Protein Molecular Weight Calculator
Paste a peptide, get the bill: twenty residue masses summed, one terminal water added, the arithmetic shown line by line.
Protein Molecular Weight Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Unmodified twenty-residue chains — no PTMs, disulfides priced as reduced
- Average-isotope masses; monoisotopic runs lower
In short: Met-Ala-Gly — three residues, one bond’s worth of chemistry — prices at 277.338580 Da: 131.1926 + 71.0788 + 57.0519 plus the single terminal water of 18.015280 that caps the chain. Residue masses already paid their peptide-bond water, so a chain of n residues adds exactly one water back, not n. The 110-Da rule of thumb says 330.000000; the table says 277.338580 — thumbs count, tables weigh. Insulin’s A chain (21 residues) computes 2,383.706080 Da.
Formula
MW = Σ residue masses + H₂O (18.015280) · thumb: ~110 Da per residue
Each residue mass is the free amino acid minus the water its peptide bond released, so the chain’s mass is the sum of residues plus exactly one terminal water for the free N- and C-termini. The table is the conventional average-isotope set — quoted approximate; monoisotopic masses run a few tenths lower per residue. Isoleucine and leucine are isomers and share 113.1594 honestly: the alphabet cannot tell them apart, only a mass spectrometer’s fragments can.
Worked Example
- Paste the sequence in one-letter code.
- Read the mass — residue sum plus one terminal water.
- Scan the bill: the heaviest residues carry the mass.
- Cross-check the 110-Da thumb against the true figure.
Defaults: MAG → 277.338580 Da (the printed cross-check: 131.1926 + 71.0788 + 57.0519 + 18.015280). PEPTIDE → 799.832780 Da. Insulin A chain, 21 residues → 2,383.706080 against a thumb of 2,310.000000.
Strengths & Limits Of This Model
Where this engine is strong
- The bill printed, heaviest residues first
- Rule-of-thumb check against the true figure
Where it stops
- No post-translational modifications
- No signal-peptide cleavage logic
Practical Use Cases
Expression planning
expected band on a gel
Mass spectrometry
the average-mass target
Teaching
residue masses vs free amino acids
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Protein Molecular Weight Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why add only one water to the whole chain?
Because the residue masses already had their bond waters subtracted — each is the free amino acid minus H₂O for the bond it will form. A chain of n residues formed n−1 bonds, so exactly one water remains to be added for the free termini. Adding water per residue would double-count the chemistry the table already paid for.
Why do I and L weigh the same?
They are isomers: identical formulas, different connectivity. The one-letter alphabet cannot price connectivity, so both carry 113.1594 honestly. The page prints identical masses rather than inventing a difference — only fragmentation data separates them in practice.
What is the 110-Da rule for?
Fast counting, not weighing: average across the twenty residues sits near 110, so residue count times 110 estimates a protein’s size in kilodaltons from a genome annotation. The thumb card prints the gap between estimate and truth — glycine-rich chains run light, tryptophan-rich chains heavy.
Why does the page refuse B, J, O, U, X and Z?
They are not among the twenty standard residues the table prices: B and Z are ambiguous (asparagine/aspartate and glutamine/glutamate), J is ambiguous leucine/isoleucine, U is selenocysteine, O is pyrrolysine, X is any. Ambiguity needs a choice, and the rare residues need their own masses — the page refuses with the letter and position named instead of guessing.
Average or monoisotopic masses?
Average, on purpose: gels, buffers and most lab arithmetic live on the average scale, and the table matches what a balance and a mole count. Mass spectrometrists doing high-resolution work switch to the monoisotopic set, a few tenths of a dalton lower per residue — a different table for a different instrument.
What is the bill card actually for?
Audit. Sorting residues by mass contribution shows which letters carry the molecule — tryptophan’s 186.2133 dominates any chain it appears in, glycine nearly vanishes — and the top share doubles as a paste check: a sequence whose heaviest residue changed since yesterday is not the same sequence. For MAG the single methionine holds 50.590363% of the residue mass.
Why is the thumb deliberately wrong-ish?
Because 110 Da per residue is a counting tool, not a weighing tool: it estimates a gene product’s size from a genome annotation in one line. The thumb card prints the gap between estimate and table — 330.000000 against 277.338580 for MAG — so the habit stays honest: thumbs plan gels, tables go on orders.
How is this different from the DNA page?
Alphabet and chemistry. Twenty letters against four, residue masses that already paid their bond water against nucleotide monophosphates that need a terminal adjustment subtracted, one water added against one adjustment taken back. Same discipline — count, weigh, show the convention — on a chemistry that builds up instead of trimming down.