Calculate the molecular weight (MW) and isoelectric point (pI) of a protein from its amino acid sequence. Paste the sequence below to get MW in Da/kDa, the predicted pI, and the net charge at pH 7.
Average residue masses; pI by net-charge titration (EMBOSS pKa values). Only the 20 standard amino acids are read; other characters are ignored.
What these numbers mean
- Molecular weight is the sum of the amino acid residue masses plus one water molecule. It’s what you compare against a band on an SDS-PAGE gel (remember post-translational modifications and SDS binding can shift apparent size).
- Isoelectric point (pI) is the pH at which the protein carries no net charge. It predicts behaviour in isoelectric focusing and ion-exchange chromatography, and tells you the pH at which a protein is least soluble.
How the pI is calculated
The tool titrates the net charge across pH using pKa values (EMBOSS set) for the ionisable groups — the N- and C-termini and the side chains of Asp, Glu, Cys, Tyr, His, Lys and Arg — and finds the pH where the net charge is zero. Different pKa sets (Expasy/Bjellqvist, EMBOSS, etc.) give slightly different pI values, so treat it as an estimate.
Common uses
- Predicting migration in 2D gels and isoelectric focusing.
- Choosing the buffer pH for ion-exchange purification (bind below pI on a cation exchanger, above pI on an anion exchanger).
- Estimating the size for SDS-PAGE and Western blots.
Frequently asked questions
Is this MW average or monoisotopic?
Average mass (the relevant value for SDS-PAGE and most lab work). Monoisotopic mass, used in high-resolution mass spectrometry, differs slightly.
Why does my pI differ from another tool?
pI depends on the pKa value set used. This tool uses the EMBOSS values; Expasy’s Compute pI/MW uses the Bjellqvist set, which can shift the result by a few tenths of a pH unit.
Does it account for modifications?
No — the calculation is from the bare amino acid sequence. Phosphorylation, glycosylation and other modifications change the real MW and pI.
