# Protein Molecular Weight Calculator

> Protein molecular weight calculator: estimate a protein's approximate mass in kilodaltons from amino acid chain length. Free, runs in your browser, no signup.

- **Canonical URL:** https://boringtoolskit.com/protein-molecular-weight-calculator/
- **Category:** Biology
- **Evidence level:** authored-methodology
- **Reviewed:** July 26, 2026
- **Last verified:** 2026-10-12
- **Privacy:** Inputs and results stay in this browser unless the page clearly labels an external request.

## How it works

Weight (Da) = chain length (residues) x average residue mass (about 110 Da).

## Worked example

A 300-residue protein has an estimated molecular weight of about 33,000 Da (33 kDa) using the 110 Da average.

## Assumptions

- The 110 Da average ignores the water lost in each peptide bond (about 18 Da per bond).
- For exact mass, sum the actual residue sequence masses.

## Common questions

### Why 110 Da per residue?

It is the average mass of the 20 amino acids minus the water lost in peptide bonding.

### How accurate is the estimate?

Within about 10% of the true mass for typical sequences; use the exact sequence for precise work.

## Related tools

- /dilution-calculator/
- /molarity-calculator/
- /unit-converter/

## Sources

- [Protein molecular weight data](https://www.uniprot.org/)
- [Calculation methodology](/methodology/)

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