Convert between the mass, the amount and the concentration of a protein. Enter its molecular weight in kDa, pick what you know and read off moles, molecules or molarity in the unit you need.
Enter the molecular weight, choose a conversion and the result updates as you type.
The amount of a substance in moles is its mass divided by its molar mass. For a protein the molar mass in g/mol is the molecular weight in daltons, so a 50 kDa protein weighs 50 000 g per mole. One milligram of it is therefore 0.001 ÷ 50 000 = 2 × 10⁻⁸ mol, or 20 nmol.
One mole of anything contains Avogadro’s number of particles, 6.022 × 10²³. Multiplying the amount by it gives the number of molecules: those 20 nmol of a 50 kDa protein are about 1.2 × 10¹⁶ molecules. The count is what matters when you compare a protein with a ligand or a DNA template one to one.
Protein stocks are usually labeled in mg/mL, but binding constants, stoichiometry and enzyme kinetics are written in molar units. Dividing the mass concentration by the molar mass converts one into the other: 1 mg/mL of a 50 kDa protein is 0.001 ÷ 50 000 mol per mL, that is 2 × 10⁻⁵ M or 20 µM. A smaller protein at the same mg/mL is more concentrated in molar terms.
Take it from the sequence (about 110 Da per residue, or an exact value from a tool such as ProtParam), from the supplier’s datasheet or from mass spectrometry. Use the weight of the species actually in solution: a tagged construct weighs more than the native protein, glycosylation adds mass, and a dimer counts double if you want the concentration of the complex rather than of its chains.
Amount n = m ÷ (MW × 1000), with the mass m in grams and the molecular weight MW in kDa; the number of molecules N = n × 6.02214076 × 10²³ (the exact SI value of the Avogadro constant); the molar concentration c = n ÷ V with the volume V in liters. Inputs are converted to base units (g, L, mol, M) before the calculation and the result is converted back to the unit you select, so the prefix you pick never changes the arithmetic.
The calculator treats the molecular weight as that of the species in solution and makes no correction for purity, bound water or counter-ions. It is tested against the worked examples on this page before every release.
This calculator is for general laboratory reference. Verify critical preparations — such as dosing, clinical or safety-relevant work — against your protocol and a qualified professional.
Short answers to the questions people ask most about protein amounts and molarity.