Protein Concentration Calculator: mg/mL to µM From Molecular Weight

This calculator converts a protein concentration in mg/mL into a molar concentration. Enter the concentration, the molecular weight in kilodaltons (kDa) and a volume in mL; you get the concentration in µM and nM and the nanomoles of protein in that volume.

Formula

µM = c (mg/mL) ÷ MW (kDa) × 1000; nmol = µM × V (mL).

• c: mass concentration in mg/mL (equal to g/L)
• MW: molecular weight in kDa; 1 kDa = 1000 g/mol
• g/L ÷ g/mol = mol/L, so mg/mL ÷ kDa gives mmol/L, and × 1000 gives µM
• V: volume in mL; µmol/L × mL = nmol

Worked Example

A protein of 66.5 kDa at 1.5 mg/mL; 2 mL of the solution.

1. 1.5 ÷ 66.5 = 0.022556 mM
2. × 1000 = 22.556 µM, or 22,556.4 nM
3. Amount: 22.556 × 2 = 45.11 nmol

Assumptions

• The molecular weight is that of the protein as it is in solution: one chain, or the whole complex if you want the concentration of complexes.
• The mass concentration is accurate; values from absorbance depend on the extinction coefficient used.
• The protein is pure; other proteins in the sample are counted as if they were this one.

How to read the result

The molar concentration counts protein molecules, which is what binding and enzyme experiments compare. A larger protein at the same mg/mL gives fewer molecules, so a lower µM. The molecular weight can be estimated from the sequence (about 110 Da per amino acid on average) or measured. For the molar mass of small molecules, use the molar mass calculator.

Limits and common mistakes

• Entering the molecular weight in Da instead of kDa gives a result 1000 times too small.
• Using the monomer weight for a protein that forms a dimer doubles the concentration of the real particles.
• Mixing µg/mL and mg/mL: 1 mg/mL is 1000 µg/mL.

Frequently Asked Questions

How do I convert mg/mL to µM for a protein?

Divide the mg/mL by the molecular weight in kDa and multiply by 1000.

What is 1.5 mg/mL of a 66.5 kDa protein in µM?

About 22.556 µM, or 22,556.4 nM.

What is a kilodalton?

A unit of molecular mass equal to 1000 daltons, which corresponds to 1000 g/mol.

Does it work for DNA?

The maths is the same, but use the DNA molarity calculator for double-stranded DNA, which works from length in base pairs.

in c | Protein concentration | mg/mL | 1.5 | 0
in kda | Molecular weight | kDa | 66.5 | 0.001
in v | Volume | mL | 2 | 0
out um | Molar concentration | µM | c / kda * 1000 | 3
out nm | Molar concentration in nM | nM | um * 1000 | 1
out nmol | Amount in the volume | nmol | um * v | 2