Protein Molecular Weight Calculator
Calculate the molecular weight of a protein from its amino acid sequence or number of amino acids.
Enter Your Values
Enter your values and press Calculate: you will get the result, its method, the inputs used and confidence, plus copy, print and share options.
How this calculator works
Protein molecular-weight summationv1.0.0
Molecular weight of a peptide or protein computed by summing amino-acid residue masses plus water, from the one-letter sequence.
- Applies to
- Students and researchers working with peptide sequences
- Assumptions
- Unmodified residues; no post-translational modifications, salts or counterions
- Limitations
- Modified residues (phosphorylation, glycosylation) are not accounted for
- Output
- Da (daltons)
- Last reviewed
- 2026-06-01
Evidence Sources & Literature
- IUPAC. Amino-acid residue average masses used for peptide molecular-weight calculations. IUPAC-IUB Commission on Biochemical Nomenclature, 2009.
About the Protein Molecular Weight Calculator
This is a scientific tool rather than a nutrition one: it calculates the molecular weight of a protein from its amino acid sequence. It is useful for students, researchers and anyone working in biochemistry or molecular biology, where knowing a protein's molecular weight is essential for lab work like running gels, preparing solutions and interpreting results. If you arrived here looking for dietary protein, the protein calculator is the one you want instead.
I include this because Macro and Meals serves a curious, science-minded audience too, and molecular weight is one of those fundamental numbers people need quickly. Here is how it works and why it matters in the lab.
What molecular weight means here
A protein is a chain of amino acids, and each amino acid has a known mass. The molecular weight of the whole protein is essentially the sum of its amino acid residues (accounting for the water molecules released when they link together). It is usually expressed in daltons (Da) or kilodaltons (kDa). This single number tells researchers a lot about how a protein will behave in experiments.
How the calculation works
The calculator takes a protein's amino acid sequence (the string of letters representing each residue) and adds up the masses of those residues, subtracting the mass of water lost at each peptide bond. The result is the protein's estimated molecular weight. It is a straightforward computation, but doing it by hand for a long sequence is tedious and error-prone, which is exactly why a tool like this is handy.
Why it matters in the lab
Molecular weight is a workhorse number in molecular biology. It helps you predict where a protein will appear on an SDS-PAGE gel, calculate molar concentrations from a mass, work out the amounts needed for assays, and confirm a protein's identity against an expected value. Getting it right underpins a lot of everyday experimental work, so a quick, accurate calculation saves time and reduces mistakes.
A note on accuracy
The calculated value is based on the standard masses of the amino acids in your sequence, so it reflects the unmodified protein. Real proteins often carry modifications (such as added sugar groups or phosphate) that change their actual measured weight, which is why an experimental result can differ from the theoretical figure. Treat the calculator's number as the theoretical molecular weight and account for any known modifications separately.
Dietitian Insight: Calculating peptide molecular mass allows precise biochemical analysis of amino acid polymers. To bridge molecular biology with real-world nutritional requirements, explore our dietary macro calculation framework or determine personal needs with our daily protein calculator.
Sources & method
This calculator uses established, peer-reviewed formulas and reference ranges from recognized health and nutrition authorities. Results are estimates for general education, not a medical diagnosis. For decisions about your health, consult a qualified clinician. Reviewed by Macro & Meals Editorial Team, Content editor.
Frequently Asked Questions
What does this protein molecular weight calculator do?
It calculates the molecular weight of a protein from its amino acid sequence, summing the masses of the residues and accounting for the water released as they link together. The result, usually in daltons or kilodaltons, is a fundamental value for biochemistry and molecular biology work.
Is this the same as the dietary protein calculator?
No. This is a scientific tool for computing the molecular mass of a specific protein from its sequence, used in lab settings. If you want to know how much protein to eat each day for health or fitness, the protein calculator is the right tool instead.
Why is a protein's molecular weight important?
It is widely used in the lab: to predict where a protein runs on an SDS-PAGE gel, to calculate molar concentrations from a mass, to prepare assays, and to confirm a protein's identity against an expected value. It underpins a lot of everyday experimental work.
Why might the measured weight differ from this calculation?
The calculator gives the theoretical weight of the unmodified protein based on its amino acid sequence. Real proteins often carry modifications, such as added sugar or phosphate groups, that change the actual measured weight, so experimental results can differ from the theoretical figure.
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