Enter the protein molecular mass in kDa to convert between mass concentration (such as mg/mL or µg/µL) and molar concentration (µM or nM), or between protein mass, moles, and molecule count.
Enter the protein molecular mass first. Each conversion row calculates independently and updates automatically.
For example, 1 mg/mL of a 50 kDa protein is 20 µM.
µg/µL and mg/mL have the same numerical value, so use the same conversion. A 2 µg/µL solution of a 50 kDa protein is 40 µM.
Divide the concentration in µg/mL by the molecular mass in kDa, then multiply by 1,000 to obtain nM. For example, 1 µg/mL of a 50 kDa protein is 20 nM.
Divide protein mass in µg by molecular mass in kDa, then multiply by 1,000,000 to obtain pmol. Thus, 1 µg of a 50 kDa protein is 20 pmol.
To prepare a protein solution, multiply concentration in µM by volume in µL and molecular mass in kDa, then divide by 1,000 to obtain µg. For example, 10 µM in 100 µL for a 50 kDa protein requires 50 µg.
The calculator uses protein molecular mass to connect mass, amount in moles, molecule count, and molar concentration. Use the first conversion for common mg/mL to µM, µg/µL to µM, or µg/mL to nM calculations. If you have a mass and volume rather than a prepared concentration, use the mass-and-volume conversion. The remaining sections find the mass in a known volume or convert protein mass to moles and back. Each conversion is independent, so only the values required for that section affect its result.
The same 50 kDa protein is used below so the relationships between all direct mass-concentration conversion and the remaining calculations are easy to compare.
| Conversion | Example result |
|---|---|
| Mass concentration → molar concentration | 1 mg/mL = 20 µM |
| Molar concentration → protein mass | 100 µL at 10 µM contains 50 µg |
| Protein mass → moles | 1 µg = 20 pmol ≈ 1.204 × 1013 molecules |
| Moles → protein mass | 1 nmol = 50 µg |
Formula: molar concentration = mass concentration in g/L ÷ molar mass in g·mol⁻¹.
A 1 mg/mL solution contains 1 g/L. For a protein with a molecular mass of 50 kDa, the corresponding molar mass is approximately 50,000 g·mol⁻¹.
1 g/L ÷ 50,000 g·mol⁻¹ = 2 × 10⁻⁵ mol/L = 20 µM
Formula: protein mass = molar concentration × volume × molar mass.
10 µM × 100 µL × 50,000 g·mol⁻¹ = 50 µg
Formula: amount in moles = mass ÷ molar mass.
1 µg ÷ 50,000 g·mol⁻¹ = 20 pmol ≈ 1.204 × 10¹³ molecules
Formula: protein mass = amount in moles × molar mass.
1 nmol × 50,000 g·mol⁻¹ = 50 µg
The molecule count refers to the molecular species represented by the entered molecular mass. Use the intact-complex mass when the relevant entity is an oligomer or stable molecular complex.
For detailed examples, common mistakes, and FAQs, see the Protein Molarity Guide.
Molecular mass is expressed in Da or kDa, whereas molar mass is expressed in g·mol⁻¹. Relative molecular mass—often called protein molecular weight in informal laboratory usage—is a dimensionless ratio.
For routine biochemical calculations, a protein molecular mass in Da has approximately the same numerical value as its molar mass in g·mol⁻¹. Therefore, a 50 kDa protein has a molar mass of approximately 50,000 g·mol⁻¹. The calculator uses the standard approximation molar mass in g·mol⁻¹ = molecular mass in kDa × 1,000; the post-2019 SI difference is negligible relative to ordinary protein molecular-mass uncertainty.
Molarity is the familiar name for amount concentration, which is amount in moles divided by solution volume. Mass concentration is protein mass divided by solution volume.
No. kDa describes molecular mass, not amount of substance, so a sample mass is also required. An 80 kDa protein has an approximate molar mass of 80,000 g·mol⁻¹. For 1 µg of that protein, the amount is 12.5 pmol and the molecule count is approximately 7.53 × 1012.
No. kDa alone cannot determine molarity because mass concentration is also required. At 1 mg/mL, which is 1 g/L, an 80 kDa protein has a molar concentration of 12.5 µM.
Terminology follows the IUPAC definitions of mass concentration, amount concentration, and relative molecular mass, together with BIPM guidance on the mole.