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Protein Molecular Weight Calculator

Calculate protein molecular weight with our free science calculator. Uses standard scientific formulas with unit conversions and explanations.

Reviewed by Daniel Agrici, Founder & Lead Developer

Reviewed by Daniel Agrici, Founder & Lead Developer

Formula

MW = Sum of all residue weights + 18.015 Da (water)

Each amino acid residue contributes its specific molecular weight (ranging from 57 Da for glycine to 186 Da for tryptophan). One water molecule (18.015 Da) is added to account for the free N-terminal amino group and C-terminal carboxyl group. For estimates, use 110 Da per residue average. The extinction coefficient uses the Pace method: E280 = nTrp x 5500 + nTyr x 1490 + nCys x 125.

Worked Examples

Example 1: Human Insulin B-Chain

Problem:Calculate the molecular weight of the insulin B-chain with sequence: FVNQHLCGSHLVEALYLVCGERGFFYTPKT (30 residues).

Solution:Sum individual residue weights:\nF(147.18) + V(99.13) + N(114.10) + Q(128.13) + H(137.14) + L(113.16) + C(103.14) + G(57.05) + S(87.08) + H(137.14) + L(113.16) + V(99.13) + E(129.12) + A(71.08) + L(113.16) + Y(163.18) + L(113.16) + V(99.13) + C(103.14) + G(57.05) + E(129.12) + R(156.19) + G(57.05) + F(147.18) + F(147.18) + Y(163.18) + T(101.11) + P(97.12) + K(128.17) + T(101.11)\nSum + H2O (18.02) = 3,400.9 Da

Result:MW: ~3,401 Da (3.4 kDa) | E280: 4,470 M-1cm-1 | 30 residues

Example 2: Estimating MW from Residue Count

Problem:A protein has 450 amino acid residues. Estimate its molecular weight.

Solution:Using average residue weight of 110 Da:\nMW = 450 x 110 + 18 (water) = 49,518 Da\nRound: ~49.5 kDa\nThis is typical of a medium-sized enzyme.

Result:Estimated MW: ~49.5 kDa | Typical range for 450 residues: 47-52 kDa

Frequently Asked Questions

How is protein molecular weight calculated from an amino acid sequence?

Protein molecular weight is calculated by summing the molecular weights of all individual amino acid residues in the sequence, then adding the molecular weight of one water molecule (18.015 Da). During peptide bond formation, one water molecule is lost per bond, but the N-terminus retains an extra hydrogen and the C-terminus retains an extra hydroxyl group, which together equal one water molecule. The average molecular weight of an amino acid residue is approximately 110 Da (after water loss), so a rough estimate is: MW = number of residues x 110. For precise calculations, each residue type has a specific weight ranging from 57 Da (Glycine) to 186 Da (Tryptophan).

What is the average molecular weight of an amino acid?

The average molecular weight of a free amino acid is about 128 Da, but in a protein (after peptide bond formation and water loss), the average residue weight is approximately 110-111 Da. This average varies slightly depending on the amino acid composition. Glycine-rich proteins (e.g., collagen) have lower average residue weights (~85 Da for Gly residue), while tryptophan-rich proteins are heavier. The 110 Da average is commonly used for quick estimates: a 300-residue protein is approximately 33 kDa. For precision, always use the exact sequence-based calculation rather than the estimate.

References

Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy