Millimolar to Micromolar Converter

µM

Common Conversions

mM	µM
0.001	1
0.005	5
0.01	10
0.05	50
0.1	100
0.25	250
0.5	500
1	1000
2	2000
5	5000
10	10000
100	100000

Why this conversion matters in chemistry

Multiplying by 1000 sounds like the kind of arithmetic that shouldn't deserve its own conversion page, and yet this is one of the most consequential factor-of-1000 errors in bench chemistry. Assay plates run at micromolar; substrate and cofactor concentrations are quoted in micromolar in the literature; but if you grab a 1 mM reading off a reagent label and treat it as 1 µM without thinking, your working concentration lands a thousand-fold too low and a reaction that should have gone in minutes doesn't detectably happen. A 1 mM ATP stock is 1000 µM — so a 100 µM working ATP in a kinase reaction takes 10 µL of stock per 100 µL of reaction, not the 100 µL you'd add if you confused the units.

Formula

µM = mM × 1000

Where the factor comes from

Everything in this pair rides on typography. In mM the lowercase m is the prefix milli, a defined 10⁻³, while the capital M is the molar, one mole of solute per liter of solution; in µM the Greek mu carries micro, 10⁻⁶. Subtract the exponents and the factor is 10³, exact, because prefixes are defined multipliers rather than measured quantities. Both the liter and the mole cancel, so nothing about the dissolved substance participates. Worth knowing alongside it: M is not an SI unit symbol at all — mol/L and mol dm⁻³ are the SI forms, and the molar is tolerated in practice rather than adopted. That is why clinical and analytical journals tend to insist on mmol/L where a biochemistry paper writes mM for precisely the same quantity.

Precision and significant figures

The decimal shift preserves every figure it is handed. 0.15 mM is 150 µM, two significant figures either way, with the zero as a placeholder. Trouble comes from the balance rather than the arithmetic. A millimolar stock made from 12.3 mg of solid weighed on a four-place balance carries three figures at best, and the glassware adds its own contribution — a class A 100 mL volumetric flask is specified to roughly ±0.08 mL, about 0.08%. Neither justifies the six digits a calculator hands back after multiplying by 1000. Round the micromolar answer to what the weighing supported, and do not let the longer number suggest a better-characterized solution than you have.

Worked Examples

1 mM = 1000 µM

A standard ATP stock. Most kinase reactions pull from something close to this and dilute a hundred-fold into the working mix.

0.1 mM = 100 µM

A common substrate working concentration — roughly where many enzymes sit at or above their Km.

0.001 mM = 1 µM

The kind of IC50 you'd look at and think the inhibitor is worth developing further.

5 mM = 5000 µM

Fasting blood glucose in a healthy person is right around here — a useful mental anchor for the scale.

Common mistakes

A µ typed as u, or lost entirely

Instrument exports, LIMS fields and older text files cannot hold the µ character, so µM arrives as uM or umol/L. Neither is ambiguous alone, but an export that substitutes an m for the mu shifts the value a thousandfold, and one that drops the prefix outright shifts it a millionfold. When a concentration comes off a machine rather than a handwritten label, confirm which prefix the file meant.

Which species the millimolar figure describes

A 1 mM CaCl₂ solution is 1000 µM in calcium and 2000 µM in chloride, and a label carrying only the formula-unit concentration will not tell you which. Moving the prefix resolves nothing — it faithfully carries whatever species the original number described. When a buffer recipe and an ionic-strength calculation disagree by a small whole-number factor, this is very often the reason.

A mg/mL stock is not millimolar

Reagent labels alternate between mass concentration and molar concentration with little signaling, and getting from one to the other needs the molar mass, which this prefix step does not supply. 1 mg/mL of a 500 g/mol compound is 2 mM; the same 1 mg/mL of a 50 kDa protein is 0.02 mM. Convert onto a molar basis first, then move the prefix.

Frequently Asked Questions

How do I convert mM to µM?

Multiply by 1000. So 0.5 mM becomes 500 µM, and 2 mM becomes 2000 µM. The milli and micro prefixes are three orders of magnitude apart (10⁻³ and 10⁻⁶), so the factor is the same every time.

When is it natural to use mM versus µM?

Millimolar tends to show up for bulk things you prepare — buffer components, electrolyte concentrations, clinical metabolites. Micromolar is more common once you're dosing: drug IC50s, enzyme Km values, working-strength reagents. Most benches keep stocks in mM and do the division when they set up an experiment.

How do I prepare a µM solution from a mM stock?

C₁V₁ = C₂V₂ does all the work. For 10 mL of 100 µM from a 10 mM stock, you need (100 µM × 10 mL) / 10,000 µM = 0.1 mL of stock, topped up with diluent to 10 mL. Keeping everything in the same units before you start the arithmetic saves a surprising number of mistakes.

How do mM and µM fit into the full molarity scale?

1 M is 1000 mM, which is a million µM, which is a billion nM. So 1 mM is 10⁻³ M and 1 µM is 10⁻⁶ M. Each prefix is a clean factor of 1000 — the reason the scale is so forgiving to move around on once you're used to it.