Peptide Molecular Weight Calculator

How to calculate peptide molecular weight

Start with the exact sequence and chemical form. For a linear chain with a free amino terminus and a free carboxyl terminus, add the formulas of the amino-acid residues, then add one water molecule. A residue formula here is the formula of a free amino acid minus H₂O. This bookkeeping accounts for the water formally lost when peptide bonds connect the residues.

An equivalent approach starts with the free amino acids: add their masses and subtract one water mass for every peptide bond. An unbranched, acyclic chain containing n residues has n − 1 backbone peptide bonds. Using residue masses and then subtracting n − 1 water masses again would double-count the loss.

The calculator uses the residue-formula approach. It accepts the 20 standard amino acids in uppercase one-letter notation, followed by your selected terminal groups and intramolecular disulfide count. It does not infer a sequence from a product name. If you need to decode a symbol, use the amino-acid code chart before calculating.

Amino Acid Code Chart: One-Letter and Three-Letter Codes

Use the peptide molecular weight calculator

Worked example: a four-residue sequence

AGKS means alanine–glycine–lysine–serine, read from the N-terminus toward the C-terminus. For this illustration, both termini are unmodified and there are no disulfide bonds. The four residue formulas total C₁₄H₂₅N₅O₅; adding H₂O gives the neutral peptide formula C₁₄H₂₇N₅O₆.

With this calculator’s published atomic-weight convention, the average molar mass is 361.399 g/mol and the neutral monoisotopic mass is 361.19613 Da. These are calculated values for the specified molecular model. They are not a measurement of a supplied sample or a statement about its purity.

Average mass and monoisotopic mass answer different questions

Average molar mass uses atomic weights that represent an isotope-weighted average. This tool uses the central values in CIAAW’s 2024 abridged table for carbon, hydrogen, nitrogen, oxygen and sulfur. Calculators using other published weight tables may differ slightly in their last digits.

The neutral monoisotopic result uses the NIST masses for ¹²C, ¹H, ¹⁴N, ¹⁶O and ³²S. It describes a particular isotope composition, rather than an average across the natural isotope distribution. A mass-spectrometry peak is usually reported as mass-to-charge ratio, m/z; charge, added or removed protons and other adducts must be considered before comparing a peak with a neutral mass.

For concentration calculations, use a molar mass and mass basis consistent with the material documentation. The number of decimal places on a calculator does not establish the accuracy of a balance, an assay or a mass spectrum. No numerical result here verifies that the requested sequence is present.

Terminal modifications and disulfide bonds change the formula

Terminal labels describe chemistry beyond the one-letter residue string. Acetylating a free N-terminus adds C₂H₂O to the neutral formula. Converting a free C-terminal acid to an amide adds NH and removes O. The tool handles these as separate selections, so terminal annotations should not be pasted into the sequence field.

Formation of one intramolecular disulfide bond removes two hydrogen atoms from two cysteine thiols. The calculator therefore subtracts H₂ per specified bond and rejects counts that require more cysteine residues than the sequence contains. The count alone does not establish which cysteines are connected.

What this sequence calculation leaves out

The model is one chain with the selected termini and disulfide bonds. It excludes counterions such as trifluoroacetate or acetate, residual water, solvent, isotopic labels, metal complexes, nonstandard residues and other covalent modifications. Head-to-tail cyclization and branched or linked multichain structures also require a different structural model.

A salt name is not enough to establish a fixed counterion stoichiometry. Do not add an assumed number of counterions simply to make a calculated result resemble a label. Record the peptide entity, salt or formulation and analytical basis separately, and resolve any disagreement against the supplier’s documents.

Sequence order has another important limitation: AGKS and SGKA have the same residue composition. With identical termini and modifications, they have the same molecular formula and calculated mass. A matching mass therefore does not uniquely identify sequence order. The calculator also does not establish stereochemistry.

Connect the result to a laboratory record

Keep the sequence, terminal groups, disulfide assumption, mass convention and calculation date with the result. If a report gives a different mass, check these inputs before interpreting the difference. A report for another lot or a related sequence cannot settle the identity of your sample.

The amount corresponding to one milligram follows from the molar mass: nmol per mg = 1,000,000 ÷ molar mass in g/mol. For the modeled AGKS molecule, this is about 2,767.02 nmol per mg. This conversion assumes the milligram is the modeled peptide mass; a milligram of gross lyophilized material may include other components.

Use the companion molarity and dilution calculator when you have a documented mass basis and final solution volume. NorthVerity’s molecular-weight and COA guides explain the records to compare when reviewing a catalog item or requesting documents for an offered lot.

Molarity, Dilution and Serial Dilution Calculator

Molecular weight, CAS and PubChem identity fields

How to read a peptide COA

Frequently asked questions

Can I enter a FASTA record? Enter only the sequence. Remove the header, numbering and annotations. Spaces and line breaks are ignored, but unsupported symbols and lowercase text are rejected so that the input format stays explicit.

Can I calculate a modified or cyclic peptide? Only the terminal options and intramolecular disulfide bonds shown in the tool are supported. Other structures need a formula derived from the complete molecular structure.

Does a matching mass prove purity? No. A calculated mass is a structural reference. Identity, chromatographic purity and net peptide content are separate questions requiring suitable evidence.