How to Calculate Protein Molecular Weight

A protein’s molecular weight is the mass of one mole of it, in g/mol — numerically identical to daltons (Da). It is the bridge between what a balance or a gel tells you (mass) and what a reaction cares about (moles), and you need it to resuspend a lyophilized protein to a known molarity, to convert an A280 reading into µM, and to know how much to load. This guide derives the mass from a sequence by hand, explains why you sum residue masses and add back one water, works two real examples through the arithmetic, and covers the average-versus-monoisotopic and theoretical-versus-measured distinctions that cause most disagreements between tools. The protein molecular weight calculator runs the same sum and shows its residue counts so you can check the arithmetic.

Why the mass is residues plus one water

A free amino acid carries both an α-amino group and an α-carboxyl group. When two amino acids join, the amino group of one and the carboxyl group of the other react and lose a water molecule to form the peptide bond — a condensation reaction. So the unit that actually sits inside a chain is the amino acid minus one water: this is its residue mass.

A chain of n residues has n − 1 peptide bonds, so n − 1 waters were lost in building it. But the two ends are still intact: the N-terminus keeps its free amino group (an H) and the C-terminus keeps its free carboxyl (an OH). Those two end groups together reconstitute exactly one water molecule. That is why the formula is “sum the residue masses, then add back one water”:

MW = Σ(count of each residue × its residue mass) + 18.0153

If you instead summed free amino-acid masses you would over-count by (n − 1) waters. If you summed residues and forgot the +18.0153 you would under-count by exactly one water. Both errors are common and both are wrong.

The average residue masses

These are the 20 standard average residue masses (in Da), the values the calculator sums. “Average” means weighted over natural isotope abundances — see the next section for how this differs from monoisotopic.

Residue	Mass (Da)	Residue	Mass (Da)
A (Ala)	71.0788	M (Met)	131.1926
R (Arg)	156.1875	F (Phe)	147.1766
N (Asn)	114.1038	P (Pro)	97.1167
D (Asp)	115.0886	S (Ser)	87.0782
C (Cys)	103.1388	T (Thr)	101.1051
E (Glu)	129.1155	W (Trp)	186.2132
Q (Gln)	128.1307	Y (Tyr)	163.1760
G (Gly)	57.0519	V (Val)	99.1326
H (His)	137.1411	K (Lys)	128.1741
I (Ile)	113.1594	L (Leu)	113.1594

Plus the terminal water, 18.0153 Da. Selenocysteine (U, 150.0388) and pyrrolysine (O, 237.3018) are added when present. The masses trace back to standard atomic weights (NIST/CIAAW) and are the same set Expasy publishes.

A worked example by hand: a pentapeptide

Take the short peptide MKVLA. Count the residues: M 1, K 1, V 1, L 1, A 1.

1. Sum the residue masses:
   • M: 1 × 131.1926 = 131.1926
   • K: 1 × 128.1741 = 128.1741
   • V: 1 × 99.1326 = 99.1326
   • L: 1 × 113.1594 = 113.1594
   • A: 1 × 71.0788 = 71.0788
   • Subtotal = 131.1926 + 128.1741 + 99.1326 + 113.1594 + 71.0788 = 542.7375
2. Add the terminal water: 542.7375 + 18.0153 = 560.7528 Da ≈ 0.561 kDa.

That is the average mass of one mole of the unmodified pentapeptide as a free acid.

A real protein: ubiquitin (76 residues)

Human ubiquitin is MQIFVKTLTGKTITLEVEPSDTIENVKAKIQDKEGIPPDQQRLIFAGKQLEDGRTLSDYNIQKESTLHLVLRLRGG. Its composition is A 2, R 4, N 2, D 5, C 0, E 6, Q 6, G 6, H 1, I 7, L 9, K 7, M 1, F 2, P 3, S 3, T 7, W 0, Y 1, V 4 — 76 residues in total. Summing each count against the table above and adding 18.0153 gives:

MW = 8564.84 Da = 8.565 kDa

which matches the ~8.6 kDa the literature quotes for ubiquitin. Notice there is no cysteine (C 0), so this protein has no disulfide bonds and no cystine contribution — relevant when you later quantify it by A280.

Average versus monoisotopic mass

Two numbers describe the same molecule, and mixing them up is a classic source of “the calculator is wrong”:

• Average mass weights each element by its natural isotope mix (carbon as 12.011, etc.). It is what a low-resolution measurement, an SDS-PAGE position or a typical ESI envelope reports. The table above is average.
• Monoisotopic mass uses the lightest stable isotope of every atom (¹²C exactly 12, etc.). It is what a high-resolution mass spectrometer reports for the first peak of the isotope envelope. Monoisotopic is always slightly lower than average, and the gap grows with size — for a small peptide they are close, for a 50 kDa protein they differ by tens of daltons.

The calculator here reports the average mass. If your MS gives a monoisotopic peak, do not compare it directly to this number for a large protein; convert using a monoisotopic residue table instead.

Theoretical versus measured: why the real mass differs

The sequence sum is the mass of the unmodified polypeptide chain as typed. The protein you actually purify rarely matches it exactly, because:

• Post-translational modifications add mass: phosphorylation (+79.97 Da), glycosylation (often hundreds to thousands), acetylation (+42.01), methylation (+14.02 each).
• Signal-peptide cleavage removes an N-terminal stretch, so the mature protein is lighter than the full translation product.
• Fusion and affinity tags add mass: a His₆ tag is roughly +0.84 kDa, GST +26 kDa, common fluorophores a few hundred Da each.
• Disulfide bonds do not change the mass — two cysteines lose two hydrogens (−2.016 Da total) when they form a cystine, a change far too small to see on a gel but worth knowing for exact MS work.
• Bound cofactors, metals and ligands (heme, zinc, a substrate) add their own mass.

So treat the sequence MW as the theoretical backbone and adjust for whatever your construct and processing actually carry.

Ambiguous and non-standard codes

Sequences from databases and alignments sometimes contain codes that are not one of the 20:

• B = Asx (Asp or Asn), Z = Glx (Glu or Gln), X = unknown, J = Xle (Leu or Ile). These are ambiguous — there is no single mass — so the calculator rejects them and names the offending position rather than guessing.
• U = selenocysteine (150.0388 Da) and O = pyrrolysine (237.3018 Da) are real, mass-bearing residues and are summed.

Resolve ambiguous codes to a definite residue before calculating; a single X in a short peptide swings the answer by tens of daltons.

From mass to moles: Da, kDa, mg/mL and µM

Once you have the MW, two conversions make it useful at the bench:

• µg per nmol = MW / 1000
• nmol per µg = 1000 / MW

For ubiquitin (MW 8564.84): µg/nmol = 8.565 and nmol/µg = 0.1168. So 100 µg of ubiquitin is 100 × 0.1168 = 11.68 nmol, and to make a 100 µM stock you dissolve that in 11.68 nmol ÷ 100 nmol·mL⁻¹ = 0.1168 mL ≈ 117 µL. This is ordinary molarity arithmetic — the molarity calculator does the same relation for any solute. The MW also feeds the protein concentration calculator, which turns an A280 absorbance into mg/mL and µM (see the how to determine protein concentration from A280 guide).

Common mistakes

• Summing free amino-acid masses instead of residue masses — over-counts by one water per peptide bond.
• Forgetting the +18.0153 terminal water — under-counts by exactly one water, which matters most for short peptides.
• Comparing an average mass to a monoisotopic MS peak and concluding a tool is broken; it is a convention difference that grows with size.
• Using the sequence MW for a tagged or modified construct without adding the tag or PTM mass.
• Trusting an average-per-residue estimate (~110 Da/residue × length) for an exact number — fine for a rough size, wrong for stoichiometry.
• Leaving an ambiguous code (B/Z/X) in the sequence, which has no single mass.

Where this fits in the wider workflow

Molecular weight is the hinge between sequence and everything quantitative. Get the exact mass first with the protein molecular weight calculator, then predict the isoelectric point to choose a purification or focusing condition (the how to calculate protein isoelectric point guide covers that), then carry the MW into the protein concentration calculator so an A280 reading becomes a usable mg/mL. All three live in the protein calculators hub, and if you are working from a nucleic-acid sequence instead, the DNA molecular weight calculator is the parallel tool for DNA.