Free tool
Peptide molar conversion
Where a protocol is specified in molar rather than mass terms, molecular weight is the bridge. Enter a molecular weight and a mass, or a concentration, and the conversions follow.
The molecular weight, and your numbers
Pick a compound below to fill the molecular weight, or type one in.
Your answer
Mass to moles, and concentration to molarity.
Show the other figures
- Same in nanomoles
- — nmol
- Molarity at that concentration
- — mM
- Same in micromolar
- — µM
- Mass of one micromole
- — mg
Arithmetic only. This tool converts between mass, volume and concentration for laboratory preparation and dilution planning. It is not dosing guidance, and nothing here is a therapeutic claim. All materials are supplied for in-vitro laboratory research use only.
How the arithmetic works
moles = mass ÷ molecular weight
Molecular weight varies enormously across this catalog — from around 160 g/mol for the smallest materials to over 5,000 for the long-chain analogues. That means a given mass contains wildly different molar quantities depending on the compound, which is exactly why mass-based comparisons between compounds mislead.
Two cautions. Use the average molecular weight for weighing and molar arithmetic; the monoisotopic mass is for comparing against a mass-spectrometry result and is a different number. And these conversions assume pure peptide — if you are working from label mass rather than peptide mass, correct for peptide content first.
Frequently asked
How do I convert peptide mass to moles?
Divide mass by molecular weight. Milligrams divided by g/mol gives millimoles; multiply by 1000 for micromoles. A 10 mg vial of a 1419.5 g/mol peptide contains about 7.045 µmol.
Should I use average or monoisotopic molecular weight?
Average, for anything involving weighing or molar concentration. Monoisotopic mass is the figure a mass-spectrometry result is compared against and diverges from the average as molecules get larger.
Where do I find a molecular weight?
Every compound page on this site lists it alongside the molecular formula and PubChem CID, sourced from PubChem. It is also stated on most certificates of analysis.