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Peptide Molecular Weight Calculator MOST POPULAR

Calculate the molecular weight, molecular formula, net charge, and extinction coefficient of a peptide from its amino acid sequence. Supports N-terminal acetylation, C-terminal amidation, and disulfide bridges.

Keywords: peptide molecular weight calculator, peptide mass calculator, amino acid sequence calculator, monoisotopic mass, average mass peptide

Calculator

Enter one-letter amino acid codes. Supports standard 20 amino acids (ACDEFGHIKLMNPQRSTVWY).
Each disulfide bridge reduces mass by 2.02 Da (loss of 2 H atoms).

Results

Molecular Weight—
Molecular Formula—
Net Charge (pH 7)—
Extinction Coefficient (280nm)—
Sequence Length—
Molar Absorptivity—
A260/A280 Ratio—

What This Calculator Does

Enter a peptide sequence in one-letter code and the calculator returns the numbers you need for downstream work: monoisotopic and average molecular weight, molecular formula, net charge at pH 7, extinction coefficient at 280 nm, molar absorptivity, and the parsed sequence length. N-terminal acetylation, pyroglutamate formation, formylation, C-terminal amidation, and up to three disulfide bridges can be applied before calculating, so the reported values can track an actual construct rather than the bare sequence.

Everything runs in the browser — nothing is uploaded or stored. The sections below set out the mass conventions, modification values, and rounding rules behind each output, so you can judge whether a calculated value fits your application.

How the Calculation Works

Amino Acid Monoisotopic Masses

Amino Acid Code Monoisotopic Mass (Da) Average Mass (Da) Formula Contribution
Alanine A 71.03711 71.0788 C₃H₅NO
Arginine R 156.10111 156.1876 C₆H₁₂N₄O
Asparagine N 114.04293 114.1039 C₄H₆N₂O₂
Aspartic Acid D 115.02694 115.0886 C₄H₅NO₃
Cysteine C 103.00919 103.1430 C₃H₅NOS
Glutamic Acid E 129.04259 129.1155 C₅H₇NO₃
Glutamine Q 128.05858 128.1307 C₅H₈N₂O₂
Glycine G 57.02146 57.0519 C₂H₃NO
Histidine H 137.05891 137.1412 C₆H₇N₃O
Isoleucine I 113.08406 113.1594 C₆H₁₁NO
Leucine L 113.08406 113.1594 C₆H₁₁NO
Lysine K 128.09496 128.1742 C₆H₁₂N₂O
Methionine M 131.04049 131.1926 C₅H₉NOS
Phenylalanine F 147.06841 147.1766 C₉H₉NO
Proline P 97.05276 97.1167 C₅H₇NO
Serine S 87.03203 87.0782 C₃H₅NO₂
Threonine T 101.04768 101.1039 C₄H₇NO₂
Tryptophan W 186.07931 186.2133 C₁₁H₁₀N₂O
Tyrosine Y 163.06333 163.1760 C₉H₉NO₂
Valine V 99.06841 99.1326 C₅H₉NO

Modifications

Modification Mass Change (Da) Formula Change
Acetylation (N-term) +42.01056 +C₂H₂O
Amidation (C-term) -0.98474 -OH + NH₂
Pyroglutamic Acid (N-term) -17.02655 -H₂O
Formylation (N-term) +28.01007 +CO
Disulfide Bridge -2.01565 per bridge -2H

Net Charge Calculation

At pH 7.0, net charge is estimated from ionizable side chains:

  • Positive charges: Arg (+1), Lys (+1), His (+0.1 at pH 7)
  • Negative charges: Asp (-1), Glu (-1)
  • N-terminus: +1 (if unmodified)
  • C-terminus: -1 (if unmodified; 0 if amidated)

Extinction Coefficient

Estimated at 280 nm using the Edelhoch method:

  • Trp (W): 5,500 M⁻¹cm⁻¹
  • Tyr (Y): 1,490 M⁻¹cm⁻¹
  • Cys (C, disulfide): 125 M⁻¹cm⁻¹ per bond

Principles of Peptide Mass Calculation

Peptide Bond Formation: Peptides are linear chains of amino acids linked by peptide bonds (amide bonds). Each peptide bond forms through a condensation reaction between the carboxyl group (—COOH) of one amino acid and the amino group (—NH₂) of the next, releasing a water molecule (H₂O).

Water Loss During Synthesis: For a peptide containing \(n\) amino acid residues, exactly \(n-1\) water molecules are lost during peptide bond formation. Each water molecule has a mass of 18.01056 Da (monoisotopic) or 18.0153 Da (average). This water loss is the single most important correction applied in peptide mass calculation.

General Relationship:

\[ \text{MW} = \sum_{i=1}^{n} \text{AA}_i - (n-1) \times \text{H}_2\text{O mass} + \sum \text{modifications} \]

Monoisotopic vs. Average Mass:

Aspect Monoisotopic Mass Average Mass
Definition Exact mass of the most abundant isotope of each element (e.g., ¹²C, ¹H, ¹⁴N, ¹⁶O, ³²S) Weighted average of all naturally occurring isotopes for each element
Typical Use High-resolution mass spectrometry (FT-ICR, Orbitrap, Q-TOF) Lower-resolution instruments, preparative work, gravimetric calculations
Difference Scale ~0.1–0.5 Da for peptides under 1,500 Da Larger differences for larger peptides
Accuracy More precise for individual molecular species Represents bulk isotopic distribution

Monoisotopic mass is the default for most peptide mass spectrometry applications, while average mass is more commonly used for determining reconstitution volumes and molar concentrations.

Example Calculations

Sequence Modifications MW (Monoisotopic) MW (Average)
YQVAD None 581.27 Da 581.62 Da
GLP-1(7-37) None 3,297.62 Da 3,299.62 Da
GHK Amidated C-term 402.20 Da 402.46 Da

Core Formula

Complete Molecular Weight Formula:

\[ \text{MW} = \sum_{i=1}^{n} \text{AA}_i - (n-1) \times 18.01056 + \sum \text{modifications} - 2.01565 \times \text{disulfide_count} \]

Where:

Variable Definition
\(\sum_{i=1}^{n} \text{AA}_i\) Sum of monoisotopic (or average) masses of all \(n\) amino acid residues in the sequence
\(n\) Total number of amino acids in the peptide
\(18.01056\) Mass of one water molecule (H₂O) in daltons — one H₂O is lost per peptide bond formed
\((n-1) \times 18.01056\) Total water loss from all peptide bond condensation reactions
\(\sum \text{modifications}\) Sum of all modification mass changes (positive for additions like acetylation, negative for losses like amidation)
\(2.01565\) Mass of two hydrogen atoms lost per disulfide bridge
\(\text{disulfide_count}\) Number of disulfide bridges (each S–S bond replaces two Cys–SH groups, losing 2H)

Simplified for unmodified linear peptides:

\[ \text{MW} = \sum_{i=1}^{n} \text{AA}_i - (n-1) \times 18.01056 \]

Worked Example: GHK (Gly-His-Lys)

Let's calculate the molecular weight of the tripeptide GHK (Glycine-Histidine-Lysine), a copper-binding peptide found in human plasma.

Step 1: Sum the amino acid monoisotopic masses

Amino Acid One-Letter Code Monoisotopic Mass (Da)
Glycine G 57.02
Histidine H 137.06
Lysine K 128.09
Total 322.17 Da

Step 2: Account for water loss from peptide bonds

GHK has 3 amino acids, therefore \(n-1 = 2\) peptide bonds. Each bond formation releases one H₂O.

\[ \text{Water loss} = 2 \times 18.01056 = 36.02 \text{ Da} \]

Step 3: Calculate unmodified molecular weight

\[ \text{MW} = 322.17 - 36.02 = 286.15 \text{ Da} \]

Step 4: Apply modifications (optional)

Modification Calculation Result
None (free N- and C-termini) 286.15 286.15 Da
C-terminal amidation (−0.98 Da) 286.15 − 0.98 285.17 Da
N-terminal acetylation (+42.01 Da) 286.15 + 42.01 328.16 Da
Acetylation + Amidation 286.15 + 42.01 − 0.98 327.18 Da

Verification: A mass spectrometer analyzing synthetic GHK (amidated) should show a monoisotopic peak at approximately 285.17 m/z ([M+H]⁺ at 286.17).

Frequently Asked Questions

**What is the difference between monoisotopic and average mass?**

Monoisotopic mass refers to the exact mass of a molecule calculated using the most abundant isotope of each element. For example, carbon is taken as ¹²C (exactly 12.00000 Da), hydrogen as ¹H (1.00783 Da), nitrogen as ¹⁴N (14.00307 Da), and oxygen as ¹⁶O (15.99491 Da). This represents the mass of a single, specific isotopic variant and is what you see in high-resolution mass spectra as the lowest-mass peak of the isotopic envelope.

Average mass uses the weighted average of all naturally occurring isotopes for each element. For carbon, this includes ¹²C (98.9%) and ¹³C (1.1%), giving an average atomic mass of 12.0107 Da. The average mass represents the center of the isotopic distribution and is typically 0.1–0.6 Da higher than the monoisotopic mass for most peptides.

When to use which: Monoisotopic mass is standard for mass spectrometry and precise analytical work. Average mass is used for calculating molar concentrations, reconstitution volumes, and other bench-scale quantities.

**How does acetylation affect molecular weight?**

Acetylation adds an acetyl group (CH₃CO−) to the N-terminus of the peptide, replacing the terminal hydrogen atom. The net mass change is +42.01 Da (monoisotopic) or +42.04 Da (average). This modification also neutralizes the positive charge of the free N-terminus, which can affect the peptide's overall net charge and isoelectric point. Acetylation is commonly used to improve peptide stability by protecting the N-terminus from enzymatic degradation by aminopeptidases.

**Why does my peptide's MW differ from the calculated value?**

Several factors can cause discrepancies between calculated and experimentally observed molecular weights:

  • Isotopic distribution: Mass spectrometry shows a distribution of isotopic peaks, not a single value. The monoisotopic peak (lowest mass) should match the calculated monoisotopic mass.
  • Salt adducts: Sodium (Na⁺, +22.99 Da), potassium (K⁺, +38.10 Da), or other counterions from buffers can form adducts.
  • Post-translational modifications: Phosphorylation (+79.97 Da), glycosylation, oxidation of methionine (+15.99 Da), or other in-vivo modifications may be present.
  • Sequence errors: A single amino acid substitution or deletion changes the mass significantly.
  • Truncation products: Incomplete synthesis can produce shorter peptide fragments.
  • Counterions: Peptides purified as TFA salts or acetate salts carry additional mass from the counterion.

Always confirm your sequence and consider the purification method when interpreting mass spectrometry data.

**What is the extinction coefficient and why does it matter?**

The extinction coefficient (ε) is a measure of how strongly a peptide absorbs light at a particular wavelength, typically 280 nm for aromatic amino acids. It is calculated using the Edelhoch method:

Amino Acid ε₂₈₀ (M⁻¹cm⁻¹)
Tryptophan (W) 5,500
Tyrosine (Y) 1,490
Cystine (C–C, disulfide) 125 per bond

The extinction coefficient is essential for: - Quantifying peptide concentration using UV spectrophotometry (A = ε × c × l) - Determining yield after synthesis and purification - Normalizing samples for biochemical assays - Quality control in peptide manufacturing

Peptides lacking Trp, Tyr, and disulfide bonds have negligible absorbance at 280 nm and require alternative quantification methods (e.g., BCA assay, tryptophan fluorescence at 295 nm).

**How do disulfide bridges affect mass calculation?**

A disulfide bridge (cystine bond) forms between the thiol groups (—SH) of two cysteine residues. During formation, each cysteine loses one hydrogen atom, and the two hydrogens combine and are released. This means each disulfide bridge reduces the molecular weight by 2.01565 Da (the mass of two hydrogen atoms).

The formula impact is straightforward:

\[ \text{MW}{\text{with disulfide}} = \text{MW}) \]}} - (2.01565 \times \text{disulfide_count

Disulfide bridges also: - Introduce structural constraints that affect peptide folding and bioactivity - Contribute to UV absorbance at 280 nm (125 M⁻¹cm⁻¹ per bond) - Protect peptides from proteolytic degradation - Are critical for the stability of many therapeutic peptides (e.g., insulin, oxytocin)

When to Use This Tool

The Peptide Molecular Weight Calculator is essential for a wide range of research and development applications:

  • Peptide Synthesis: Verify the mass of synthesized peptides before and after purification. Calculate expected [M+H]⁺ peaks for HPLC-MS and LC-MS quality control.
  • Mass Spectrometry: Predict monoisotopic and average masses for database searching, spectral interpretation, and fragment ion analysis in proteomics workflows.
  • Drug Discovery: Design therapeutic peptides with precise molecular weights. Calculate masses for modified peptides including acetylated, amidated, and cyclized variants.
  • Biopharmaceutical Development: Determine molar extinction coefficients for accurate quantification of peptide-based drug substances and drug products.
  • Reconstitution and Dosing: Use calculated molecular weights alongside the Dilution Calculator and Molarity Calculator to prepare precise stock solutions and dosing formulations.
  • Characterization of Natural Peptides: Analyze bioactive peptides from venoms, hormones, growth factors, and antimicrobial peptides by comparing calculated masses against observed MS data.
  • Education and Training: Teach students the fundamental principles of peptide chemistry, including condensation reactions, water loss, and the impact of post-translational modifications on mass.

For comprehensive physicochemical characterization including isoelectric point, hydrophobicity, and instability index, visit the Peptide Properties Calculator.

Assumptions and Rounding

A few conventions sit behind every result, and the same ones apply to both mass models:

  • Residue mass basis. Monoisotopic values are computed from the exact masses of the most abundant isotopes (¹²C, ¹H, ¹⁴N, ¹⁶O, ³²S); average values from natural-abundance atomic weights. The two are reported separately and never mixed.
  • Chain model. A linear chain of the 20 proteinogenic L-amino acids is assumed, with free termini unless a modification is selected. Cyclic, branched, and D-amino-acid constructs are out of scope.
  • Modification set. Only the dropdown modifications are modeled (acetylation, pyroglutamate, formylation, amidation, disulfide bridges), each applied as a fixed mass delta to the full-length chain.
  • Charge model. Net charge at pH 7 counts Arg and Lys as +1, Asp and Glu as −1, and His as +0.1; termini follow their modification state. It is a fixed-pH estimate, not a titration.
  • Extinction model. Edelhoch-style estimate (Trp 5,500, Tyr 1,490 M⁻¹cm⁻¹) with 125 M⁻¹cm⁻¹ added per disulfide bridge.
  • Rounding. Displayed values are rounded for readability — mass to 2 decimals, charge to 1 decimal, formula as whole-number atoms, extinction to the nearest integer. The sums behind them keep full precision.

Input Definitions

Input What it means Units Allowed values
Amino acid sequence One-letter code for the peptide chain; any character outside the 20 standard codes is stripped before calculation — Standard one-letter codes (ACDEFGHIKLMNPQRSTVWY)
N-terminal modification Chemistry applied to the free N-terminus — None; acetylation (+42.01 Da); pyroglutamate (−17.03 Da); formylation (+28.01 Da)
C-terminal modification Chemistry applied to the free C-terminus — None; amidation (−0.98 Da)
Disulfide bridges Number of S–S bonds assumed to be formed bridges 0, 1, 2, or 3
Mass type Isotopic model used for the reported mass — Monoisotopic; average

Output Interpretation

Each result row answers a different question about the same chain:

Output How to read it
Molecular weight The chain mass in daltons for the selected model — monoisotopic (lowest-mass isotope peak) or average (center of the isotopic envelope); the label after the number tells you which
Molecular formula Whole-number atom counts for the modified chain; a quick sanity check against a vendor's stated formula
Net charge (pH 7) Estimated charge at near-neutral pH; the sign tells you whether the peptide behaves cation- or anion-like in a neutral buffer
Extinction coefficient (280 nm) Estimated molar absorptivity; reports 0 when no Trp, Tyr, or disulfide is present
Sequence length Number of residues parsed from the input
Molar absorptivity Extinction coefficient expressed per gram of peptide (L·g⁻¹·cm⁻¹), for when you weigh out material rather than count moles
A260/A280 ratio A rough aromatic-content estimate; read it as an indicator, not a measurement

Limitations

  • Standard residues only. D-amino acids, norleucine, selenomethionine, and other non-standard residues are not recognized — letters outside the 20 standard codes are silently ignored.
  • Fixed modification inventory. Phosphorylation, oxidation, methylation, glycosylation, and other post-translational or synthetic marks are not modeled; add their mass deltas by hand when needed.
  • Charge is an estimate. The pH 7 figure uses fixed side-chain contributions rather than a full Henderson–Hasselbalch charge curve; read it as a coarse guide to ion-exchange and solubility behavior.
  • Extinction assumes simple composition. The Edelhoch estimate reflects aromatic content and disulfide count only.
  • No counterions or salt forms. TFA/acetate counterion mass, metal complexes, and bound water are not included; a measured mass for a salt form will differ.
  • Linear chains only. Cross-linked, branched, or conjugated constructs fall outside the model.
  • Check before relying on it. This tool is provided for laboratory research and educational use; for critical applications, confirm masses against your own mass spectrometry data.

The Author's Take

Position — in my view, the mass model should be picked by the task before the first calculation, not after the discrepancy shows up.

Reasoning. Monoisotopic mass is the number that pairs with a high-resolution mass spectrum, because that is what the instrument resolves as the lowest-mass peak. Average mass is the number that pairs with the balance: gravimetric conversions between mass and moles use the bulk isotopic distribution. Most mismatches trace back to comparing numbers computed under different conventions, or to a salt form that was never subtracted. Decide the convention at the start, write it next to the value, and the ambiguity disappears.

Disclosure. This is the author's working opinion from laboratory practice, not a verified fact — follow your instrument's and protocol's conventions.

Mass values only mean something in context. These references go deeper on measurement and interpretation:

Data source: IUPAC-IUBMB biochemical nomenclature. Calculations follow ExPASy ProtParam methodology.