Protein Molecular Weight Calculator

Average mass is the weighted average of all naturally occurring isotopes, used for SDS-PAGE and general biochemistry. Monoisotopic mass is the mass of the molecule containing only the most abundant isotope of each element, used for high-resolution mass spectrometry.

When amino acids form peptide bonds, a water molecule (H₂O, 18.015 Da) is removed for each bond. However, the N-terminus retains an extra H and the C-terminus retains an extra OH, so one water molecule is effectively added back to calculate the intact protein mass.

Each disulfide bond between two cysteine residues removes 2 hydrogen atoms (2.016 Da) as the S-S bond forms. For each disulfide bond, subtract 2.016 Da from the calculated molecular weight.

B represents Asx (average of D/N), Z represents Glx (average of E/Q), and J represents I/L (identical mass). X represents any amino acid. These are used when the exact identity is unknown.

For proteins under 100 kDa, the calculator is accurate to within 0.01% when using monoisotopic masses. For very large proteins (>100 kDa), average masses are preferred because monoisotopic isotope envelopes overlap.

For SDS-PAGE, use the average mass option and compare your calculated molecular weight to the apparent molecular weight on your gel. Note that post-translational modifications (glycosylation, phosphorylation) can shift apparent mass by 5—30% from predicted values.

Common reference proteins: Insulin (5.8 kDa), Lysozyme (14.3 kDa), BSA (66.5 kDa), Myosin (200 kDa). Use these as molecular weight markers in SDS-PAGE experiments to estimate unknown protein sizes.

Post-translational modifications add mass: glycosylation (+0.5—15 kDa), phosphorylation (+80 Da per site), ubiquitination (+8.5 kDa), acetylation (+42 Da). Always account for known modifications when predicting molecular weight for mass spectrometry validation.

Yes, this calculator works for synthetic peptides. Enter the exact sequence and account for any terminal modifications (e.g., acetylation, amidation) by adjusting the calculation. Most peptide synthesis companies provide mass calculations, but this tool verifies their accuracy.

Molecular weight and isoelectric point (pI) are independent properties. MW depends on amino acid composition, while pI depends on the ratio of acidic to basic residues. Both are essential for protein characterization, but knowing one does not predict the other.

To convert Da to kDa, divide by 1,000. To convert kDa to Da, multiply by 1,000. For example: 50,000 Da = 50 kDa. Most biochemistry papers report proteins >10 kDa in kDa, while peptides <10 kDa are typically reported in Da.

Differences may result from: post-translational modifications not accounted for, different amino acid residue mass tables, experimental vs. theoretical values, or protein variants. Always check if the published value includes modifications or reflects experimental mass.

Non-standard amino acids (selenocysteine, hydroxyproline) are not directly supported. For modified residues, calculate the standard sequence mass first, then manually add or subtract the mass difference for each modification. Consult residue modification tables for exact mass shifts.

Theoretical molecular weight calculations are accurate to within 0.01% for unmodified proteins. Experimental mass spectrometry may differ by 0.1—1% due to instrument calibration, ionization effects, and sample preparation. Post-translational modifications account for most discrepancies between predicted and observed masses.

Cross-validate using: SDS-PAGE with molecular weight markers, mass spectrometry (MALDI-TOF, ESI-MS), analytical ultracentrifugation, or gel filtration chromatography. Compare your calculated value with experimental data and published literature values for the same protein.

Yes, enter each isoform sequence separately. Isoforms resulting from alternative splicing will have different molecular weights based on their amino acid composition. Compare the calculated masses to determine which isoform you are working with in your experiments.

Common average residue masses: Glycine (57.05), Alanine (71.08), Valine (99.13), Leucine (113.16), Isoleucine (113.16), Proline (97.12), Phenylalanine (147.18), Serine (87.08), Threonine (101.10), Cysteine (103.14), Methionine (131.20), Tryptophan (186.21), Tyrosine (163.18), Histidine (137.14), Lysine (128.17), Arginine (156.19), Aspartic acid (115.09), Glutamic acid (129.12), Asparagine (114.10), Glutamine (128.13). These values account for the water loss during peptide bond formation.

Molecular weight influences protein diffusion rate, sedimentation behavior, and interaction partners. Smaller proteins (<30 kDa) diffuse faster and often serve as signaling molecules, while larger proteins (>100 kDa) form complexes and structural components. MW is essential for predicting protein behavior in purification and characterization experiments.

Molecular weight is a dimensionless ratio relative to carbon-12, while molecular mass has units of Daltons or g/mol. In practice, the terms are used interchangeably in biochemistry. Strictly speaking, molecular weight is unitless, but most papers report values in Da or kDa for clarity.

If the sequence is unknown, estimate molecular weight using: SDS-PAGE migration, gel filtration chromatography, analytical ultracentrifugation, or mass spectrometry. These experimental methods provide apparent molecular weights that can be compared to databases like UniProt for protein identification.

Molecular weight determines drug-like properties: proteins <60 kDa cross cell membranes more easily, while larger proteins require special delivery methods. MW affects pharmacokinetics, bioavailability, and manufacturing feasibility. The Lipinski Rule of 5 (MW <500 Da for small molecules) illustrates how mass influences drug design.

The Protein Molecular Weight Calculator uses verified mathematical formulas processed entirely in your browser. Calculator inputs and results are never sent to external servers. We do use Google Analytics and AdSense for standard website operation — see our Privacy Policy for details.

Yes. All calculations happen locally in your browser. We never store, transmit, or log any input data you enter into the calculator.

Double-check that all inputs are valid numbers within reasonable ranges. If you believe there is an error, please contact us with your input values and we will investigate.