protein size calculator

What does “protein size” actually mean?

Protein size can mean different things depending on context. In computational biology, we often start with sequence length (number of amino acids). In biochemistry labs, we usually discuss molecular weight in Daltons (Da) or kilodaltons (kDa). In structural biology, we care about physical dimensions such as radius or diameter in nanometers.

This calculator gives all three practical views of size in one place so you can quickly move from sequence to useful lab-level estimates.

How this calculator works

1) Molecular weight estimation

If you provide a sequence, the script sums residue masses and adds terminal water mass (approximately 18.015 Da). If you do not provide a sequence, it uses:

• Estimated MW (Da) = amino acid length × average residue mass + 18.015

2) Contour length

The fully extended peptide backbone contributes roughly 0.36 nm per residue:

• Contour length (nm) = residues × 0.36

This is not folded size; it is an upper-bound chain length if stretched out.

3) Approximate folded diameter

For globular proteins, hydrodynamic radius scales roughly with molecular weight to the one-third power. We use:

• Radius (nm) ≈ 0.066 × MW(Da)^1/3 × shape factor
• Diameter (nm) = 2 × radius

This gives a practical estimate for planning chromatography, filtration, and rough structural expectations.

When to use sequence vs. length input

• Use sequence when you have a known amino acid string and want a better mass estimate.
• Use length for quick screening, construct design, or when only residue count is known.
• Keep in mind that tags, signal peptides, cleavage products, glycosylation, phosphorylation, and disulfide status can shift real experimental mass.

Practical examples

Example A: 300 aa enzyme

With the default 110 Da per residue, mass is approximately 33 kDa. That fits well with what many SDS-PAGE gels would show for a mid-sized soluble enzyme.

Example B: 1200 aa structural protein

This lands near 132 kDa by rough estimate. At this scale, multidomain architecture is common, and the compact-size assumption may underpredict dimensions if regions are disordered.

Why this is useful in real workflows

• Designing expression constructs and fusion tags
• Choosing gel percentage for SDS-PAGE bands
• Estimating dialysis and filtration cutoff strategy
• Planning SEC (size exclusion chromatography) ranges
• Generating sanity checks for mass spectrometry outputs

Important limitations

This tool is intentionally lightweight and fast. It does not calculate post-translational modifications, oligomerization states, membrane insertion effects, glycan mass additions, or exact charge-state behavior. Treat results as first-pass planning values, not final analytical measurements.

Bottom line

If you need a quick, practical estimate of protein molecular size, this calculator gives you a strong starting point in seconds. For publication-grade values, always validate with experimental methods such as mass spectrometry, analytical ultracentrifugation, SEC-MALS, or high-quality structural data.