Protein Molarity Calculator
Convert protein mass concentration to molarity (M, mM, µM, nM) using molecular weight.
Reverse Calculator (Target molarity → Required concentration)
Useful for preparing buffer stocks and dilution plans.
What is protein molarity?
Protein molarity is the number of moles of a protein per liter of solution (mol/L). Unlike small molecules, proteins are often reported by mass concentration (such as mg/mL), so converting to molarity requires the protein’s molecular weight.
Core formula
The conversion is straightforward:
Molarity (M) = concentration (g/L) ÷ molecular weight (g/mol)
From there:
- mM = M × 1,000
- µM = M × 1,000,000
- nM = M × 1,000,000,000
How to use this calculator
1) Enter your protein concentration
Use the concentration exactly as measured in your assay or stock record, then pick the matching unit.
2) Enter molecular weight
Use the expected molecular weight for the specific construct in solution. If you know the value in kilodaltons, select kDa. If you have g/mol, select Da.
3) Calculate and review outputs
The tool reports molarity across common scales (M, mM, µM, nM) so you can quickly choose the unit that makes sense for your experiment.
Worked example
If your protein is at 2 mg/mL and has molecular weight 50 kDa:
- 2 mg/mL = 2 g/L
- 50 kDa = 50,000 g/mol
- M = 2 / 50,000 = 0.00004 M
- So the concentration is 40 µM
Why this matters in lab workflows
Molarity is often the unit needed for binding experiments, enzyme kinetics, stoichiometric planning, and dosing calculations. Two protein samples with the same mg/mL value can represent very different molar concentrations if their molecular weights differ.
Common mistakes to avoid
- Mixing units: mg/mL and mg/L differ by a factor of 1,000.
- Wrong molecular weight basis: monomer vs. multimer assumptions can change molarity dramatically.
- Ignoring tags or fusions: His-tags, GST, Fc fusions, or linkers affect total molecular weight.
- Over-rounding: excessive rounding early can create avoidable prep errors.
Quick unit notes
- 1 mg/mL = 1 g/L
- 1 µg/mL = 0.001 g/L
- 1 kDa = 1,000 Da
- 1 Da is numerically equivalent to 1 g/mol for molecular weight usage
Final tip
For best reproducibility, report both mass concentration and molarity in your notebook. This makes it much easier to compare results across proteins, constructs, and assay conditions.