Enter protein molecular weight in kDa (kilodalton) to convert mass to moles, calculate molar concentration (µM, nM), and convert between concentration units.
Results update automatically when MW and row inputs are filled.
Protein work often moves back and forth between mass, moles, molecule count, and molar concentration. This calculator helps connect those units quickly when you know the protein molecular weight, whether you are preparing a stock solution, planning an assay, comparing yields, or converting a concentration from mg/ml into µM for experimental design.
These are the quick conversions people often need when moving from protein mass concentration to molar concentration, or from a target molarity back to the amount of protein in a tube.
For a deeper explanation of these shortcuts, see the Protein Molarity Guide.
Converts a known mass of protein into the equivalent number of moles (or molecules) using the formula:
n (mol) = m (g) ÷ MW (g·mol-1)
Suppose you purified 1 mg of E. coli RpsA protein, which has a molecular weight of 61.2 kDa. How many moles does that correspond to?
n = 1 × 10-3 g ÷ 61,200 g·mol-1
n = 1.634 × 10-8 mol
n = 16.34 nmol
In other words, 1 mg of E. coli RpsA is equivalent to 16.34 nmol of protein.
Converts a known molar amount of protein into the equivalent mass using the same molecular weight.
m (g) = n (mol) × MW (g·mol-1)
Suppose you have 1 nmol of E. coli RpsA protein with a molecular weight of 61.2 kDa. What mass does that correspond to?
m = 1 × 10-9 mol × 61,200 g·mol-1
m = 6.12 × 10-5 g
m = 61.2 µg
In other words, 1 nmol of E. coli RpsA is equivalent to 61.2 µg of protein.
Converts a protein mass in a known volume into molar concentration.
concentration (M) = moles ÷ volume (L)
Suppose you have 1 µg of E. coli RpsA protein in 1 µl, and the protein molecular weight is 61.2 kDa. What molar concentration is that?
n = 1 × 10-6 g ÷ 61,200 g·mol-1
n = 1.634 × 10-11 mol
concentration = 1.634 × 10-11 mol ÷ 1 × 10-6 L
concentration = 16.34 µM
In other words, 1 µg of E. coli RpsA in 1 µl is 16.34 µM.
Converts a molar concentration and volume into the protein mass needed.
m (g) = concentration (M) × volume (L) × MW (g·mol-1)
Suppose you want 1 µl of 1 µM E. coli RpsA protein, and the protein molecular weight is 61.2 kDa. What mass of protein is present?
m = 1 × 10-6 mol/L × 1 × 10-6 L × 61,200 g·mol-1
m = 6.12 × 10-8 g
m = 6.12 × 10-2 µg
In other words, 1 µl of 1 µM E. coli RpsA contains 6.12 × 10-2 µg of protein.
The dalton (Da) is the standard unit of molecular mass in biochemistry. Because proteins are large, their molecular weight is usually expressed in kilodaltons (kDa). The key relationship is simple: 1 kDa = 1000 g/mol. This means a protein's molar mass in g/mol is numerically equal to its molecular weight in Da — so a 50 kDa protein has a molar mass of 50,000 g/mol. This calculator uses that relationship to convert between mass, moles, and concentration.