Peptide Molecular Weight Calculator

Calculate the molecular weight of any peptide from its amino acid sequence — in Daltons (Da) — for synthesis planning, mass spectrometry, and bioassay applications.

Try:
Molecular Weight
0.000 g/mol
Calculation Breakdown
Element Atomic Mass Atoms Total %
Total Molecular Weight
Elements in this compound
In formula
Other elements

Quick Examples

Click any compound to calculate its molecular weight instantly.

Water
H₂O
18.015 g/mol
Carbon Dioxide
CO₂
44.009 g/mol
Sodium Chloride
NaCl
58.443 g/mol
Glucose
C₆H₁₂O₆
180.156 g/mol
Ethanol
C₂H₅OH
46.069 g/mol
Sulfuric Acid
H₂SO₄
98.077 g/mol

How to Use the Peptide Molecular Weight Calculator

To find the molecular weight of a peptide, enter its amino acid sequence or chemical formula into the input field and click "Calculate." The calculator returns the molecular weight in Daltons with a full breakdown of each residue's mass contribution.

Follow these 5 steps to calculate peptide molecular weight:

  1. Enter the peptide sequence using standard one-letter amino acid codes (e.g., ACDEFG). Write the sequence from N-terminus to C-terminus, left to right. Remove any spaces, numbering, or header lines.
  2. For peptides with non-standard amino acids or chemical modifications, enter the chemical formula instead. The calculator parses element symbols and subscripts to compute the total molecular weight.
  3. Press "Calculate" or hit Enter. The tool assigns each amino acid its residue weight, sums them, and adds 18.02 Da for the intact peptide's terminal water molecule.
  4. Review the molecular weight result. For peptides under 4,000 Da, the monoisotopic mass is commonly reported. For longer peptides approaching protein range, the average mass is more practical.
  5. Check the element-level breakdown table for total atom counts of C, H, N, O, and S. These values help predict isotope patterns for mass spectrometry identification.

This calculator is designed for peptide chemists, mass spectrometry analysts, pharmaceutical researchers working with therapeutic peptides (such as GLP-1 agonists, insulin analogs, or antimicrobial peptides), and biochemistry students studying peptide bond formation and polypeptide chemistry.

How the Peptide Molecular Weight Calculator Works

The peptide molecular weight calculator computes the mass of a linear peptide chain by summing amino acid residue weights. When two amino acids form a peptide bond, one water molecule (18.02 Da) is released. Residue weights already account for this condensation loss. The calculator adds back one water molecule for the complete peptide chain, representing the H at the N-terminus and the OH at the C-terminus.

Key components of the calculation:

MW
Peptide MW Formula
MW = Σ(amino acid residue weights) + 18.02 Da. For a tripeptide Ala-Gly-Val: 71.04 + 57.02 + 99.07 + 18.02 = 245.15 Da (monoisotopic).
m/z
Mass Spectrometry Prediction
m/z = (MW + z × 1.0073) ÷ z. Predicts the observed mass-to-charge ratio in ESI-MS for each charge state z. Proton mass ≈ 1.0073 Da.
mod
Common Modifications
N-terminal acetylation: +42.04 Da. C-terminal amidation: -0.98 Da. Disulfide bond: -2.02 Da per bond. Phosphorylation: +79.97 Da per site.
iso
Monoisotopic vs Average
Monoisotopic mass uses lightest stable isotopes (¹²C, ¹H, ¹⁴N, ¹⁶O, ³²S). Average mass uses natural isotope abundance-weighted averages. Choose based on your MS instrument's resolution.

For the neuropeptide Met-enkephalin (YGGFM), the calculator sums residue weights: Tyr (163.06) + Gly (57.02) + Gly (57.02) + Phe (147.07) + Met (131.04) + H₂O (18.02) = 573.23 Da (monoisotopic). This matches the expected [M+H]⁺ peak at m/z 574.24 in positive-mode ESI mass spectrometry.

Peptide molecular weight calculations are directly applied in solid-phase peptide synthesis (SPPS) quality control, LC-MS peptide mapping of protein digests, pharmacokinetic dosing of peptide drugs, and custom peptide ordering from synthesis vendors (where pricing and purification requirements scale with molecular weight).

Common Compound Molecular Weights

Reference table of frequently searched chemical compounds.

Chemical Formula Compound Name Molecular Weight
H₂O Water 18.015 g/mol
CO₂ Carbon Dioxide 44.009 g/mol
NaCl Sodium Chloride 58.443 g/mol
NH₃ Ammonia 17.031 g/mol
CH₄ Methane 16.043 g/mol
H₂SO₄ Sulfuric Acid 98.077 g/mol
CaCO₃ Calcium Carbonate 100.086 g/mol
C₆H₁₂O₆ Glucose 180.156 g/mol
C₂H₅OH Ethanol 46.069 g/mol
HCl Hydrochloric Acid 36.461 g/mol
NaOH Sodium Hydroxide 39.997 g/mol
Ca(OH)₂ Calcium Hydroxide 74.092 g/mol
HNO₃ Nitric Acid 63.012 g/mol
KMnO₄ Potassium Permanganate 158.032 g/mol
CuSO₄ Copper(II) Sulfate 159.607 g/mol

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Peptide Molecular Weight Calculator FAQs and Common Questions

Answers to common questions about calculating peptide molecular weight, mass spectrometry predictions, synthesis modifications, and practical applications.

How is peptide molecular weight calculated from an amino acid sequence?
Peptide molecular weight is calculated by summing the residue molecular weights of each amino acid in the sequence and adding 18.02 Da for the water molecule retained at the peptide's N-terminus and C-terminus. For a peptide with N residues: MW = Σ(residue weights) + 18.02 Da. This method uses the monoisotopic or average residue masses of the 20 standard amino acids.
What is the typical molecular weight range for peptides?
Peptides typically range from approximately 200 Da (dipeptides) to about 10,000 Da (roughly 80–90 amino acids). Molecules larger than ~10 kDa are generally classified as proteins rather than peptides. Therapeutic peptides commonly fall in the 500–5,000 Da range, which corresponds to sequences of about 4–45 amino acid residues.
Does this calculator account for peptide modifications like acetylation or amidation?
The base calculation covers the unmodified peptide chain. N-terminal acetylation adds 42.04 Da. C-terminal amidation subtracts 0.98 Da (replacing -OH with -NH₂). Disulfide bond formation removes 2.02 Da per bond. Users should manually adjust the calculated molecular weight for specific chemical modifications applied during peptide synthesis.
What is the difference between monoisotopic and average peptide molecular weight?
Monoisotopic molecular weight uses the mass of the most abundant isotope of each element (e.g., ¹²C, ¹H, ¹⁶O, ¹⁴N, ³²S). Average molecular weight uses the weighted average of all naturally occurring isotopes. For small peptides (<2,000 Da), mass spectrometers typically resolve the monoisotopic peak. For larger peptides, the average mass better matches the observed centroid peak.
How do I use peptide molecular weight for mass spectrometry analysis?
Calculate the peptide molecular weight, then predict its mass-to-charge (m/z) ratios for different charge states. For electrospray ionization (ESI): m/z = (MW + z × 1.0073) ÷ z, where z is the charge state and 1.0073 Da is the proton mass. A peptide with MW = 1,500 Da at charge state +2 appears at m/z = 751.5. These predicted values help identify peptide peaks in LC-MS/MS data.
Can I calculate the molecular weight of a cyclic peptide?
Yes, with a manual adjustment. A cyclic peptide loses one additional water molecule during ring closure compared to a linear peptide. Calculate the linear peptide molecular weight first, then subtract 18.02 Da for the cyclization. For a head-to-tail cyclic peptide: MW(cyclic) = MW(linear) - 18.02 Da.