Protein Molecular Weight Calculator
Calculate the molecular weight (Da and kDa) of a protein from its amino-acid sequence. Computed from average residue masses + one water. 100% in-browser — your sequence is never uploaded.
- 1Sum of residue masses
Σ residues = 8287.47 Da - 2Add one water (peptide bond hydrolysis)
MW = 8287.47 + 18.01528 = 8305.49 Da
🔒 100% client-side — your data is computed in the browser and never uploaded.
Cite this tool
ToolJolt. Protein Molecular Weight Calculator. ToolJolt Chemistry & Lab Tools; 2026. https://tooljolt.comProtein Molecular Weight Calculator for protein biochemists, structural biologists and assay developers. Enter your values and read a sourced, step-by-step result instantly, right in your browser.
About Protein Molecular Weight Calculator
Calculate the molecular weight (Da and kDa) of a protein from its amino-acid sequence. Computed from average residue masses + one water. 100% in-browser — your sequence is never uploaded. The calculation uses MW = Σ residue masses + H₂O. Why accuracy here pays off: The extinction coefficient sets every A280 concentration you report; the pI decides your ion-exchange and solubility strategy. These numbers underpin purification and quantitation. Protein: Computed from average residue masses + one water. Mistakes that trip people up: ignoring post-translational modifications; forgetting cystine (disulfide) contributions to ε280; using reduced vs oxidized ε in the wrong context. No account, no upload, no tracking of your inputs — the result is generated on your machine, which makes it reproducible, private and citable in published work.
How to use Protein Molecular Weight Calculator
- 1Enter your input values.
- 2Read the headline result and the supporting figures, which recompute as you type.
- 3Open “Worked example with your numbers” to see the substituted formula step by step.
- 4Copy the result, or use the cite-this-tool snippet for your methods section.
Why use Protein Molecular Weight Calculator?
- ✓Designed for protein biochemists, structural biologists and assay developers who need a trustworthy answer fast
- ✓Instant, client-side result — works offline once loaded and keeps your data private
- ✓Shows the worked example step by step with your own numbers, not just a final figure
- ✓Pre-filled with sensible, niche-specific defaults so it is useful the second it loads
- ✓Mobile-friendly and completely free, with no sign-up or usage caps
Frequently asked questions
Any tips specific to this calculation?+
Protein: Computed from average residue masses + one water. Also watch out for: ignoring post-translational modifications and assuming pI from sequence equals experimental pI exactly.
Is this protein molecular weight calculator free to use?+
Yes. It is completely free, needs no sign-up, and runs entirely in your browser — there are no usage limits.
What formula does it use?+
It uses MW = Σ residue masses + H₂O The full worked example is shown beneath the result so you can verify each step.
What are the most common mistakes here?+
In protein and peptide characterization, watch for: forgetting cystine (disulfide) contributions to ε280; using reduced vs oxidized ε in the wrong context; assuming pI from sequence equals experimental pI exactly; ignoring post-translational modifications. This tool shows the working so you can catch these before they cost an experiment.
Does my data leave my device?+
No. All computation happens locally in your browser. Nothing you enter — sequences, concentrations or measurements — is uploaded to any server, so it is safe for confidential work.
Can I cite this tool?+
Yes — use the “Cite this tool” snippet on the page. Many users link these calculators from methods sections, lab SOPs and teaching materials.
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