Molarity versus Mass Concentration

mg/mL and mM are both written as a number over a millilitre, and they are frequently read as interchangeable. They are not. One measures mass per volume, the other counts molecules per volume, and the relationship between them is the molecular weight.

Two different measurements

Mass concentration is mass per volume, written mg/mL or g/L. It is what a vial label gives you and what reconstitution produces. It says nothing about how many molecules that mass contains, because different substances pack different amounts of molecule into the same mass.

Molarity is moles per litre, written M, mM or uM. A mole is a fixed number of particles, 6.02214076e23 of them, so a molarity counts molecules and is directly comparable between substances.

Why they differ, and by how much

Converting between them requires the molar mass, which is why this conversion is not a units conversion and not all calculators can do it:

mM = (mg/mL x 1000) / molecular weight

The factor of one thousand converts mg/mL to g/L, and dividing by the molar mass in g/mol then gives mol/L.

Worked, on a 2.5 mg/mL solution of a 1500 g/mol peptide:

2.5 mg/mL x 1000 = 2500 mg/L = 2.5 g/L
2.5 g/L / 1500 g/mol = 1.6667 mmol/L = 1.6667 mM

The shortcut that is nearly right

Notice how close 2.5 and 1.67 are. That is not a coincidence, and it is where a lot of avoidable error comes from. For a peptide of about 1000 g/mol, mg/mL and mM are numerically close, so people start treating them as the same number.

The shortcut fails in proportion to the molecular weight. For the same 2.5 mg/mL solution:

at 1500 g/mol it is 1.67 mM
at 3755 g/mol it is 0.67 mM
at 711 g/mol it is 3.52 mM

A factor of five spread across three plausible peptide sizes. Reading one as the other is wrong by that factor, silently, and the resulting error looks reasonable because the number is the right order of magnitude.

When each unit is the right one

Use mg/mL when you are working from a vial label, reconstituting, or computing a draw volume. The label is in milligrams and reconstitution arithmetic is naturally in mass per volume. This is most of peptide work.

Use molarity when you are comparing across substances, following a published method that specifies molar units, or calculating anything that depends on molecule count rather than mass: extinction coefficients, binding, enzyme substrate concentrations. A molarity is meaningless without a molecular weight, so you need the sequence or a trusted specification first.

The average molecular weight calculator gives you the molar mass from a sequence, and the mass to molarity converter then crosses between the two families once you have it.

One caution on the conversion

The conversion assumes the substance is the free peptide. Salt forms, counter ions and hydrates change the mass without changing the molecule count, so a molarity computed from a gross mass is proportionally off by the same amount. The purity correction calculator is the step that handles a certificate of analysis figure.