Protein Molecular Weight Calculator Updated Aug 11, 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.
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 = 1,209.47 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. The average
across all twenty amino acids is close to 110 Da, which is where the
"110 Da per residue" rule of thumb comes from.
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
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.
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, the
average residue mass. 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.
We build free, fast, accurate online tools that explain
the answer, not just show a number. This calculator uses standard average
residue masses and is validated against known peptides such as the
insulin A chain before it goes live.
This protein molecular weight calculator from Anchor AI
Tools 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.
