What this protein kDa calculator does
This calculator estimates protein molecular weight in both Daltons (Da) and kilodaltons (kDa). You can calculate mass from a full amino acid sequence for higher accuracy, or use a quick estimate from sequence length when the exact sequence is not available.
In most lab workflows, you’ll see protein mass reported in kDa. For example, a protein listed as “55 kDa” has a molecular weight of about 55,000 Da. This is useful for SDS-PAGE planning, Western blot interpretation, chromatography setup, and recombinant construct design.
How kDa is calculated
1) Sequence-based calculation (recommended)
When you provide a one-letter amino acid sequence, the calculator sums residue masses and adds the mass of water to represent the full polypeptide termini:
- Total mass (Da) = Σ(residue masses) + H2O + extra mass adjustment
- kDa = total mass / 1000
This approach is much better than simple length approximation, especially for proteins rich in heavy residues like tryptophan (W), tyrosine (Y), or arginine (R).
2) Length-only estimate (quick approximation)
If sequence is unknown, a common rule of thumb is ~110 Da per amino acid residue:
- Estimated mass (Da) = (number of residues × average residue mass) + H2O + extra mass
This is convenient for rough planning, but sequence-based values are preferred whenever possible.
How to use this calculator
- Paste your protein sequence (plain or FASTA), or enter only the amino acid length.
- Choose average or monoisotopic mass mode.
- Optionally add a custom mass offset for affinity tags, labels, or known modifications.
- Click Calculate kDa to get Da and kDa instantly.
Why predicted mass may differ from observed band size
It is normal for calculated molecular weight and experimental migration to differ. Apparent mass on gels can shift due to structure and chemistry, not just true atomic mass.
- Post-translational modifications: glycosylation, phosphorylation, ubiquitination, lipidation.
- Disulfide status: reducing vs non-reducing conditions can change mobility.
- Protein conformation: membrane proteins and highly acidic/basic proteins often migrate atypically.
- Tags and linkers: His-tags, GST, MBP, Fc fusions, and cleavage scars alter expected mass.
- Proteolysis/processing: signal peptide removal or truncation affects final mature size.
Common quick references
Dalton vs kilodalton
- 1 kDa = 1000 Da
- 25 kDa = 25,000 Da
- 150 kDa = 150,000 Da
Back-of-the-envelope estimate
For quick mental math, multiply amino acid count by 0.11 to estimate kDa. Example: 420 aa × 0.11 ≈ 46.2 kDa (before PTMs/tags).
FAQ
Should I use average or monoisotopic mass?
Use average mass for general protein reporting and many routine biology contexts. Use monoisotopic mass when matching high-resolution mass spectrometry peaks.
Can I paste FASTA directly?
Yes. Header lines starting with > are ignored automatically, and line breaks/spaces are removed.
Can I include non-standard amino acids?
This calculator is built for the 20 canonical residues in sequence mode. If your construct includes unusual residues or chemical modifications, use the additional mass field to account for them.
Bottom line
For best accuracy, use full sequence input and then adjust for known modifications. For quick planning, length-based estimation is perfectly fine. Either way, this protein kDa calculator gives you immediate, practical values for experimental design.