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Molecular Weight Calculator

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Molecular Weight Calculator

Compute peptide MW from single-letter amino acid sequence.

Inputs

Result
Molecular weight— Da (g/mol)
Residues—
Sequence—
Uses average residue masses. Add −18.01 Da per missing terminus (e.g. C-terminal amide).
Informational Use Only. Molecular weight values are calculated from published average residue masses. Results are estimates — cross-reference with certified analytical data for research accuracy.
FAQ

Frequently Asked Questions

Q1What is molecular weight and why is it important in peptide research?

Molecular weight (MW) is the sum of the atomic weights of all atoms in a molecule, expressed in daltons (Da) or g/mol. In peptide research, MW is used to verify compound identity, calculate molar concentrations, and convert between mass (mg/mcg) and molar amounts (nmol/pmol).

Q2How is the molecular weight of a peptide calculated from its sequence?

Sum the residue molecular weights of each amino acid, add 18.02 Da for the water molecule at the termini (H at N-terminus + OH at C-terminus), and the result is the peptide's MW. This calculator performs the summation automatically from your entered amino acid sequence.

Q3How do I convert between mcg and nmol for a peptide?

Use the relationship: mass (g) = moles × molecular weight (g/mol). For example, 1 nmol of a 3,500 Da peptide = 3,500 ng = 3.5 mcg. Conversely, 100 mcg of the same peptide = 100,000 ng ÷ 3,500 g/mol = 28.6 nmol. The calculator handles these conversions automatically.

Q4Does molecular weight affect how a peptide is absorbed or excreted?

Yes. Larger peptides (higher MW) generally have lower oral bioavailability due to poor GI absorption and enzymatic degradation. Renal clearance is also MW-dependent—peptides under ~30 kDa can be filtered by the kidneys, affecting half-life and excretion rate.