This protein molecular weight calculator sums a protein’s residue masses from its amino acid sequence (one-letter code) and reports the molecular weight in daltons/kDa — with composition counts alongside.
MW = Σ Residue Masses + Water — Bookkeeping at Atomic Scale
Residue masses already subtract the water lost in each peptide bond; adding one water back closes the molecule's ends. Sequences themselves come from translation — our Codon Calculator goes DNA → protein. Part of our Biology Calculators collection.
Worked Example (Verified)
- Human insulin A chain (default, 21 aa: GIVEQCCTSICSLYQLENYCN): residue sum + water = ≈ 2,384 Da — matching the published A-chain weight; its 4 cysteines form 2 disulfides in the mature hormone (fold the −2.02 Da each into precise work)
- Scale check: an average 50 kDa protein runs ~450 residues — the estimator (length × ~110 Da) every gel reader carries, and which this page replaces with the exact sum
- Composition asymmetry: tryptophan (186.21) weighs 3.3× glycine (57.05) — two same-length proteins can differ by kilodaltons on composition alone
- Bovine serum albumin (583 aa, famously) sums to 66.4 kDa — the "66 kDa band" anchoring a million gels
From Sequence Mass to Real Sample
- Tags add up: His₆ ≈ +0.84 kDa, GST ≈ +26 kDa, MBP ≈ +42 kDa — recombinant expectations must include them before calling a band wrong.
- N-terminal Met cleavage removes 131.19 Da on many cytosolic proteins — databases' predicted vs mature weights differ by design.
- Phosphorylation/phosphatases shift bands measurably on gels despite being only ~80 Da — charge cooperates with mass on SDS-PAGE.
- For molar conversions (mg → nmol for assays), this MW is the divisor — errors here propagate linearly into every stoichiometry downstream.
Related Tools
- Codon Calculator — translate DNA/RNA to protein
- DNA Concentration Calculator — the nucleic-acid counterpart
- Biology Calculators — the full collection
Frequently Asked Questions
How do you calculate protein molecular weight from sequence?
Sum the average residue masses of all amino acids (each already net of the water lost forming peptide bonds), then add 18.02 Da for the terminal H and OH. Insulin's A chain: 21 residues summing to ≈2,384 Da. This calculator performs the sum and reports Da and kDa.
What is a residue mass vs an amino acid mass?
A free amino acid's mass minus water (18.02 Da) = its residue mass — the contribution it makes inside a chain, since peptide-bond formation releases H₂O per bond. Summing residues and adding one water back at the end equals summing free amino acids minus (n−1) waters; both ledgers agree.
Why is my gel band heavier than the calculated weight?
Candidates in order: uncleaved tags/signal peptides (check your construct), modifications (glycosylation adds kDa-scale mass), incomplete denaturation or odd SDS binding (basic/membrane proteins migrate anomalously), or the protein is a complex that survived the gel buffer. The sequence mass is the floor truth — explain the gap, don't average them.
How do disulfide bonds change the weight?
Each bond oxidizes two cysteine thiols, removing 2 H atoms = −2.02 Da. Proteins like insulin (3 disulfides across both chains) sit ~6 Da below the naive sum — trivial for gels, decisive for high-resolution mass spectrometry.
What average mass does one amino acid contribute?
About 110 Da weighted by natural abundance (the famous rule of thumb), though the unweighted mean of the 20 residues is ~118 Da — composition decides: glycine-rich structural proteins run light, aromatic-rich ones heavy. The calculator uses exact per-residue values instead of either average.
Is this protein molecular weight calculator free?
Yes — free, instant, no sign-up.