Tool
Peptide molecular weight calculator
Type or paste a sequence using the one-letter amino-acid codes. Everything updates as you type.
Letters: A C D E F G H I K L M N P Q R S T V W Y. Spaces, numbers and other characters are ignored.
- Average mass
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- Monoisotopic mass
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- Formula
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- Length
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How the calculation works
Each amino acid in a chain is a residue: the amino acid minus the water lost when it joined the chain. A peptide's mass is the sum of its residue masses plus one molecule of water (18.015 g/mol average, 18.011 Da monoisotopic) for the two free ends.
- Average mass uses the natural mix of isotopes. It's the number used for weighing and making solutions.
- Monoisotopic mass uses only the most common isotope of each element. It's the number a mass spectrometer reports for a peptide's main peak.
- Acetylation adds 42.04 g/mol and amidation subtracts 0.98 g/mol. Both are common chemical caps on the ends of a chain.
Disulfide bridges link two cysteines together, and each one removes two hydrogen atoms (about 2.02 g/mol). Oxytocin has one, which closes its ring, so the Oxytocin example sets it for you. The insulin examples show each chain on its own, before the chains are joined.
For full protein analysis, including isoelectric point, the Expasy ProtParam tool from the SIB Swiss Institute of Bioinformatics is the standard reference.