Protein Molecular Weight Calculator By Amara Taylor · Updated Sep 13, 2026
A free protein molecular weight calculator. Paste an amino
acid sequence in single-letter code and the tool returns the molecular
weight in daltons (Da) and kilodaltons (kDa), the residue count, the
average mass per residue, and the amino acid composition. It uses standard
average residue masses, the same method behind tools like ExPASy ProtParam.
✓ Free, no signup✓ Da and kDa output✓ Composition breakdown✓ Runs in your browser
Quick answer: A protein's molecular weight is the sum of the
average residue masses of its amino acids plus one water molecule
(18.015 Da) for the free ends. The formula is MW = Σ residue masses +
18.015. For example, the peptide MKTAYIAKQR has a
molecular weight of about 1,209.5 Da (1.21 kDa). A quick estimate
for any protein is roughly 110 Da per amino acid, so a
300-residue protein is about 33 kDa.
🧬 Paste an amino acid sequence (single-letter code)
FASTA headers, spaces, numbers, and line breaks are ignored
How to use this protein molecular weight calculator
Paste your sequence. Enter the protein or peptide in
single-letter amino acid code, for example MKTAYIAKQR.
FASTA headers, spaces, numbers, and line breaks are ignored by
default.
Click Calculate. The tool sums the average residue mass of
every valid amino acid and adds one water molecule for the free N-
and C-termini.
Read the result. You get the molecular weight in daltons and
kilodaltons, the residue count, and the average mass per residue,
plus the amino acid composition.
The Copy button saves the result for a lab notebook, methods
section, or spreadsheet. Everything runs in your browser, so your
sequence is never uploaded.
The protein molecular weight formula
A protein is a chain of amino acids joined by peptide bonds. Every time
two amino acids join, the reaction releases one water molecule.
So the molecular weight is not the sum of the free amino acid masses.
It is the sum of their residue masses (each free amino acid minus
one water), with a single water added back for the two free ends of the
finished chain:
MW = Σ (residue masses) + 18.015 Da
This calculator uses average residue masses, which reflect the
natural mix of isotopes and match the values used by the widely cited
ExPASy
ProtParam tool. Average masses are the standard choice for reporting a
protein's overall molecular weight. Mass spectrometry sometimes uses
monoisotopic masses instead, which are slightly lower.
Worked example. For MKTAYIAKQR,
add the residue masses: 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. Add one water: 1,191.46 + 18.015 = 1,209.48 Da, or
about 1.21 kDa.
Amino acid average residue masses
These are the average residue masses this calculator sums. A residue
mass is the amino acid mass after losing one water in the peptide bond, so
it is what each amino acid contributes to the chain:
Amino acid
1-letter
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. Averaging
the twenty masses in this table gives about 119 Da, not 110 — the
familiar 110 figure comes from weighting by how often each amino acid
actually appears in proteins.
Quick estimate: the 110 Da per residue rule
If you only need a rough figure and know the length, multiply the
number of amino acids by about 110 Da:
Approx. MW (Da) ≈ number of residues × 110
The 110 works because the small residues are the common ones. Glycine
(57), alanine (71) and serine (87) appear far more often than tryptophan
(186), so the frequency-weighted average across real proteins lands near
110 rather than the 119 you get by averaging the table itself.
Example. A 100-residue protein is roughly
100 × 110 = 11,000 Da, or about 11 kDa. A 607-residue protein such
as serum albumin estimates to about 67 kDa, close to its true value of
66.5 kDa. For an exact figure, paste the real sequence into the
calculator above.
Common uses for a protein molecular weight calculator
Reading an SDS-PAGE gel — comparing a band's position
against the calculated molecular weight to confirm a protein's
identity.
Planning experiments — converting between mass and moles for
buffers, dilutions, and molar concentrations.
Writing methods sections — reporting the molecular weight of
a construct or peptide in a paper or thesis.
Biology and biochemistry coursework — checking homework
answers and understanding how residue masses build a protein.
Ordering synthetic peptides — confirming the expected mass
before a supplier makes the peptide.
What this calculator does and does not include
This tool calculates the molecular weight of the plain amino acid
chain. It gives an accurate figure for a standard, unmodified protein or
peptide, but a few real-world features shift the true mass:
Disulfide bonds each remove about 2 Da (two hydrogens), so a
protein with several bridges weighs slightly less than the sum
shown.
Post-translational modifications such as glycosylation,
phosphorylation, or acetylation add mass that a sequence alone cannot
predict.
Terminal modifications such as C-terminal amidation change
the end groups and shift the mass by a few daltons.
Non-standard residues outside the twenty common amino acids
are skipped, so include only standard single-letter codes for an
exact result.
Protein molecular weight calculator FAQs
How is protein molecular weight calculated?
Add the average residue mass of every amino acid in
the sequence, then add one water molecule (18.015 Da) for the free
ends. The formula is MW = Σ residue masses + 18.015. Residue masses
are used because each peptide bond releases one water.
What is a dalton and a kilodalton?
A dalton (Da) is the standard unit of molecular
mass, roughly the mass of one hydrogen atom. A kilodalton (kDa) is
1,000 daltons. Protein weights are often reported in kDa, so a 33,000
Da protein is written as 33 kDa.
Should I use average or monoisotopic mass?
Use average mass for reporting a protein's overall
molecular weight, which is what this calculator gives. Monoisotopic
mass, based only on the most common isotope of each element, is used
in high-resolution mass spectrometry and is slightly lower.
Why is the residue mass lower than the amino acid mass?
Because forming a peptide bond releases one water
molecule. The residue mass is the free amino acid minus that water,
so it is what each amino acid actually adds to the chain. One water
is added back at the end for the two free termini.
Why is the rule of thumb 110 Da and not 119?
Averaging the twenty residue masses equally gives
about 119 Da. In real proteins the small residues appear far more
often than the large ones, so the frequency-weighted average is lower
— close to 110 Da, which is the figure used for quick length-based
estimates.
Does this account for disulfide bonds or modifications?
No. It calculates the plain chain. Each disulfide
bond removes about 2 Da, and modifications like glycosylation or
phosphorylation add mass that a sequence alone cannot predict. Treat
the result as the unmodified backbone weight.
How do I estimate weight without the full sequence?
Multiply the number of residues by about 110 Da. A
250-amino-acid protein is roughly 27,500 Da, or about 27.5 kDa. For
an exact figure, paste the real sequence into the calculator.
Is this protein molecular weight calculator free?
Yes. The protein molecular weight calculator from
Anchor AI Tools is completely free, with no signup and no limits. It
runs entirely in your browser, so your sequence is never
uploaded.
Residue masses checked against standard average values and validated against the insulin A chain on September 13, 2026 ·
How we check our tools
This tool computes the unmodified chain. Confirm critical values with a dedicated laboratory tool.
Written by Amara
Taylor for Anchor AI Tools · Last reviewed September 13, 2026. This protein
molecular weight calculator provides estimates for general and educational use.
It computes the unmodified amino acid chain and does not account for disulfide
bonds, post-translational modifications, or non-standard residues. Confirm
critical values with a dedicated laboratory tool.
