Protein Molecular Weight · Amino Acid Length · CDS Size
kDa to Amino Acid Calculator
The kDa to amino acid calculator converts protein molecular mass in kilodaltons (kDa) or daltons (Da) into estimated amino acid residue count and open reading frame (CDS) length in base pairs, and converts residue counts back to molecular weight.
Interactive Protein Mass & Residue Converter
110 Da/residue is the standard biochemical average; weighted natural proteome average is ~111.1 Da.
Subtracts the selected fusion tag mass before estimating native protein residues.
Residues (N) ≈ (Mass_Da − Tag_Da − 18.015) ÷ Avg_Residue_Da; CDS = (N + 1) × 3 bp.
Formula: How to Convert kDa to Amino Acids (and Back)
When free amino acids condense into a polypeptide chain, each peptide bond releases one water molecule (H₂O = 18.015 Da). Across typical proteomes, the average amino acid residue mass in a polypeptide chain is approximately 110 Da (0.110 kDa).
kDa to Amino Acid & CDS Length Quick Reference Table
Standard conversion table using the 110 Da/residue biochemical rule of thumb, including the corresponding coding DNA sequence (CDS) length with one stop codon:
| Protein Mass (kDa) | Mass in Daltons (Da) | Estimated Length (110 Da/aa) | CDS Length (incl. stop) |
|---|---|---|---|
| 10 kDa | 10,000 Da | ~91 aa | ~276 bp |
| 25 kDa | 25,000 Da | ~227 aa | ~684 bp |
| 37 kDa | 37,000 Da | ~336 aa | ~1,011 bp |
| 50 kDa | 50,000 Da | ~455 aa | ~1,368 bp |
| 55 kDa | 55,000 Da | ~500 aa | ~1,503 bp |
| 75 kDa | 75,000 Da | ~682 aa | ~2,049 bp |
| 100 kDa | 100,000 Da | ~909 aa | ~2,730 bp |
| 150 kDa | 150,000 Da | ~1,364 aa | ~4,095 bp |
| 250 kDa | 250,000 Da | ~2,273 aa | ~6,822 bp |
Practical Factors Affecting SDS-PAGE & Mass Estimates
Amino acid composition bias
Proteins enriched in small residues such as Glycine (57.05 Da) or Alanine (71.08 Da) contain more residues per kDa, whereas proteins rich in Tryptophan (186.21 Da), Tyrosine (163.18 Da), or Arginine (156.19 Da) have a higher average residue weight.
Epitope & affinity fusion tags
Recombinant expression constructs frequently include N- or C-terminal tags. Small tags like 6×His (~0.84 kDa) or FLAG (~1.01 kDa) shift mass slightly, while GST (~26.0 kDa), EGFP (~26.9 kDa), and MBP (~42.5 kDa) add hundreds of residues.
Post-translational modifications & gel migration
N-linked glycosylation, phosphorylation, ubiquitination, signal peptide cleavage, or highly acidic/proline-rich regions can cause apparent SDS-PAGE band mobility to differ from the unmodified polypeptide formula weight.
Related Protein & Gel Analysis Calculators
Continue your protein workflow with sequence-specific physicochemical calculations, polyacrylamide gel recipe casting, or individual residue mass tables:
Frequently Asked Questions
How many amino acids are in a 50 kDa or 55 kDa protein?
Using the standard 110 Da per residue average, a 50 kDa protein contains approximately 455 amino acids (~1,368 bp CDS), and a 55 kDa protein contains approximately 500 amino acids (~1,503 bp CDS).
How do you convert amino acid length to kDa?
Multiply the number of amino acid residues by 110 Da (or 0.110 kDa) and add 18.015 Da for the terminal H and OH groups. For example, a 300-amino-acid protein has an estimated molecular weight of about 33.0 kDa.
Why is 110 Da used instead of 128 Da per amino acid?
The average molecular weight of the 20 free amino acids is ~128 Da, but peptide bond formation removes one water molecule (18.015 Da) per linkage. Subtracting 18 Da leaves ~110 Da as the effective residue mass inside a protein chain.
How do I estimate gene coding sequence (CDS) length from protein kDa?
First divide the protein mass in Da by 110 to estimate the number of amino acids (N), then multiply (N + 1) by 3 nucleotides per codon to include the translation stop codon.
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