Quick answer: Molecular weight = sum of average residue masses + 18.015 Da for one water at the free ends. The peptide MKTAYIAKQR weighs about 1,209.5 Da (1.21 kDa). A quick estimate for any protein is about 110 Da per amino acid, so 300 residues is about 33 kDa.
The protein molecular weight formula
MW = Σ residue masses + 18.015 DaEach peptide bond releases one water; one is added back for the terminiThis uses average residue masses, reflecting natural isotope mixes and matching the values behind ExPASy ProtParam. Mass spectrometry sometimes uses slightly lower monoisotopic masses.
MKTAYIAKQR
- Met 131.19 + Lys 128.17 + Thr 101.11 + Ala 71.08 + Tyr 163.18
- + Ile 113.16 + Ala 71.08 + Lys 128.17 + Gln 128.13 + Arg 156.19 = 1,191.46
- + water 18.015 = 1,209.48 Da
Result: 1,209.48 Da (1.21 kDa)
Amino acid average residue masses
| Amino acid | Code | Residue mass (Da) |
|---|---|---|
| Glycine | G | 57.05 |
| Alanine | A | 71.08 |
| Serine | S | 87.08 |
| Proline | P | 97.12 |
| Valine | V | 99.13 |
| Threonine | T | 101.11 |
| Cysteine | C | 103.14 |
| Leucine / Isoleucine | L / I | 113.16 |
| Asparagine | N | 114.10 |
| Aspartate | D | 115.09 |
| Glutamine | Q | 128.13 |
| Lysine | K | 128.17 |
| Glutamate | E | 129.12 |
| Methionine | M | 131.19 |
| Histidine | H | 137.14 |
| Phenylalanine | F | 147.18 |
| Arginine | R | 156.19 |
| Tyrosine | Y | 163.18 |
| Tryptophan | W | 186.21 |
Glycine is the lightest residue and tryptophan the heaviest. The simple average of the table is about 119 Da.
Quick estimate: the 110 Da per residue rule
MW ≈ residues × 110 DaFrequency-weighted average across real proteinsSmall residues such as glycine, alanine and serine are far more common than tryptophan, which pulls the real-world average to about 110. A 607-residue protein such as serum albumin estimates to about 67 kDa against its true 66.5 kDa.
Common uses
- Reading SDS-PAGE gels against the expected size.
- Planning experiments: converting mass to moles for buffers and dilutions.
- Writing methods sections for papers and theses.
- Coursework in biology and biochemistry.
- Ordering synthetic peptides with the expected mass.
What this calculator does not include
- Disulfide bonds each remove about 2 Da.
- Post-translational modifications such as glycosylation add unpredictable mass.
- Terminal modifications such as amidation shift the mass by a few Da.
- Non-standard residues are skipped.
Frequently asked questions
How is protein molecular weight calculated?
Add the average residue mass of every amino acid, then add one water (18.015 Da) for the free ends: MW = Σ residue masses + 18.015.
What is a dalton and a kilodalton?
A dalton (Da) is the unit of molecular mass, about one hydrogen atom. A kilodalton (kDa) is 1,000 Da.
Should I use average or monoisotopic mass?
Average mass for reporting overall molecular weight, as here. Monoisotopic mass is used in high-resolution mass spectrometry and is slightly lower.
Why is residue mass lower than amino acid mass?
Each peptide bond releases one water. The residue mass is the amino acid minus that water.
Why is the rule of thumb 110 Da and not 119?
Averaging the 20 residues equally gives about 119 Da, but small residues are far more common in real proteins, so the frequency-weighted average is near 110.
Does this account for disulfide bonds or modifications?
No. Each disulfide removes about 2 Da, and glycosylation or phosphorylation add mass a sequence cannot predict.
Related tools
Accuracy note: Residue masses are standard average values, validated against the insulin A chain. This computes the unmodified chain, so confirm critical values with a dedicated laboratory tool.
Content Lead, Anchor AI Tools, California, USA. Spotted an error? Email hello@anchoraitools.com and it will be checked and corrected. See our editorial standards.