Protein Molecular Weight Calculator

The protein molecular weight calculator turns a raw amino acid sequence into a usable molecular mass in daltons (Da) and kilodaltons (kDa), along with the supporting properties that biochemists and molecular biologists reach for every day. Paste a sequence written in the standard single-letter amino acid code and the tool counts each residue, sums the residue masses, corrects for the water released during peptide-bond formation, and reports the finished polypeptide mass. Because the calculation is deterministic, the same sequence always returns the same answer, which makes this protein MW calculator a quick sanity check when you order peptides, design constructs, or interpret a mass-spectrometry result.

Beyond raw mass, the calculator surfaces the metrics you actually need at the bench: the molar extinction coefficient at 280 nm for spectrophotometric concentration measurements, the count of positively and negatively charged residues, the aliphatic index as a proxy for thermostability, and GRAVY (Grand Average of Hydropathy) for a hydrophobicity read. It also lists the full amino acid composition so you can spot cysteine pairs, count tryptophans, or check that your codon-optimized gene encodes the protein you expect. Whether you are estimating the mass of an antibody fragment, a fusion tag, or a synthetic peptide, this amino acid sequence molecular weight calculator gives a reproducible starting number in a single step.

A protein is a chain of amino acids joined by peptide bonds. Each peptide bond forms by condensation, releasing one molecule of water. So the mass of the finished chain is the sum of the individual amino acid masses, minus the water lost for every bond, plus one water molecule to cap the free N-terminal amine and C-terminal carboxyl. For a chain of n residues there are n − 1 peptide bonds, giving the relationship below.

This calculator uses average isotopic masses for the twenty standard amino acids and a water mass of 18.015 Da. The average residue mass it reports is simply the total mass divided by the chain length, which for typical proteins lands near 110 Da per residue, a handy rule of thumb when you only know the residue count.

The calculator reports four derived properties that depend only on amino acid composition, so they are fast to compute and useful for planning experiments.

The molar extinction coefficient at 280 nm predicts how strongly your protein absorbs ultraviolet light, which you use with the Beer-Lambert law to convert an A280 reading into concentration. The tool weights each chromophore by its contribution, assuming all cysteines are reduced:

• Tryptophan (W): 5500 M⁻¹cm⁻¹ each
• Tyrosine (Y): 1490 M⁻¹cm⁻¹ each
• Cysteine (C): 125 M⁻¹cm⁻¹ each

The charged residue counts tally positive residues (lysine K, arginine R, histidine H) and negative residues (aspartate D, glutamate E). Comparing these counts hints at the protein's net charge and helps you anticipate behavior on ion-exchange columns. The aliphatic index measures the relative volume occupied by aliphatic side chains (alanine, valine, isoleucine, leucine) and correlates with thermostability: higher values suggest a more heat-tolerant protein. Finally, GRAVY averages the Kyte-Doolittle hydropathy value across every residue; positive GRAVY points to a hydrophobic, possibly membrane-associated protein, while negative GRAVY indicates a soluble, hydrophilic one.

Using the protein molecular weight calculator takes only a few seconds:

1. Enter your sequence in the text box using the one-letter amino acid code (for example, M for methionine, W for tryptophan, K for lysine).
2. Let the tool clean the input. Spaces, numbers, line breaks, and any character that is not one of the twenty standard amino acid letters are automatically stripped before counting, so you can paste straight from a FASTA file.
3. Read the molecular weight in both daltons and kilodaltons at the top of the results panel.
4. Review the properties below it: chain length, average residue mass, extinction coefficient, charged residue counts, aliphatic index, and GRAVY.
5. Inspect the composition grid to see how many of each amino acid appear and what fraction of the protein they represent.

Because only the twenty standard amino acids are recognized, non-standard residues, modifications, and stop symbols are ignored. If your sequence contains a selenocysteine (U), pyrrolysine (O), or an ambiguous letter such as X, remove or substitute it before relying on the mass for a critical calculation.

The molecular weight this protein MW calculator returns is an average mass, the value you compare against a deconvoluted electrospray mass spectrum or an estimate from an SDS-PAGE ladder. Keep a few caveats in mind. SDS-PAGE often reports an apparent mass that can differ from the true mass by 10 percent or more, especially for highly charged, glycosylated, or membrane proteins, so a mismatch with the gel is not automatically an error. Post-translational modifications such as phosphorylation (+80 Da each), glycosylation (often hundreds to thousands of Da), disulfide bonds (−2 Da per bond), or removal of an initiator methionine all shift the experimental mass away from the sequence-only number computed here.

A good validation habit is the 110 Da check: multiply the residue count by roughly 110 Da and confirm the calculator's total lands in that ballpark. For the example 51-residue fragment, 51 × 110 ≈ 5610 Da, very close to the calculated 5644 Da. If your protein is far from that range, double-check that you pasted the full sequence and that unusual residues were not silently dropped. For concentration work, remember that the extinction coefficient assumes reduced cysteines; if your protein forms cystines (disulfides), the true 280 nm absorption is slightly higher.