Protein Molecular Weight Calculator

Compute protein molecular weight from a custom list of amino acids.

Supports all 20 standard amino acids with automatic peptide bond water loss correction.

Updated July 30, 2026

CreatorFrank Zhao

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Introduction / overview

The Protein Molecular Weight Calculator estimates the molecular mass of a peptide or small protein from its amino acid sequence. You pick which amino acids make up your protein — up to 20 residues — and the calculator sums their individual weights while automatically accounting for the water molecules lost when each peptide bond forms.

✅ Results are shown in both daltons (u) and kilodaltons (kDa) — the two units you'll encounter most often in biochemistry literature and lab protocols.

Who is this for?

Lab researchers & students who need a quick molecular weight for SDS-PAGE, Western blotting, or mass spectrometry interpretation.
Bioinformatics beginners learning how peptide sequences translate into physical properties.
Anyone preparing protein-based solutions who needs weight-to-mole conversions.

The calculator uses standard biochemical residue weights and the accepted water loss value of $18.0153\ \mathrm{u}$ per peptide bond. If you also work with nucleic acids, you might find our DNA Concentration Calculator or Protein Concentration Calculator useful for downstream applications.

How to use / quick start

1Select the first amino acid from the dropdown — the calculator starts with Alanine (Ala, A) already chosen as a placeholder.
2Add more amino acids by selecting from each subsequent dropdown. After you pick the 3rd residue, a new empty slot automatically appears — keep going until your sequence is complete (up to 20 residues).
3Read your results in the Results section. The calculator shows the molecular weight in both kilodaltons (kDa) and daltons (u). The sequence display below lists your chosen residues in FASTA-like format.

Example: a simple tripeptide

Let's build the tripeptide Ala–Arg–Asn (Alanine, Arginine, Asparagine).

Step 1: Select the three amino acids:

1st: Alanine (Ala, A) — molecular weight 89.094 u
2nd: Arginine (Arg, R) — molecular weight 174.203 u
3rd: Asparagine (Asn, N) — molecular weight 132.119 u

Step 2: The calculator sums the weights and subtracts the water lost during bond formation.

\text{Sum} = 89.094 + 174.203 + 132.119

=

395.416\ \mathrm{u}

\text{Water loss} = 2 \times 18.0153

=

36.0306\ \mathrm{u}

\text{MW}_\text{u} = 395.416 - 36.0306

\approx

359.39\ \mathrm{u}

\text{MW}_\text{kDa} = 359.39 \div 1000

\approx

0.359\ \mathrm{kDa}

The calculator shows both values. You can use the daltons figure for precise stoichiometric calculations and the kilodalton figure for comparing with gel markers or literature references.

Real-world examples / use cases

1) Predicting Western blot band position

Background: You're running an SDS-PAGE gel to detect a small peptide hormone with the sequence Gly–Leu–Phe (GLF).

Inputs: Select Glycine (75.067 u), Leucine (131.175 u), and Phenylalanine (165.192 u).

Result: The calculator gives $\approx 353.42\ \mathrm{u}$ (0.353 kDa). You can estimate the band will migrate just above the 0.35 kDa mark on your gel.

2) Designing a synthetic peptide

Background: A researcher wants a custom peptide with antimicrobial activity containing 8 residues: Ala–Lys–Leu–Trp–Gly–Ile–Arg–Val.

Inputs: Enter all eight amino acids into the calculator.

Result: With 8 residues, the calculator subtracts 7 water molecules. The final weight helps determine how much peptide powder to weigh for your stock solution. Combined with our Protein Concentration Calculator, you can then prepare accurate molar solutions.

3) Checking post-translational modification impact

Background: You're studying a phosphorylation event on a small peptide. The unmodified peptide is Ser–Tyr–Thr (SYT).

Inputs: Enter serine, tyrosine, and threonine (total $\approx 289.15\ \mathrm{u}$ after water loss).

Application: A phosphorylation adds roughly 80 Da ( $\mathrm{HPO_3}$ ). By comparing the unmodified and modified weights, you can estimate the mass shift expected in your mass spectrum.

4) Classroom exercise: peptide bond counting

Background: A biochemistry instructor wants students to verify that $n$ amino acids form $n-1$ peptide bonds.

Inputs: Add 1, then 2, then 3 residues and watch how the water loss increases from 0 to $18.02\ \mathrm{u}$ to $36.04\ \mathrm{u}$ .

Takeaway: The calculator makes the "water loss per bond" concept tangible — each new residue adds its weight but also adds one more water molecule to subtract.

Common scenarios / when to use

SDS-PAGE & Western blot band identification

Before running a gel, estimate your protein's molecular weight to predict where bands should appear. This is especially useful when working with synthetic peptides or recombinant fragments where the exact sequence is known.

Molar solution preparation

Once you know the molecular weight, you can convert between mass and moles. For example, to prepare a 1 mM solution of a 0.5 kDa peptide, you'd need 0.5 mg per mL of solution.

Mass spectrometry data interpretation

Compare the calculated monoisotopic or average mass of your peptide with peaks in your mass spectrum. Small discrepancies may indicate post-translational modifications, truncations, or adducts.

Biochemistry homework & exam preparation

Rapidly check your manual peptide weight calculations. The step-by-step water loss logic reinforces the peptide bond concept — a common stumbling block for students first learning protein chemistry.

Tips & best practices

1
Know when to use average vs. monoisotopic mass
This calculator uses average isotopic masses, which are suitable for most lab applications (SDS-PAGE, Western blotting, routine solution prep). For high-resolution mass spectrometry, you may need a monoisotopic mass calculator that considers the most abundant isotope of each element.
2
Double-check the number of residues
The calculator handles up to 20 amino acids at a time. For longer proteins, calculate the weight of smaller peptide fragments separately. The displayed protein sequence helps you verify that the correct residues are in the right order.
3
Account for modifications separately
Post-translational modifications (phosphorylation, glycosylation, disulfide bridges) add or subtract mass. Calculate the unmodified weight first, then add the known mass shift of each modification. A single phosphorylation adds roughly 80 Da.
4
Use the results with concentration tools
Once you have the molecular weight, you can calculate molar concentrations, dilution factors, and extinction coefficients using our Protein Concentration Calculator. This is especially useful when working with custom peptides where the exact weight per vial is provided by the manufacturer.

Calculation method / formula explanation

The molecular weight of a protein is calculated by summing the individual residue masses of all amino acids in the sequence, then subtracting the mass of water lost when each peptide bond forms.

\text{MW}_{\text{u}} = \sum_{i=1}^{n} \text{AA}_i - (n - 1) \times 18.0153

MW in daltons (u), where n = number of amino acid residues

Where the numbers come from

Each of the 20 standard amino acids has a known molecular weight derived from its atomic composition. These values are well established in biochemistry (see the table below).

\text{AA}_i

=

\text{sum of atomic masses in residue } i

The water loss factor

When two amino acids form a peptide bond, a water molecule ( $\mathrm{H_2O}$ , MW = 18.0153 u) is released. For a chain of $n$ amino acids, exactly $n-1$ peptide bonds are formed, so $(n-1) \times 18.0153\ \mathrm{u}$ is subtracted.

\text{Water loss} = (n - 1) \times 18.0153\ \mathrm{u}

Converting to kilodaltons

Since 1 kilodalton = 1000 daltons, the conversion is straightforward:

\text{MW}_{\text{kDa}}

=

\frac{\text{MW}_{\text{u}}}{1000}

Standard amino acid molecular weights

The values used by this calculator (average isotopic masses, in daltons):

Amino acid	3-letter code	1-letter code	MW (u)
Alanine	Ala	A	89.094
Arginine	Arg	R	174.203
Asparagine	Asn	N	132.119
Aspartic acid	Asp	D	133.104
Cysteine	Cys	C	121.154
Glutamine	Gln	Q	146.146
Glutamic acid	Glu	E	147.131
Glycine	Gly	G	75.067
Histidine	His	H	155.156
Isoleucine	Ile	I	131.175
Leucine	Leu	L	131.175
Lysine	Lys	K	146.189
Methionine	Met	M	149.208
Phenylalanine	Phe	F	165.192
Proline	Pro	P	115.132
Serine	Ser	S	105.093
Threonine	Thr	T	119.119
Tryptophan	Trp	W	204.228
Tyrosine	Tyr	Y	181.191
Valine	Val	V	117.148

Related concepts / background info

Dalton vs. kilodalton vs. g/mol

A dalton (Da) is the unified atomic mass unit (u) — it's approximately the mass of a single proton or neutron. A kilodalton (kDa) is 1,000 daltons. In practice, daltons and g/mol are numerically identical: 1 Da = 1 g/mol. So a protein with a molecular weight of 50,000 Da (50 kDa) also has a molar mass of 50,000 g/mol. This is why you can directly convert between kDa and molar concentration — a 50 kDa protein at 1 g/L is a $20\ \mu\mathrm{M}$ solution.

Peptide bonds and water loss

Every time two amino acids join, a condensation reaction occurs: the carboxyl group ( $-\mathrm{COOH}$ ) of one amino acid reacts with the amine group ( $-\mathrm{NH_2}$ ) of another, releasing a water molecule. This means the total mass of the protein is always less than the sum of its free amino acid masses. The longer the chain, the more water molecules are lost.

Average vs. monoisotopic mass

This calculator uses average isotopic masses, which account for the natural abundance of all isotopes of each element (e.g., carbon includes both $^{12}\mathrm{C}$ and $^{13}\mathrm{C}$ ). For most wet-lab applications — gel electrophoresis, Western blotting, buffer preparation — average mass is the appropriate choice. For high-resolution mass spectrometry, you may need monoisotopic mass (using only the most abundant isotope), which is typically about 0.01–0.1% lower than the average mass.

Explore related tools

Protein Concentration Calculator DNA Concentration Calculator Atom Calculator Atomic Mass Calculator

Frequently asked questions (FAQs)

Why does the calculator subtract water from the total weight?

When two amino acids form a peptide bond, a molecule of water ( $\mathrm{H_2O}$ ) is released. Since the protein no longer contains that water, its mass is less than the sum of the individual amino acid masses. This is standard biochemistry — every textbook and research paper accounts for this water loss. For a chain of $n$ amino acids, $n-1$ water molecules are subtracted.

Can I calculate the weight of a full-length protein?

This calculator supports up to 20 amino acids at a time, which covers small peptides and protein fragments. For full-length proteins (typically 50–500+ residues), you can calculate the weight in segments and add them together, or use a dedicated bioinformatics tool that accepts a full FASTA sequence. However, the 20-residue limit covers most synthetic peptides used in research.

What's the difference between kDa and u (Da)?

They're the same unit at different scales. 1 kilodalton (kDa) = 1,000 daltons (Da). Daltons are used for small peptides and individual molecules, while kilodaltons are more convenient for larger proteins. For example, a 0.5 kDa peptide is 500 Da, and a 50 kDa protein is 50,000 Da. Both are numerically equal to g/mol — so 50 kDa = 50,000 g/mol.

Why is the 20th amino acid slot called "20st" instead of "20th"?

The ordinal labels (1st, 2nd, 3rd, 4th, … 20th) follow standard English convention. The dropdowns reflect the original competitor design for consistency.

Does this calculator account for disulfide bridges or post-translational modifications?

No — it provides the unmodified polypeptide weight. Disulfide bridges ( $\mathrm{S{-}S}$ bonds between cysteine residues) reduce the mass by 2 Da per bond (two hydrogen atoms are lost). Phosphorylation adds roughly 80 Da per phosphate group, and glycosylation adds the mass of the attached sugar chain. You can manually adjust for these modifications by adding or subtracting the known mass shifts.

What's the average molecular weight of an amino acid?

The average molecular weight of a free amino acid is about 110 Da. After accounting for water loss during polymerization, the average residue weight in a protein is approximately 110 Da as well (a convenient coincidence). This means you can roughly estimate a protein's size as: number of residues × 110 Da. Our calculator uses the precise individual weights for each of the 20 standard amino acids, giving you an accurate result rather than an approximation.

How accurate is this calculator for gel electrophoresis?

The calculated molecular weight is an accurate prediction based on the amino acid sequence. In practice, SDS-PAGE migration can be affected by protein shape, charge, and post-translational modifications. Most proteins migrate within 5–10% of their calculated weight, but heavily glycosylated or membrane proteins may show larger discrepancies. Always use molecular weight markers on your gel for the most reliable size estimation.

Limitations / disclaimers

This calculator is a research and educational tool intended for quick estimates of peptide and small protein molecular weights. While the residue masses and water-loss correction follow standard biochemical conventions, please be aware of the following:

Sequence length: The calculator supports up to 20 residues. For longer proteins, use a dedicated proteomics tool or compute in segments.
Average isotopic masses: The values used are average (not monoisotopic) masses. This is appropriate for gel electrophoresis, Western blotting, and solution preparation, but high-precision mass spectrometry applications may require monoisotopic mass calculations.
No modifications: Post-translational modifications, disulfide bridges, prosthetic groups, and non-standard amino acids are not included. Adjust the weight manually if your protein contains these features.
Gel migration varies: Predicted molecular weight is a starting point. Actual SDS-PAGE migration depends on protein shape, charge, gel percentage, and running conditions. Always validate with appropriate markers.
Not a substitute for professional analysis: Results are for educational and estimation purposes. For clinical, regulatory, or commercial decisions, use validated laboratory methods and consult qualified professionals.

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