About this tool
The Protein Molecular Weight & pI Calculator is a free, in-browser tool that computes the molecular weight and theoretical isoelectric point of a protein or peptide from its one-letter amino-acid sequence. Paste the sequence and it returns the weight in daltons and kilodaltons, the length in residues, the estimated pI, and the amino-acid composition. All calculation happens locally in your browser and the sequence is never sent anywhere.
Molecular weight is the sum of the 20 standard average residue masses (for example Ala 71.08, Gly 57.05, Trp 186.21 Da) plus 18.02 Da of water for the free N- and C-termini. The isoelectric point is found by writing the net charge as a function of pH with the Henderson–Hasselbalch equation over the ionizable groups — the N-terminus, C-terminus, and the side chains of Asp, Glu, Cys, Tyr, His, Lys, and Arg — using standard pKa values, then bisecting pH between 0 and 14 until the net charge crosses zero.
Positive groups (N-terminus, His, Lys, Arg) contribute 1/(1+10^(pH−pKa)) and negative groups (C-terminus, Asp, Glu, Cys, Tyr) contribute −1/(1+10^(pKa−pH)). The pI is a theoretical estimate; real values shift with temperature, ionic strength, and post-translational modifications, which this sequence-only method does not model.