Peptide unit conversions span four families, mass, moles, volume, and concentration, and the one rule that trips people up is that crossing between mass and moles requires a molecular weight: moles = mass / molecular weight. This post lays out how the four families relate and which calculator on this site handles each step.
How do you convert between peptide units?
Mass units (nanograms through grams), mole units (picomoles through moles) and volume units (microliters through liters) each convert within their own family by a simple scale factor: a milligram is always a thousandth of a gram, a milliliter is always a thousandth of a liter, regardless of what substance is involved. The mass unit converter, mole unit converter, and volume unit converter each handle one of these scale conversions directly.
Crossing between a mass unit and a mole unit is different. A gram of one peptide is not the same number of moles as a gram of another, because moles depend on molecular weight:
moles = mass / molecular weight
mass = moles x molecular weightThis is the one step in the whole system that cannot be skipped or approximated: without a molecular weight, a mass figure and a mole figure are not interchangeable, full stop. If the molecular weight is not already known, it can be derived from a sequence, covered in how to calculate peptide molecular weight from a sequence.
Concentration is mass or moles divided by volume
Concentration ties the other three families together by dividing an amount (mass or moles) by a volume. That produces two different kinds of concentration unit that look similar but are not interchangeable without, again, a molecular weight:
- Mass concentration (milligrams per milliliter, micrograms per milliliter): amount expressed as a mass divided by volume.
- Molar concentration (molar, millimolar, micromolar, nanomolar, picomolar): amount expressed as moles divided by volume.
The concentration unit converter converts within the mass-concentration family, and the molar concentration converter converts within the molar-concentration family. Neither one crosses between the two families on its own, for the same reason mass and moles do not convert without a molecular weight.
Crossing from mass concentration to molar concentration
When a conversion does need to cross from a mass-based concentration to a molar one, or back, a molecular weight has to be supplied. The mass-molarity converter does exactly this crossing, computing a molar concentration from a mass concentration and a molecular weight, or the reverse, in one step rather than requiring a separate mass to moles conversion followed by a separate division by volume.
Working out a molar concentration directly
Sometimes the starting point is a known mass of solute dissolved in a known volume, and the question is simply what molar concentration that makes. The molarity calculator computes:
molarity = (mass / molecular weight) / volumewhich is the mass-to-moles step and the divide-by-volume step combined, given a mass, a molecular weight, and a volume.
A worked example of where this matters
Suppose a vial label states 5 milligrams of peptide with a molecular weight of 1000 daltons, reconstituted in 2 milliliters. Converting the mass to moles first: 5 milligrams divided by 1000 daltons per mole is 0.005 mmol, or 5 micromoles. Dividing by the 2 milliliter volume gives 2.5 micromoles per milliliter, which is the same as 2.5 millimolar. Skipping the molecular weight step, and instead treating 5 milligrams in 2 milliliters as “2.5 millimolar” directly, would only be correct by coincidence for a compound whose molecular weight happened to be exactly 1000; for any other molecular weight it is simply the wrong unit crossed without the conversion factor it actually needs.
The arithmetic behind converting a vial concentration to a syringe reading, rather than a molar figure, is what the reconstitution calculator on this site’s home page does, and is covered separately in how to reconstitute a peptide vial and in how to convert mg to units on an insulin syringe, which apply the same mass-and-volume arithmetic to a syringe barrel instead of a molar figure.
Where this shows up elsewhere on this site
Unit conversion is not an isolated topic; it is a step inside most other calculations here. Deriving a molecular weight from a sequence, working through a serial dilution series, and reading a Beer-Lambert absorbance result as a concentration all eventually depend on correctly crossing between these unit families, which is why each of those posts links back here rather than restating the conversion rules on their own.
In short: converting within a family (mass, moles or volume) is simple scaling, but crossing between mass and moles always requires a molecular weight, moles = mass / molecular weight.