mM
Meaning of mM: Easy to confuse with mm for millimeter and with mmol for a plain amount, the capital M is the part that signals a concentration.
Definition of mM
mM is the symbol for millimolar, a unit of concentration equal to one millimole of a substance dissolved in one liter of fluid. It states how much of something is present in a given volume rather than how much exists in total. Laboratory reports more often write the same quantity as mmol/L.
What is a mole, and why use it?
A mole is a count, not a weight. It is a fixed and very large number of particles, chosen so that working with atoms and molecules produces manageable figures. Counting rather than weighing matters because chemistry happens particle by particle: one molecule reacts with one molecule regardless of how heavy either is. A millimole is one thousandth of a mole, and it is the practical size for biology, where the quantities in a sample of blood or urine are small. Attach that count to a volume and it becomes a concentration, so one millimole per liter is one millimolar.
How does it differ from the units beside it?
| Unit | What it measures | Where it appears |
|---|---|---|
| mM, written mmol/L | Concentration by particle count | Sodium and potassium in blood |
| mmol | An amount by particle count, with no volume attached | The quantity in a dose or a daily intake |
| mg/dL | Concentration by mass in a volume | Glucose and cholesterol, in the countries using these units |
| mm | Length in millimeters | The size of a lesion |
| mmHg | Pressure | Blood pressure |
Case carries meaning here. A small m stands for milli and a capital M stands for molar, so mM and mm mean entirely different things while differing by a single keystroke.
Where does it appear, and what does it not tell?
Concentrations written this way run through laboratory testing: sodium, potassium, chloride and bicarbonate, along with calcium, magnesium, urea, creatinine and glucose in the countries reporting in these units. Which unit a country uses is a matter of convention rather than accuracy, so one blood sample can be reported in millimoles per liter in one place and in milligrams per deciliter in another. Converting between them requires the molecular weight of the substance, which is exactly why the two cannot be compared by eye. The unit also says nothing about whether a value is usual, since that judgment comes from the reference range for the particular substance. And a concentration measured in blood describes the fluid tested only, not the total amount held in the body, much of which sits inside cells or in bone rather than in circulation.
Also known as
Used in a sentence
The laboratory reported the electrolyte concentration in mM.
Good to know
mM is a concentration, millimoles per liter, rather than an amount, which mmol denotes on its own, and it is distinct again from mg, a unit of mass, so values in these units cannot be compared without converting through the substance's molecular weight.