Protein Molecular Weight Calculator

Estimate protein molecular weight from amino acid sequence length and composition.

Number of amino acids (length) (AA residues)

Average MW per residue (Da) (Da)

Average = 110 Da; use 128 for MW in kDa

Number of disulfide bonds

Each disulfide loses 2 Da (2H)

Please check your inputs and try again.

Worked Examples

Example 1: Typical enzyme: 350 AA, avg 110 Da, 2 SS bonds

MW = 350×110 − 18×349 − 4

Result: 32,076 Da ≈ 32 kDa

Mid-range enzyme

Example 2: Antibody: 1320 AA (IgG), avg 110 Da

MW = 1320×110 − 18×1319

Result: ≈ 122 kDa

IgG literature value: ~150 kDa (with glycosylation)

Frequently Asked Questions

Why subtract 18 Da per peptide bond?

Each peptide bond formation loses one water molecule (H₂O = 18 Da). A protein of n amino acids has n-1 peptide bonds, losing 18(n-1) Da from the sum of free amino acid masses.

How accurate is this calculation?

This gives approximate MW. Actual MW depends on exact amino acid composition (MW ranges from 57 Da for Gly to 186 Da for Trp). Use sequence-based tools (ProtParam, ExPASy) for exact calculation.