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 mass in kDa. Every example can be checked in the Protein Molarity Calculator.

The quick version

Molecular mass (often informally called molecular weight) connects protein mass to amount of substance. For routine biochemical calculations, the numerical value of molecular mass in Da corresponds approximately to the molar mass in g·mol⁻¹. Thus, a 50 kDa protein has an approximate molar mass of 50,000 g·mol⁻¹. Once you know the molar mass, the core relationship is amount in moles = mass (g) ÷ molar mass (g·mol⁻¹).

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 Mass concentration to molar concentration and enter:

  • Protein molecular mass: 50 kDa
  • Mass concentration: 1 mg/mL
  • 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 Molar concentration to protein mass and enter:

  • Protein molecular mass: 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 Protein mass to moles and enter:

  • Protein molecular mass: 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 mass in kDa. If you only have a sequence, estimate it with the Protein Properties Calculator.
  3. Use the row that matches what you need: mass concentration to molarity, mass and volume to molarity, moles to mass, or mass to moles.
  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 a molar mass. A 50 kDa protein has an approximate molar mass of 50,000 g·mol⁻¹, not 50 g·mol⁻¹. The calculator uses this routine biochemical approximation when you enter molecular mass 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 mass. Tags, fusion partners, cleavage products, glycosylation, and multimeric states can change which molecular mass is appropriate for your calculation.

Frequently asked questions

Should I use monomer or complex molecular mass?

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

Can I convert absorbance or A280 directly to µM?

Not directly from molecular mass 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: