Peptide Solution Concentration Calculator

Every lyophilized vial arrives with a single number on the label, and that number is not the concentration of anything. Before a stock can go into a sample sequence or a notebook entry, the labeled mass has to be corrected for salt and moisture, divided by the volume you added and, for most analytical work, converted into molar terms. This page does the conversion for you and shows the working so you can check it.

Use it to prepare HPLC and mass-spectrometry samples, to set up in-vitro assay stocks, to plan a reference standard, or simply to document what is in a tube. ExoLabz supplies research compounds to US laboratories for research use only, and this tool is purely analytical chemistry.

Preset molecular weights come from PubChem and describe the free molecule. Three choices need a word of explanation. “TB-500 fragment” means the acetylated heptapeptide Ac-LKKTETQ at 889.0 g/mol; if your material is the full 43-residue thymosin beta-4, pick Custom. The GHK-Cu entry is the 1:1 copper complex at 402.9 g/mol. NAD+ is listed as nadide, the free acid, at 663.4 g/mol.

Why the label mass is only a starting point

Peptides made by solid-phase synthesis come off the purification column as salts. Positively charged side chains and the N-terminus pair up with counter-ions from the mobile phase, typically trifluoroacetate or, after exchange, acetate. Freeze-drying removes bulk water but leaves some bound moisture behind, and the powder absorbs more whenever the vial is opened. The result is that a vial labeled 10 mg often contains something like 7 to 9 mg of actual peptide.

The figure that captures this is net peptide content, reported as a percentage and measured by amino acid analysis or nitrogen determination. The calculator defaults to 80% because that is a middle-of-the-road value for purified research peptides. If you have a measured number for your lot, type it in; if you don’t, record that you used the default so anyone reading your notebook later knows the concentration is nominal.

Purity is a separate correction

Net content answers “how much of this powder is peptide?” Purity answers “how much of that peptide is the sequence I want?” A reversed-phase chromatogram separates the target from closely related byproducts such as deletion or truncated sequences, and the purity figure is the target peak’s share of the total. ExoLabz material runs 99%+ purity, so for routine work leaving the field at 100 changes the answer very little. For quantitative calibration standards, enter the exact chromatographic value.

Picking the right molecular weight

Molarity depends entirely on the molecular weight you divide by. The presets use average mass rather than monoisotopic mass, which is correct for weighing and solution work; monoisotopic values belong in your mass spectrometer’s extraction window, not in concentration math. They also describe the unsalted molecule. Since the counter-ion has already been stripped out by the net content step, using a salt-form weight here would correct for it twice.

The math, step by step

peptide mass = labeled mass × net content % × purity %
mg/mL = peptide mass ÷ solvent volume
mM = mg/mL ÷ MW × 1000
nmol in vial = peptide mass ÷ MW × 10^6

Check it against a real case. Take a 10 mg vial of BPC-157 (1,419.5 g/mol) at 80% net content, fully pure, dissolved in 2 mL. Peptide mass is 8 mg, mass concentration is 4 mg/mL, molar concentration is 2.818 mM or 2,818 µM, and the whole vial holds about 5,636 nmol. Because a milligram per milliliter equals a microgram per microliter, 4 mg/mL is also 4 µg/µL.

Diluting to a working standard

The lower tool solves C1V1 = C2V2. Choose whether you are working in µM or µg/mL, press the button to carry over the stock value, then enter the target concentration and the final volume. It returns the aliquot of stock and the volume of diluent to reach that total. A few habits help: prepare dilutions in the same matrix as your samples, pipette the smaller volume into the larger one, and split very large dilution factors into two steps rather than one heroic jump. A stock aliquot under 2 µL is a sign you should add an intermediate step.

What this calculator can’t tell you

The results are only as good as the assumptions behind them: solubility limits, adsorption to plastics and degradation are not accounted for. Some sequences will not dissolve completely at high concentration in water, low-concentration solutions can lose a meaningful fraction of peptide to tube and tip surfaces, and oxidation-prone residues change over time in solution. When a concentration really matters, verify it with an orthogonal measurement such as UV absorbance or HPLC quantitation against a reference.

Keep the lot number with every calculation. Third-party HPLC and purity reports for selected lots are collected on our COA page; a certificate is not posted for every batch, so if yours isn’t listed, note in your records that the default net content and purity were used.

FAQ

What solvent should I use?

That depends on the sequence and on your downstream method. Many short, charged peptides dissolve readily in water or dilute acetic acid; hydrophobic ones may need a small proportion of acetonitrile or DMSO first. Make sure whatever you choose is compatible with your column or assay.

Does the calculator include the volume of the powder?

No. It assumes the final volume equals the solvent volume you added. At milligram-per-milliliter levels the displacement from the solid is too small to matter for most analytical purposes.

Should I use average or monoisotopic mass?

Average mass for anything involving weighing or molarity. The presets are all average values.

Why do I get a message about the target concentration?

The dilution tool only dilutes. If the target is higher than the stock, there is no volume of stock that can reach it, so the tool asks you to check the numbers.

Does the page save my entries?

No. Everything is computed locally in your browser and cleared when you leave the page.

For research use only. ExoLabz products are sold to US customers strictly for laboratory research and analytical purposes and are not for use in humans or animals. This tool is provided to support solution preparation and record keeping in the lab.