Formula Used
Residue count: N = valid residues + estimated unknown residues + linker residues.
Contour length: Lc = N × contour rise + terminal allowance.
Alpha helix length: Lh = N × helix rise + terminal allowance.
Beta strand length: Lb = N × beta rise + terminal allowance.
Custom length: Lx = N × custom rise + terminal allowance.
Peptide bonds: B = N − 1, when N is greater than zero.
Estimated mass: M = sum of residue masses + unknown estimates + linker estimates + H₂O.
Coding bases: Bases = N × 3. Add 3 bases when a stop codon is included.
How to Use This Calculator
Select sequence mode when you have a one letter amino acid sequence. Paste the sequence into the large box.
Select direct count mode when you only know the residue total. Enter the count and adjust the rise values.
Use linker residues for tags, spacers, repeats, or planned flexible regions. Set terminal allowance when extra end distance is needed.
Press the calculate button. The result appears below the header and above the form. Use the export buttons to save records.
Example Data Table
| Example |
Residues |
Contour Å |
Alpha Helix Å |
Average Mass kDa |
| Short peptide panel |
25 |
95.0 |
37.5 |
2.77 |
| Small protein construct |
120 |
456.0 |
180.0 |
13.22 |
| Large enzyme domain |
300 |
1140.0 |
450.0 |
33.02 |
Amino Acid Length Chemistry Guide
Why Residue Length Matters
An amino acid sequence can describe a small peptide or a large protein. Each residue adds a repeated backbone segment. The calculator turns that sequence into useful length estimates. It also reports mass, peptide bonds, coding bases, and composition. These values help during cloning, purification, modeling, and teaching.
Interpreting Chain Length
The main length estimate is the contour length. It treats the chain as an extended polymer. A common rise per residue is about 3.8 angstroms. This value fits a stretched peptide backbone. It is not the folded size. Folded proteins can be much smaller. Disulfide bonds, salt bridges, and packing can shorten the visible distance.
Helix, Strand, and Custom Models
Alpha helices are more compact along their axis. They usually rise about 1.5 angstroms per residue. Beta strands are more extended. They often use about 3.3 angstroms per residue. The custom field supports linkers, repeats, or measured structures. You can compare all spans in one result panel.
Mass and Composition
Mass estimation uses residue masses. A peptide bond forms when water is removed. Therefore residue masses represent amino acids after condensation. The tool adds one water molecule for the chain termini. Unknown letters may be ignored or estimated. This choice is useful for draft sequences.
Composition is also important. A cysteine rich chain may form disulfides. A proline rich chain may resist helix formation. Charged residues can alter solubility. Hydrophobic residues may suggest membrane regions. The percentage table gives a quick view before deeper analysis.
Practical Notes
Use clean one letter amino acid codes when possible. Spaces, numbers, and line breaks are removed. FASTA headers should be deleted before pasting. If the sequence is not final, use direct count mode. Add linker residues when tags or flexible segments are planned.
The calculated length is a model. It should guide early design, not replace structure data. For exact dimensions, use crystallography, NMR, cryo EM, or molecular simulation. Still, quick estimates save time. They help compare constructs and choose practical purification strategies.
For reports, export the result as a spreadsheet row or a simple document. Keep the entered rise values beside each estimate. This makes the calculation reproducible. When comparing variants, use the same settings for every construct. Small changes then show real sequence effects clearly across the full design set.
FAQs
What is amino acid length?
It is an estimated chain span based on residue count and rise per residue. It may describe contour length, helix length, strand length, or a custom structural model.
Does this calculator show folded protein size?
No. It estimates chain length models. Folded size depends on tertiary structure, domains, solvent, disulfides, and packing. Use structural data for exact folded dimensions.
Why is 3.8 Å used for contour length?
A value near 3.8 Å is often used for an extended peptide backbone. It gives a useful maximum style estimate for many sequence planning tasks.
Why is alpha helix length shorter?
An alpha helix rises about 1.5 Å per residue along its axis. The backbone coils, so axial length is shorter than an extended chain.
Can I enter FASTA format?
You can paste sequence lines. Header lines beginning with the greater than symbol are skipped. It is still best to remove headers before calculation.
How are unknown amino acid letters handled?
You can estimate them with the average residue mass or ignore them. Estimation is useful for draft sequences. Ignoring is better for strict validated inputs.
What does linker residues mean?
Linker residues are extra planned residues, such as tags, spacers, or flexible regions. They are added to the total count and length estimates.
Are CSV and PDF exports included?
Yes. After calculation, buttons appear in the result section. They download the main metrics, mass estimates, length models, and composition data.