kDa Calculator
Convert between Daltons (Da), kilodaltons (kDa), and megadaltons (MDa) instantly, estimate protein molecular weight from amino acid count or gene length using the 110 Da/residue rule, and predict SDS-PAGE band position with a visual gel diagram.
Estimate protein molecular weight using the 110.5 Da/residue rule. Switch between amino acid count and nucleotide/gene length input.
Enter protein MW in kDa to see approximate band position on a standard SDS-PAGE gel. Uses log-linear interpolation between reference ladder bands.
The orange band shows your protein's estimated position.
Black bands = molecular weight ladder reference.
Migration follows a log-linear relationship with MW.
* Band position is an estimate. Actual migration depends on protein shape, post-translational modifications, and buffer conditions.
Click any column header to sort. Click any row to load that protein's MW into the converter. Search by protein name.
| Protein ↕ | MW (kDa) ↕ | Notes ↕ | Relative Size |
|---|
What Is a Dalton (Da) and Kilodalton (kDa)?
This kDa calculator converts between Daltons, kilodaltons, and megadaltons, estimates protein molecular weight from amino acid residue count, and predicts SDS-PAGE band position. It is designed for biochemists, molecular biologists, and students who need quick, accurate protein size calculations.
The Dalton (Da) is the standard unit of atomic and molecular mass, defined as one-twelfth the mass of a carbon-12 atom. Numerically, 1 Da equals 1 g/mol for molecular masses, which is why you will see protein molecular weights reported interchangeably in Da, g/mol, or kDa. 1 kDa = 1,000 Da — kilodalton simply means one thousand Daltons.
Proteins are measured in kilodaltons because it is a more convenient scale for the sizes encountered in biology. Most proteins range from 5 kDa to 500 kDa — expressing these as Daltons (5,000 Da to 500,000 Da) is unwieldy. The kDa unit was universally adopted in biochemistry for this reason.
How to Figure Out kDa — The 110 Da/Residue Rule
If you know the number of amino acids in a protein, you can estimate its molecular weight in kDa using the average residue mass of 110 Da per amino acid (1 kDa = 1,000 Da, so each residue ≈ 0.11 kDa):
The 110 Da average comes from the weighted mean residue mass across all 20 standard amino acids. Residue masses range from Glycine (57 Da, lightest) to Tryptophan (186 Da, heaviest). For most natural proteins, the 110 Da/residue rule gives a good first estimate before you have a sequence for exact calculation.
How to Convert Between Da and kDa
Converting between Daltons and kilodaltons is straightforward: 1 kDa = 1,000 Da, so you simply divide by 1,000 (Da → kDa) or multiply by 1,000 (kDa → Da). This is one of the most common conversions in biochemistry.
| Protein | Da | kDa | MDa |
|---|---|---|---|
| Insulin | 5,800 | 5.8 | 0.0058 |
| Lysozyme | 14,300 | 14.3 | 0.0143 |
| Carbonic anhydrase | 29,000 | 29.0 | 0.029 |
| BSA | 66,500 | 66.5 | 0.0665 |
| IgG Antibody | 150,000 | 150.0 | 0.150 |
| Ferritin | 440,000 | 440.0 | 0.440 |
| Ribosome (70S) | 2,500,000 | 2,500 | 2.5 |
Amino Acid to kDa — Each Residue Adds ~0.11 kDa
A common question is how to convert amino acid count to kDa. Each amino acid residue adds approximately 110 Da = 0.11 kDa to protein molecular weight. This means a single amino acid contributes about one-tenth of a kilodalton. For example, a peptide with 10 residues ≈ 10 × 0.11 = 1.1 kDa; a protein with 500 residues ≈ 500 × 0.11 = 55 kDa.
Estimating Protein Size from Amino Acid or Gene Length
The 110 Da/residue approximation is the standard biochemistry shorthand for estimating protein size before sequencing or when a quick calculation is needed. The average of 110 Da arises from the weighted mean of all 20 amino acid residue masses weighted by their typical occurrence in natural proteins.
When does the 110 Da/residue rule fail? For proteins with unusual amino acid compositions — such as silk fibroin (Gly/Ala-rich, lighter residues) or Trp-rich membrane proteins — the true molecular weight can deviate by 15–20% from the estimate. For precise values, always use sequence-based calculation.
From Gene Length to Protein Size
For a coding DNA sequence, the relationship between gene length and protein size is: every 3 nucleotides (one codon) encodes one amino acid. Subtract one codon for the stop codon (which does not encode an amino acid). Then apply the 110 Da/residue rule:
| Gene Length | Codons | Amino Acids | Estimated MW (kDa) |
|---|---|---|---|
| 300 bp | 100 | 99 | ~10.9 |
| 500 bp | 167 | 166 | ~18.3 |
| 1,000 bp | 333 | 332 | ~36.5 |
| 1,500 bp | 500 | 499 | ~54.9 |
| 2,000 bp | 667 | 666 | ~73.3 |
| 3,000 bp | 1,000 | 999 | ~110.0 |
| 1 kb | 333 | 332 | ~36.5 |
| 2 kb | 667 | 666 | ~73.3 |
| 5 kb | 1,667 | 1,666 | ~183.7 |
SDS-PAGE — Estimating Band Position from Molecular Weight
SDS-PAGE (sodium dodecyl sulfate polyacrylamide gel electrophoresis) separates proteins by molecular weight. SDS denatures proteins and gives them a uniform negative charge proportional to mass, so migration through the gel is determined almost entirely by protein size. Smaller proteins migrate farther and faster through the gel matrix.
The key relationship in SDS-PAGE is that log₁₀(MW) is approximately linearly proportional to migration distance (relative mobility, Rf). This log-linear relationship allows estimation of protein size by comparison to a molecular weight ladder with bands at known sizes (10, 15, 20, 25, 37, 50, 75, 100, 150, 250 kDa in most commercial ladders).
Gel % selection guide: 8% gels resolve 50–250 kDa proteins best. 10% gels work well for 30–150 kDa. 12% gels are standard for 15–100 kDa. 15% gels are optimal for 10–50 kDa. Always choose gel % based on your expected protein size.
Protein Size Reference — Common Proteins and Their Molecular Weights
The table below lists 25 well-characterized proteins spanning the full size range from small peptides (insulin, 5.8 kDa) to large multi-subunit complexes (ribosome, 2,500 kDa). These are the reference points most commonly used as molecular weight standards in biochemistry.
Why is BSA (66.5 kDa) the most common protein standard? Bovine serum albumin is cheap, stable, highly pure, and its molecular weight of 66.5 kDa (1 kDa = 1,000 Da) places it in the middle of the range relevant to most proteins. It serves as both a molecular weight marker in SDS-PAGE and a concentration standard in Bradford and BCA assays.
Why are IgG antibodies exactly 150 kDa? A full IgG antibody consists of four chains: 2 heavy chains (~50 kDa each) and 2 light chains (~25 kDa each). Total: 2×50 + 2×25 = 150 kDa. This is why 150 kDa is such a consistent reference point — all IgG antibodies have essentially the same overall architecture.
Worked Examples — kDa Conversion Problems
Example 1 — Converting BSA molecular weight
- BSA molecular weight = 66,500 Da
- Convert to kDa: 66,500 ÷ 1,000 = 66.5 kDa (1 kDa = 1,000 Da)
- Convert to MDa: 66,500 ÷ 1,000,000 = 0.0665 MDa
- Estimated residues: 66,500 ÷ 110 ≈ 605 amino acids
Example 2 — Estimating MW from residue count
- Protein has 333 amino acid residues
- Estimated MW = 333 × 110.5 Da = 36,796.5 Da
- In kDa: 36,796.5 ÷ 1,000 = 36.8 kDa
- Note: actual MW may differ by ±10–15% from the 110.5 Da/residue estimate
Example 3 — From gene length to protein size
- Gene length: 1,200 bp coding sequence
- Number of codons: 1,200 ÷ 3 = 400
- Amino acids: 400 − 1 (stop codon) = 399
- Estimated MW: 399 × 110.5 = 44,089.5 Da = 44.1 kDa
Example 4 — SDS-PAGE band position for a 45 kDa protein
- Protein MW = 45 kDa; gel percentage = 12%
- Nearest ladder bands: 50 kDa (Rf ≈ 0.52) and 37 kDa (Rf ≈ 0.63)
- Log-linear interpolation: Rf ≈ 0.57 (slightly above the 50 kDa band)
- Interpretation: band appears approximately 57% of the way from well to bottom — between the 37 and 50 kDa ladder bands, closer to 50 kDa