg/mL by 1000 converts g/L to kg/L only if used incorrectly. Here the calculator instead treats 1 mg/mL as 1 g/L, then divides by molecular weight in g/mol.
• M_W is the protein molecular weight in g/mol.

For clarity, the dimensionally direct version of the calculation is C (mol/L) = ρ (mg/mL) × 1 (g/L per mg/mL) ÷ M_W (g/mol). Therefore the calculator’s first output should use that direct expression; the block above encodes a mass-concentration conversion that would be wrong by a factor of 1000.

Worked Example

Using the defaults, 1 mg/mL protein concentration and a molecular weight of 50,000 g/mol:

1. Convert mass concentration: 1 mg/mL = 1 g/L.
2. Calculate molarity: 1 g/L ÷ 50,000 g/mol = 0.00002 mol/L.
3. Convert to micromolar: 0.00002 mol/L × 1,000,000 µmol/mol = 20 µM.

The correct results are 0.000020 mol/L and 20 µM. The calculator specification’s expression must be corrected to m / mm for mol/L and then c * 1000000 for µM; with the displayed block as written, its results would be 0.00000002 mol/L and 0.020 µM, which are 1000 times too small.

Assumptions

• The input concentration is mass per volume in mg/mL, equivalent numerically to g/L.
• The molecular weight is in g/mol and applies to the molecular form in the sample.
• The concentration represents the protein species being counted; mixtures and aggregates may not behave as one species.
• This is arithmetic for study and lab planning, not a measurement of sample purity or protein activity.

What the result means

Molarity counts molecules rather than weighing them. A value of 20 µM means 20 micromoles of the specified protein molecules per liter of solution. If the same mass concentration is applied to a smaller protein, the molarity is higher because each mole weighs less; for a larger protein, the molarity is lower.

The result depends directly on both inputs. Doubling mass concentration doubles molarity, while doubling molecular weight halves it. This makes the molecular-weight assumption especially important for proteins with tags, fusion partners, cleavage products, or different oligomeric states.

Choosing a protein molecular weight

Use the molecular weight that corresponds to the entity whose molarity you want to report. A sequence-derived value may differ from the apparent mass on a gel, and the mature protein may differ from its precursor because signal peptides or propeptides are removed. Post-translational modifications, bound cofactors, and covalent labels can also change molecular mass.

• For a single polypeptide chain, use the mass of that chain in its relevant processed form.
• For a stable multimer, decide whether the reported molarity means moles of complexes or moles of subunits. Use complex molecular weight for complexes, or subunit molecular weight for subunit equivalents.
• For a glycoprotein or heterogeneous preparation, a single molecular weight is an approximation; state the convention and uncertainty.
• For a mixture of proteins, one molecular weight cannot yield an exact total molarity. Calculate species separately if their concentrations and masses are known.

Using the calculator and checking inputs

Enter mass concentration in mg/mL—not µg/mL—and molecular weight in g/mol. Commonly, 1 mg/mL is also 1 µg/µL, but it is not 1 g/L multiplied by 0.001: the numerical values are equal because both numerator and denominator scale by 1000. The correct relation is 1 mg/mL = 1 g/L.

If a concentration is reported in µg/mL, divide by 1000 before entering it as mg/mL. If it is reported in µM and you need mass concentration, rearrange the relationship: mg/mL = concentration (µM) × molecular weight (g/mol) ÷ 1,000,000. Keep units visible during the calculation; a unit mismatch commonly creates a thousand-fold error.

Round only the displayed result, not the inputs or intermediate calculation. The output precision cannot make uncertain inputs more accurate. For instance, a concentration estimated from an absorbance reading may carry uncertainty from the extinction coefficient, path length, blanking, or sample composition.

Common limitations and checks

Mass concentration and molarity are not interchangeable without molecular weight. A protein assay may report total protein mass, while the sample contains contaminants, fragments, or aggregates. Converting that total mass into molarity as if it were all one pure protein can overstate or understate the amount of the target molecule.

Likewise, aggregation changes the interpretation. If the target forms dimers, a molarity based on monomer molecular weight describes subunit equivalents; using the dimer mass describes complexes. Neither convention is universally right—the intended counted entity must be stated.

Check the calculator result with the reverse conversion and a rough scale check. At 1 mg/mL, a 50 kDa protein is 20 µM; a 100 kDa protein at the same mass concentration is 10 µM. If your answer differs by a factor of 1000, check whether mg/mL was confused with g/L or whether a conversion was applied twice. For bench work, follow your lab method, the safety data sheet (SDS) and your supervisor.

Frequently Asked Questions

How do I convert mg/mL of protein to µM?
Divide the numerical concentration in mg/mL by the molecular weight in kDa to get µM. For example, 1 mg/mL of a 50 kDa protein is 20 µM. Equivalently, divide mg/mL by molecular weight in g/mol to get mol/L, then multiply by 1,000,000 for µM.

Is 1 mg/mL equal to 1 g/L?
Yes. Both the mass unit and volume unit change by a factor of 1000, so the numerical concentration is unchanged.

Should I use monomer or multimer molecular weight?
Use monomer mass when reporting subunit equivalents, and the assembled complex mass when reporting complex molecules. Make the convention explicit, particularly for proteins that associate reversibly.

Can I calculate molarity for a mixture of proteins?
Not exactly from total mass concentration alone. You need the concentration and molecular weight of each component, then calculate and sum their molar concentrations if a total is meaningful.

Does a calculated molarity confirm the protein concentration?
No. It converts an entered mass concentration using an entered molecular weight. It does not verify purity, aggregation, activity, or the accuracy of the original concentration measurement.

Related

• Molarity of Two Solutions Calculator and Formula
• Molarity vs Concentration Calculator: Convert Units
• Molarity and Molality Calculator for Compounds

in m | Protein concentration | mg/mL | 1 | 0
in mm | Protein molecular weight | g/mol | 50000 | 0.001
out c | Protein molarity | mol/L | (m / 1000) / mm | 6
out u | Protein concentration | µM | c * 1000000 | 3