How to Convert Protein Mass, Moles, and Molarity

Protein concentrations are often measured by mass, but binding assays, enzyme work, and stoichiometry usually need molar concentration. This guide shows how to move between mg/ml, µM, pmol, molecules, and mass using the protein molecular weight in kDa. Every example can be checked in the Protein Molarity Calculator.

The quick version

Protein molecular weight connects protein mass to moles. Since 1 Da equals 1 g/mol, a protein listed as 50 kDa has a molar mass of 50,000 g/mol. Once you know the molecular weight, the core relationship is moles = mass(g) ÷ MW(g/mol).

For concentration, a useful shortcut is: µM = mg/ml × 1000 ÷ kDa. This works because 1 mg/ml is the same mass concentration as 1 g/L.

Example 1: Convert 1 mg/ml of a 50 kDa protein to µM

This is one of the most common protein concentration conversions. Open Molar concentration from weight concentration and enter:

  • Protein molecular weight (MW): 50 kDa
  • Mass: 1 µg
  • Volume: 1 µl
  • Report concentration as: µM

Since 1 µg/µl is the same as 1 mg/ml, the result is 20 µM. Using the shortcut: 1 × 1000 ÷ 50 = 20 µM.

Example 2: Find how much protein is in a 10 µM assay tube

Say you have 100 µl of a 10 µM solution of a 50 kDa protein. Open Weight from molar concentration and enter:

  • Protein molecular weight (MW): 50 kDa
  • Concentration: 10 µM
  • Volume: 100 µl
  • Report mass as: µg

The amount of protein is 50 µg. This direction is useful when a protocol gives a molar concentration but you need to prepare or audit the actual mass in the tube.

Example 3: Convert protein mass to pmol and molecules

Suppose you have 1 µg of a 25 kDa protein. Open Weight to mole and enter:

  • Protein molecular weight (MW): 25 kDa
  • Mass: 1 µg
  • Report moles as: pmol

The result is 40 pmol, or about 2.409 × 1013 molecules. This is helpful when comparing protein amount to DNA, ligand, bead, or cell counts.

Step-by-step: using the Protein Molarity Calculator

  1. Open the Protein Molarity Calculator.
  2. Enter the protein molecular weight in kDa. If you only have a sequence, estimate it with the Protein pI and MW Calculator.
  3. Use the row that matches what you need: weight to moles, moles to weight, weight plus volume to molarity, or molarity plus volume to weight.
  4. Enter the known values and choose the units beside each field. Results update automatically when the row is complete.
  5. Use the row Clear button to reset one conversion, or Reset all to clear the whole calculator.

Common mistakes to avoid

  • Using kDa as if it were g/mol. A 50 kDa protein is 50,000 g/mol, not 50 g/mol. The calculator handles this conversion when you enter MW in kDa.
  • Forgetting that mg/ml equals g/L. This is why the shortcut µM = mg/ml × 1000 ÷ kDa works for proteins.
  • Using the wrong molecular weight. Tags, fusion partners, cleavage products, glycosylation, and multimeric states can change which MW is appropriate for your calculation.

Frequently asked questions

Should I use monomer MW or complex MW?

Use the molecular weight for the species your concentration refers to. For a purified monomer, use the monomer MW. For a stable dimer or antibody-like complex measured as the intact species, use the intact complex MW.

Can I convert absorbance or A280 directly to µM?

Not directly from molecular weight alone. A280 concentration depends on the extinction coefficient. Once you have a mass concentration such as mg/ml, this guide and calculator can convert it to molarity.

Is µg/µl the same as mg/ml?

Yes. 1 µg/µl equals 1 mg/ml, and both equal 1 g/L. That equivalence is useful for protein molarity conversions.

Try it yourself: