This calculator provides an estimation on peptide solubility, with information on what strategies to try to solubilise your peptide.
The calculator, which also reports other physiochemical properties, is loaded through an Iframe, but if you are reading this, then you may access it here. Peptide solubility calculator
Remember to read the notes on peptide solubility estimation!
Peptide solubility can be roughly estimated by just looking at the charged residues of the peptide. The more positive charges, the better the water solubility of the peptide. If the peptide is difficult to get into solution at neutral pH, one can shift the pH away from the isoelectric point (pI). If the pI is >7 then the peptide is basic and the pH should be lowered in order to promote solubility, and if the pH is <7 then the peptide is acidic and the pH should be elevated. You can make a rough estimate of pI by looking at the residues in the peptide sequence. Arginine, Lysine, and the unmodified N-terminus each add +1 to the total charge. Asp, Glu and the unmodified C-terminus each add -1. If the total is positive, then the peptide is basic. If the total is negative, then the peptide is acidic.