Isometric flow from a mass-cube beaker through a molecular-weight gear icon to a cluster of molecule dots, representing converting mass concentration to molarity

Molar Concentration: A Worked Example for Peptides

Key takeaways

Molarity (mol/L) = concentration (g/L) divided by molecular weight (g/mol); 1 mg/mL equals 1 g/L.
The molecular weight comes from the Certificate of Analysis.
Worked example: 2 mg/mL and MW 1,000 g/mol gives 0.002 mol/L = 2 mM = 2,000 micromolar.
The same mass-per-volume gives different molarity for peptides of different sizes.
All framing is laboratory concentration math, in vitro only.

Converting a peptide's mass-per-volume concentration into molarity takes one value from the Certificate of Analysis, the molecular weight, and a short calculation. This guide walks through a full worked example.

What you need

You need the prepared concentration in mass-per-volume (mg/mL) and the peptide's molecular weight (MW) in grams per mole (g/mol), which appears on the COA.

The formula

Molarity (mol/L) = concentration (g/L) ÷ molecular weight (g/mol). Because 1 mg/mL equals 1 g/L, you can use the mg/mL number directly as g/L.

The worked example

Suppose you prepared a solution at 2 mg/mL and the peptide's molecular weight is 1,000 g/mol. Then: molarity = 2 g/L ÷ 1,000 g/mol = 0.002 mol/L = 2 millimolar (mM) = 2,000 micromolar (µM). If an assay calls for 10 µM, you would dilute this stock 200-fold.

Why the molecular weight matters

The same mass-per-volume corresponds to different molarities for peptides of different sizes, because a heavier molecule means fewer molecules in the same mass. That is why the conversion always depends on the molecular weight.

Related research guides

Research-use note

This describes laboratory concentration math for in vitro research use only.

Related research: Concentration calculator mg per mL and molarity.

Frequently asked questions

How do I convert mg/mL to molarity?

Divide the concentration in grams per liter (1 mg/mL = 1 g/L) by the peptide's molecular weight in grams per mole.

Where do I get the molecular weight?

From the peptide's Certificate of Analysis or spec sheet.

Can you show an example?

2 mg/mL with MW 1,000 g/mol gives 2 divided by 1,000 = 0.002 mol/L = 2 mM = 2,000 micromolar.

Why does molecular weight change the result?

A heavier molecule means fewer molecules per unit mass, so the same mg/mL is a lower molarity.

Research use only?

Yes, in vitro laboratory research use only.