What is Protein Molecular Weight?

Protein molecular weight is the sum of the atomic masses of all atoms in a protein molecule, expressed in Daltons (Da) or kilodaltons (kDa). One Dalton equals 1/12 the mass of a carbon-12 atom (approximately 1.66 × 10⁻²⁴ grams). Molecular weight is fundamental for protein identification, purification, and characterization.

The calculation accounts for peptide bond formation: when amino acids link together, each bond releases one water molecule (H₂O, 18.015 Da). For a protein with n amino acids, there are (n-1) peptide bonds, so the total molecular weight equals the sum of amino acid masses minus 18.015 × (n-1). This subtraction is critical for accurate MW determination.

Molecular weight is essential for: SDS-PAGE interpretation (proteins migrate based on MW), calculating molar concentrations, determining protein stoichiometry in complexes, mass spectrometry validation, and selecting appropriate purification columns. Typical proteins range from 5 kDa (insulin, 51 amino acids) to over 500 kDa (titin, ~27,000 amino acids).

How to Calculate Protein MW

The Formula

MW = Σ(Amino Acid Weights) - 18.015 × (n - 1)

• Σ(AA Weights) = Sum of all amino acid molecular weights

• n = Number of amino acids in the sequence

• 18.015 Da = Mass of one water molecule (H₂O)

• (n-1) = Number of peptide bonds = number of H₂O released

Amino Acid Weights (Average)

A: 89.09 Da

R: 174.20 Da

N: 132.12 Da

D: 133.10 Da

C: 121.15 Da

E: 147.13 Da

Q: 146.15 Da

G: 75.07 Da

H: 155.16 Da

I: 131.17 Da

L: 131.17 Da

K: 146.19 Da

M: 149.21 Da

F: 165.19 Da

P: 115.13 Da

S: 105.09 Da

T: 119.12 Da

W: 204.23 Da

Y: 181.19 Da

V: 117.15 Da

* Average isotopic masses (not monoisotopic)

Worked Example

Calculate MW for a small peptide: AVGC

Given:

• Sequence: AVGC (4 amino acids)

• A (Ala): 89.09 Da

• V (Val): 117.15 Da

• G (Gly): 75.07 Da

• C (Cys): 121.15 Da

Step 1:

Sum amino acid weights:

Total = 89.09 + 117.15 + 75.07 + 121.15
Total = 402.46 Da

Step 2:

Calculate peptide bonds:

Peptide bonds = n - 1
Peptide bonds = 4 - 1
Peptide bonds = 3

Step 3:

Subtract water loss:

MW = 402.46 - (3 × 18.015)
MW = 402.46 - 54.045
MW = 348.42 Da

Result:

348.42 Da

or 0.348 kDa

Frequently Asked Questions

Why subtract water molecules?

Peptide bond formation is a dehydration synthesis reaction. When two amino acids join, the carboxyl group of one (−COOH) reacts with the amino group of another (−NH₂), releasing H₂O. Each bond loses 18.015 Da, so n amino acids release (n-1) water molecules.

What is the difference between average and monoisotopic mass?

Average mass uses the weighted average of all isotopes (e.g., C is 12.011 Da). Monoisotopic mass uses only the most abundant isotope (C-12 = 12.000 Da). This calculator uses average masses. Mass spectrometry often reports monoisotopic masses for small proteins.

Does this include post-translational modifications?

No. This calculates the theoretical MW from the primary sequence only. Real proteins often have modifications (phosphorylation +80 Da, acetylation +42 Da, glycosylation +hundreds to thousands Da) that increase MW. Add these manually if known.

Why does my SDS-PAGE show different MW?

SDS-PAGE estimates MW based on migration distance using protein standards. Anomalous migration occurs with: highly charged proteins, glycoproteins (run higher), membrane proteins, unusual shapes. Use mass spectrometry for accurate MW determination.

What about disulfide bonds?

Disulfide bonds (Cys-Cys) form by removing 2H (−2.016 Da per bond). This calculator doesn't account for them automatically. If you know the number of disulfide bonds, subtract 2.016 × (number of bonds) from the result.

How accurate is this calculation?

For theoretical MW from sequence: very accurate (±0.01%). However, real protein MW differs due to: processing (signal peptide cleavage), modifications, bound ligands/metals, and isotopic variation. Mass spectrometry typically shows ±1-2 Da difference from calculated values.

Can I use three-letter codes?

This calculator uses one-letter amino acid codes only (A, R, N, D, etc.). Convert three-letter codes to one-letter: Ala→A, Arg→R, Asn→N, Asp→D, Cys→C, Glu→E, Gln→Q, Gly→G, His→H, Ile→I, Leu→L, Lys→K, Met→M, Phe→F, Pro→P, Ser→S, Thr→T, Trp→W, Tyr→Y, Val→V.

What is kDa vs Da?

Daltons (Da) and kilodaltons (kDa) measure molecular mass. 1 kDa = 1,000 Da. Small peptides use Da (insulin = 5,734 Da), larger proteins use kDa (BSA = 66 kDa). It's the same unit, just different scale like grams vs kilograms.

Protein Molecular Weight Calculator

Sequence MW Calculator