We have created a database containing measured pIs not theoretical values.

It is available here:

http://www.pip-db.org/

We are in the (slow) process of updating this to include more values and to broaden the scope of data archived.

The paper describing PIP-DB is:

http://www.ncbi.nlm.nih.gov/pubmed/25252779

http://bioinformatics.oxfordjournals.org/content/31/2/295.long

You can try to look for your protein on the Uniprot web site (http://www.uniprot.org/) by using either the name of the protein or by using its accession number ( if this protein have been already sequenced).

Once done you can access a number of structural information that includes: full sequence, number of AA, molecular mass, theoretical pI, C an N terminal sequences....These features are easily accessible by using a drop down menu in the section sequence.

If you do not have the accession number you may try to conduct your research first on PubMed to get the ID of your protein or directly select from the results options the structural features you are looking for.

Hope this can help!

Both Ibtissam and Andrei give links to excellent on-line tools where you can calculate the pI of proteins, but if your protein has post-translational modifications then the calculated pI will be wrong. 

Calculating the pI from the sequence is always going to be an approximation. If there are ionisable groups which are buried within the structure, the pI of these groups may be influenced considerably by the local environment of that residue. So the above tools give a reasonable approximation, but caveat emptor.

Hi! Colleagues are absolutely right, it is very useful to estimate pI of your protein, and it is easily done by many programs both online and standalone. However, since it is done solely from the primary structure and does not take into account many real factors, this pI value should be considered as a guidance only. Experimental pI should be checked for particular cases specifically. Also, try to check literature on pI values of your stuff.

Andrei is right. Theoretical pI obtained from the amino acid sequence can easily be obtained at expasy and other web services, but most of the proteins have post-traduccional modifications and experimental pI is different of the theoretical one. Several isoforms with different pI are also common. It depends on the protein and the source. Cheers.  

http://web.expasy.org/tagident/ - great, but it gives only theoretical isoelectric point (DTAselect model)

for more theoretical pI models see: http://isoelectric.ovh.org

And recently there was one database published: http://www.ncbi.nlm.nih.gov/pubmed/25252779

We have created a database containing measured pIs not theoretical values.

It is available here:

http://www.pip-db.org/

We are in the (slow) process of updating this to include more values and to broaden the scope of data archived.

The paper describing PIP-DB is:

http://www.ncbi.nlm.nih.gov/pubmed/25252779

http://bioinformatics.oxfordjournals.org/content/31/2/295.long

See, http://isoelectricpointdb.org

Due technical reasons the web server for pI has been moved from http://isoelectric.ovh.org to http://isoelectric.org

Details about Isoelectric Point Calculator and comparison to all other pI algorithms (>15 methods) in Article IPC – Isoelectric Point Calculator

The Proteome-pI database with predicted pI for >5k proteomes and all proteins from nr, UniProt, PDB stays where it was: http://isoelectricpointdb.org

Details about the database in: Article Proteome-pI: Proteome isoelectric point database

if you know the sequence try www.protparam.net.