Protein Molecular Weight Calculator
Enter the amino-acid residue count and average residue mass; the tool instantly estimates the protein molecular weight (Da and kDa) to quickly read SDS-PAGE bands and sequence size.
Input Data
Results
At a glance:The molecular weight (MW) of a protein is the sum of the masses of all amino-acid residues in a polypeptide chain, usually expressed in daltons (Da) or kilodaltons (kDa). The exact value requires summing the 20 amino-acid residue masses and subtracting the condensation water, but a quick estimate is common in lab and teaching: MW ≈ residue count × average residue mass, where the average residue mass is about 110 Da (water lost in peptide-bond formation already subtracted). This estimate lets you predict the molecular weight from sequence length and compare it with the migration position of protein bands on SDS-PAGE.
Formula
MW estimate: MW ≈ amino-acid residue count × average residue mass (default 110 Da).
Conversion: kDa = Da ÷ 1000.
$$MW \approx N_{aa} \times \bar{m}_{residue}$$$$MW(\text{kDa}) = \dfrac{MW(\text{Da})}{1000}$$How to Use
- Enter the amino-acid residue count; the average residue mass defaults to 110 Da but can be adjusted.
- The tool instantly gives the estimated molecular weight in both Da and kDa.
- Compare this estimate with SDS-PAGE standard protein bands to quickly judge the target protein size.
Molecular weights of common proteins
| Protein | Approx. Residues | MW (kDa) |
|---|---|---|
| Insulin (monomer) | 51 | ≈ 5.8 |
| Lysozyme | 129 | ≈ 14.3 |
| Myoglobin | 153 | ≈ 17 |
| Serum albumin | 585 | ≈ 66 |
| Immunoglobulin IgG | ≈ 1320 | ≈ 150 |
Residue count × 110 Da is a quick estimate; actual values differ slightly due to amino-acid composition, post-translational modifications (e.g. glycosylation) and multi-subunit assembly.
Case Studies
Estimate MW from sequence length
A recombinant protein has 300 amino-acid residues, average residue mass 110 Da.
MW ≈ 300 × 110 = 33,000 Da = 33 kDa.
On SDS-PAGE it should band near the 33 kDa standard, confirming the expressed product size.
Molecular weight of serum albumin
Human serum albumin has about 585 amino-acid residues.
MW ≈ 585 × 110 = 64,350 Da ≈ 64 kDa.
Close to the actual ~66 kDa; the difference comes from exact composition. Quick estimate is enough to read electrophoresis.
FAQ
Why use 110 Da as the average residue mass?
The 20 free amino acids average about 128 Da, but each residue loses one water molecule (~18 Da) in peptide-bond formation, so the average residue mass in-chain is about 110 Da. Multiplying by the residue count gives a quick, close estimate — a common lab convention.
How far is this estimate from the exact value?
Most proteins are within a few percent, enough for reading electrophoresis and rough estimates. Error comes from actual composition deviating from the average: tryptophan/tyrosine-rich proteins are underestimated, glycine/alanine-rich ones overestimated. For an exact value, sum residue masses from the sequence.
Do post-translational modifications affect MW?
Yes. Glycosylation, phosphorylation, lipid modification, disulphide bonds and proteolytic cleavage all change the actual MW. Glycosylation especially can make the apparent MW on SDS-PAGE far above the sequence estimate. If the target protein has heavy modifications, this estimate reflects only the polypeptide backbone.
Why do SDS-PAGE band positions differ from the estimate?
SDS-PAGE estimates MW by mobility after SDS binding; proteins with unusual charge, shape, heavy modification or extreme pI show 'apparent MW' deviation. Membrane and glycoproteins often run slower than expected. The estimate is a starting point; confirm with standards, mass spectrometry or western blot.
How to handle multi-subunit proteins?
This calculator gives the MW of a single polypeptide chain. If a protein has multiple subunits (e.g. IgG has two heavy and two light chains), compute each chain's MW separately and add to get the full complex MW; on denaturing SDS-PAGE you usually see separate subunit bands.
Related Tools
References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.