How to Use Peptide Calculator
The Peptide Calculator starts from the sequence itself. Type a continuous one-letter sequence such as ACDEFGHIK, or enter three-letter amino acid names separated by spaces, such as Ala Cys Asp Glu. The calculator normalizes the input and stops if any residue is not recognized.
After a valid sequence is entered, the result gives molecular weight, residue count, estimated pI, and a composition summary. The composition line is especially helpful for checking whether the sequence was interpreted correctly before using the molecular weight in a lab notebook or order form.
Use standard amino acid sequences only. If the peptide has amidation, phosphorylation, fluorescent labels, disulfide pairing assumptions, isotope labels, or other modifications, add those changes with a dedicated peptide chemistry tool.
Formula & Theory — Peptide Calculator
The Peptide Calculator uses the following formula or scoring rule:
molecular weight = Σ residue masses + H2O
pI = pH where net charge ≈ 0The Peptide Calculator uses average residue masses rather than free amino acid masses. Peptide-bond formation removes water between residues, so summing residue masses and adding one terminal water molecule gives the approximate neutral peptide mass.
The pI estimate uses common pKa values for the N-terminus, C-terminus, and ionizable side chains. The tool tests pH values between 0 and 14 until the calculated net charge approaches zero.
The composition table is a simple count of each residue symbol after sequence normalization. Leucine and isoleucine remain separate sequence letters even though they share the same average residue mass.
Use Cases for Peptide Calculator
The Peptide Calculator is useful in specific situations such as:
- checking a synthetic peptide order before submission
- estimating mass for a quick LC-MS or MALDI expectation
- teaching how residue composition influences pI
- counting amino acid content for a short peptide sequence