Average molecular weight sums each element’s mass weighted by its natural isotopic abundance; monoisotopic mass sums only each element’s single most abundant isotope. The two numbers are close for a single amino acid but diverge more as a sequence gets longer, since every residue added contributes its own small gap between the two.
What is the difference between monoisotopic mass and average molecular weight?
Most elements exist in nature as a mix of isotopes, atoms with the same number of protons but a different number of neutrons, each with a slightly different mass. Carbon, for example, is mostly carbon-12 with a small fraction of the heavier carbon-13.
Average molecular weight weights each element’s mass by its natural isotopic abundance. It is the number a standard periodic table reports for each element, and it is what a bulk sample on a benchtop balance is effectively measuring, because a real sample contains the natural mix of isotopes.
Monoisotopic mass uses only the mass of each element’s single most abundant isotope, ignoring the rarer heavier ones entirely. It answers a different question: what does the lightest, most common isotopic combination of this molecule weigh.
Where the calculators get their numbers
Both figures are built the same way in this site’s tools: sum a residue mass for every amino acid in the sequence, then add one water molecule for the whole chain (the molecular weight from a sequence post covers that sum in full). The only difference is which residue mass table and which water mass get used.
average molecular weight = sum(average residue masses) + 18.01528 Da
monoisotopic mass = sum(monoisotopic residue masses) + 18.010565 DaThe average molecular weight calculator uses the first table, the monoisotopic mass calculator the second, and both draw from the same amino acid reference data so a sequence run through either one is being measured consistently.
Why the gap grows with sequence length
For a single amino acid the two masses are close, typically within a fraction of a dalton. For a longer chain the small per-residue differences accumulate, because every residue added contributes its own small gap between its average and monoisotopic mass. A short research peptide might show a difference of well under one dalton, while a longer chain can show a difference of several daltons. Neither number is wrong; they are answering different questions, and the gap between them is not an error to be reconciled, it is a real property of how each figure is defined.
When each one matters
Average molecular weight is the number to use for anything involving a bulk mass: weighing out powder, converting a milligram quantity to moles with the mass to moles calculator, or comparing a supplied molecular weight on a label against what the sequence predicts. That mass-to-moles step, and why a molecular weight is required to take it, is covered more generally in the unit conversion guide.
Monoisotopic mass is the number mass spectrometry work generally quotes, because a mass spectrometer resolving individual isotopic peaks reports the mass of the monoisotopic species as its base peak, not a population average. If a mass spec report or a paper states a mass and you are trying to match it against a sequence, check which figure it is actually reporting before comparing; a mismatch of a few daltons on a longer peptide is often nothing more than average being compared against monoisotopic.
The molecular formula underneath both
Both mass figures are summaries of the same underlying elemental composition. The molecular formula calculator reports the actual atom counts (carbon, hydrogen, nitrogen, oxygen, sulfur) that both the average and monoisotopic masses are computed from. If two molecular weight figures for the same sequence disagree by more than the expected average-versus-monoisotopic gap, checking the formula is a useful next step, since it shows exactly what atoms are being counted rather than just a single summary number.
A quick cross-check
If you already have a published molecular weight from a supplier or a paper and want to confirm it matches a sequence, the molar mass cross-check calculator compares a supplied value against the sequence-derived average molecular weight and reports the difference directly, which is a faster way to catch a transposed residue than re-deriving the sum by hand.
In short: average molecular weight = sum(average residue masses) + 18.01528 Da, while monoisotopic mass = sum(monoisotopic residue masses) + 18.010565 Da, and the gap between the two grows with sequence length.