Protein Molecular Weight Calculator

Your details

Use single-letter amino acid codes (A, C, D, E, F, G, H, I, K, L, M, N, P, Q, R, S, T, V, W, Y). FASTA headers (lines starting with >) are ignored automatically. Spaces, digits and line breaks are stripped.
Average mass uses natural isotopic abundances - standard for gel electrophoresis and most biochemical work. Monoisotopic mass uses the most abundant isotope of each element - preferred for high-resolution mass spectrometry.
1 kDa = 1000 Da. Kilodaltons are conventional for proteins; Daltons for small peptides.
Molecular weightSmall protein
15.258

Mass of the full protein chain including terminal H and OH

UnitkDa
Residue count142aa
Extinction coefficient (280 nm)10,095M⁻¹ cm⁻¹
Most abundant residueAlanine (A) - 21 residues
Residue count142
Ext. coeff. (M⁻¹ cm⁻¹)10,095

Protein: 15.26 kDa, 142 residues

  • The protein has 142 residues and a molecular weight of 15.26 kDa (15258 Da).
  • The molar extinction coefficient at 280 nm is 10,095 M⁻¹ cm⁻¹ (Edelhoch method, reduced Cys). Use Beer-Lambert law (A = εcl) to determine concentration from absorbance.

Next stepAverage mass matches what you see on an SDS-PAGE ladder or from a Bradford assay. If you need to verify by mass spectrometry, switch to Monoisotopic mass.

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