Peptide Concentration Calculation for Laboratory Research

Every peptide calculator is one equation with good bookkeeping: C = m / (M × V) — concentration equals peptide mass divided by molar mass times volume. With m in mg, M in g/mol, and V in mL, C comes out in mmol/L. Example: 2 mg of a 2,000 g/mol peptide in 2 mL = 2 / (2000 × 0.002) = 0.5 mmol/L = 500 µM. Every "reconstitution calculator" on the internet implements this same arithmetic.

The vial label states fill mass; the peptide mass is smaller. A "5 mg" vial with 90% peptide content holds 4.5 mg of actual peptide, and using 5 mg in the equation overstates concentration by 11%. The content figure comes from the COA (water + counter-ion + residual solvent accounting) and is the difference between a label and a measurement — the distinction this site exists to teach in the certified peptides guide.

Scale linearly: halving volume doubles concentration; the molar mass column is the multiplier that adapts the table to your sequence.

Dilutions follow C1V1 = C2V2. A 2 mM stock diluted 1:10 gives 200 µM; to make 1 mL of 50 µM from a 1 mM stock, take 50 µL of stock plus 950 µL of solvent. Serial dilutions compound multiplicatively — three 1:10 steps give 1:1000 — and each transfer step should use a fresh tip to avoid carrying concentrated stock forward.

COA: 5 mg fill, 92% content, M = 2,450 g/mol. True peptide mass = 4.6 mg. Reconstituted in 2 mL: C = 4.6 / (2450 × 0.002) = 0.939 mmol/L ≈ 939 µM. Without the content correction the answer would have been 1,020 µM — an 8.7% error that propagates into every downstream dilution.

The five classics: forgetting content correction; using average instead of monoisotopic molar mass inconsistently with the COA; confusing mg/mL with µM (mass concentration versus molar concentration); ignoring the dead volume of a syringe or filter; and re-freezing a thawed aliquot whose concentration drifted with evaporation. The solvent side of the workflow is in how to reconstitute peptides.