Peptide Concentration & Molarity Calculator

Turning the milligram figure on a lyophilized vial into a concentration you can write in a lab notebook is simple arithmetic, yet it trips people up more often than you would expect. This calculator handles it in one pass. Enter the nominal vial mass, the net peptide content, the purity, the volume of solvent added and the molecular weight, and it reports mass concentration, molar concentration and the total amount of substance in the vial.

ExoLabz built it for the bench tasks our Canadian customers actually perform: preparing analytical reference standards, making up samples for HPLC and LC-MS, setting up stock solutions for in-vitro assays and keeping concentration records accurate. It is a chemistry tool only, and every compound we supply is sold for laboratory research.

About the presets: each value is the average molecular weight of the free molecule as listed on PubChem. The TB-500 option is the acetylated seven-residue fragment Ac-LKKTETQ (889.0 g/mol), not full-length thymosin beta-4. GHK-Cu uses the 1:1 copper complex (402.9 g/mol), and NAD+ uses the free acid, nadide (663.4 g/mol). For anything else, choose Custom and type the figure from your certificate.

What each field means

Nominal vial mass

This is the number printed on the label, such as 5 mg or 10 mg: the gross weight of lyophilized powder in the vial, not the weight of the peptide chain alone.

Net peptide content

A freeze-dried peptide is never 100% peptide by weight. Synthetic sequences are purified by reversed-phase HPLC and isolated as salts, so basic sites (lysine, arginine, histidine, the N-terminus) carry counter-ions such as acetate or trifluoroacetate. The dried cake also holds residual moisture that it pulls back from the air. Net peptide content is the fraction of the powder that really is peptide, usually determined by amino acid analysis or elemental nitrogen, and it commonly lands between 70% and 90%. The field starts at 80% as a reasonable placeholder; replace it with a measured value whenever you have one. Our article net peptide content explained walks through where the missing mass comes from.

Purity

Chromatographic purity tells you what share of the peptide material is the target sequence as opposed to deletion sequences, truncations and other related impurities. Leave it at 100 if you want the concentration of total peptide. Enter the HPLC figure from your certificate if you need the concentration of the target compound alone. ExoLabz material is 99%+ purity, so the correction is small.

Solvent volume and molecular weight

Volume is the total liquid added, in millilitres. For small quantities of peptide the volume taken up by the dissolved solid is negligible, so the calculator treats the added volume as the final volume. For molecular weight, the presets deliberately use the unsalted molecule: the counter-ion mass has already been removed by the net content correction, so pairing a salt-form weight with a net content figure would subtract that mass twice.

How the numbers are worked out

You can check the arithmetic by hand:

net mass (mg) = vial mass × (net content ÷ 100) × (purity ÷ 100)
concentration (mg/mL) = net mass ÷ volume
molarity (mM) = (mg/mL ÷ molecular weight) × 1,000
total amount (nmol) = (net mass ÷ molecular weight) × 1,000,000

One mg/mL is numerically identical to one µg/µL, which is convenient when your pipettes are set in microlitres. As a worked example, a 10 mg vial of BPC-157 at 80% net content and 100% purity holds 8 mg of peptide. Dissolved in 2 mL that gives 4 mg/mL. Dividing by 1,419.5 g/mol gives 2.818 mM (2,818 µM), and the vial contains roughly 5,636 nmol in total.

Making a working standard from the stock

A fresh stock is usually too concentrated to use directly. The second tool applies C1V1 = C2V2: give it the stock concentration, the concentration you want and the volume you need, and it returns how much stock to pipette and how much diluent to add. Work in µM when building a calibration series for LC-MS quantitation or a concentration series for a cell-free assay, and in µg/mL when your method or reporting sheet is mass-based. If the stock volume comes out below about 2 µL, add an intermediate dilution step, since very small volumes are where pipetting error is largest.

Limits of the calculation

This page performs arithmetic only. It does not account for solubility limits, adsorption to plastics, or degradation. A hydrophobic sequence may not fully dissolve at the concentration you request, dilute solutions of some peptides lose material to polypropylene tube walls, and residues such as methionine, cysteine and tryptophan can oxidize once the peptide is in solution. For critical standards, confirm the real concentration analytically, for example by UV absorbance where the sequence contains aromatic residues, or by quantitation against a reference material on HPLC.

When you record the result, note the lot number, the net content and purity values you used, the solvent and the date. If a certificate of analysis has been published for your lot, it will be on our COA page with the rest of our third-party HPLC and purity results. Not every lot has a published certificate, so keep your own records complete either way.

Frequently asked questions

Why is my concentration lower than label mass divided by volume?

Because the label weight includes counter-ions and water. With 80% net content, a 10 mg vial holds about 8 mg of peptide, so the true concentration is about a fifth lower than the simple division suggests.

Is mg/mL the same as µg/µL?

Yes. Numerator and denominator both scale by 1,000, so the figure is unchanged.

My certificate lists the acetate salt weight. Which should I enter?

Use one correction, never both. Either enter the free-molecule weight together with the measured net content, or enter the full salt-form weight and set net content to 100% if your supplier has specified the salt stoichiometry and water content separately.

Does the NAD+ preset work the same way?

NAD+ is a dinucleotide, not a peptide, so there is no peptide counter-ion correction. Set the content field to the assay value on your certificate, or to 100% if none is given, and the molar results will follow from the nadide weight.

Is anything I type saved or sent anywhere?

No. The calculator runs entirely in your browser. Nothing is stored between visits and no values leave your device.

Research use only. ExoLabz products are supplied in Canada exclusively for laboratory research and analytical work. They are not for use in humans or animals, and this calculator is intended solely for laboratory solution work.