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Millimolarity to molarity is a change of prefix, not a change of quantity: millimolar (mM) is one thousandth of a molar (M), so you divide a value in mM by 1,000 to get M. Micromolar (µM) is one millionth of a molar, so you divide a value in µM by 1,000,000 to get M. This page gives the formula, a full lookup table, two worked examples, spreadsheet and code snippets, and the mistakes that most often cause a thousand-fold error at the bench.
The mM to M formula
Molar (M) is the unit of molar concentration, moles of solute per litre of solution (mol/L). The prefixes milli and micro are standard SI prefixes, and the conversion factors come directly from them. The US National Institute of Standards and Technology (NIST) and the International Bureau of Weights and Measures (BIPM) both list milli (m) as 10-3, micro (µ) as 10-6, nano (n) as 10-9 and pico (p) as 10-12.
- mM to M: M = mM × 10-3 = mM ÷ 1,000
- M to mM: mM = M × 1,000
- µM to M: M = µM × 10-6 = µM ÷ 1,000,000
- µM to mM: mM = µM ÷ 1,000
The factors are exact powers of ten, so the conversion involves no rounding. Any rounding in your answer comes only from the significant figures of the number you started with.
Conversion table: M, mM, µM, nM and pM
Read across a row to see the same concentration in each unit. Each step down the table multiplies the number by 1,000. Prefix values are from the NIST and BIPM SI prefix tables, accessed 2026-10-29; the table entries themselves are arithmetic on those prefixes and are exact.
| Unit | Symbol | Value in M | 1 of this unit equals | To convert to M |
|---|---|---|---|---|
| Molar | M | 1 | 1,000 mM | × 1 |
| Millimolar | mM | 10-3 (0.001) | 1,000 µM | ÷ 1,000 |
| Micromolar | µM | 10-6 (0.000001) | 1,000 nM | ÷ 1,000,000 |
| Nanomolar | nM | 10-9 (0.000000001) | 1,000 pM | ÷ 1,000,000,000 |
| Picomolar | pM | 10-12 | 0.001 nM | ÷ 1012 |
A quick reference for common values:
| mM | M | µM |
|---|---|---|
| 0.01 mM | 0.00001 M | 10 µM |
| 0.1 mM | 0.0001 M | 100 µM |
| 1 mM | 0.001 M | 1,000 µM |
| 10 mM | 0.01 M | 10,000 µM |
| 100 mM | 0.1 M | 100,000 µM |
| 500 mM | 0.5 M | 500,000 µM |
| 1,000 mM | 1 M | 1,000,000 µM |
Worked examples
Example 1: millimolarity to molarity (100 mM to M)
A buffer is labelled 100 mM. Divide by 1,000: 100 ÷ 1,000 = 0.1, so the concentration is 0.1 M. Check the direction: the unit got larger (M is 1,000 times bigger than mM), so the number must get smaller.
Example 2: micromolarity to molarity (5 µM to M)
A working solution is 5 µM. Divide by 1,000,000: 5 ÷ 1,000,000 = 0.000005, which is 5 × 10-6 M. To express the same value in millimolar, divide the µM figure by 1,000 instead: 5 ÷ 1,000 = 0.005 mM.
The reverse: molarity to millimolarity (0.5 M to mM)
Multiply by 1,000: 0.5 × 1,000 = 500 mM. The unit got smaller, so the number gets larger.
Micromolarity (µM) to molarity
People also search for “micro molarity to molarity” and “micromolarity to molarity”; both mean micromolar (µM), the unit symbol with the Greek letter mu. The rule is the same as for millimolar with a different power of ten: M = µM ÷ 1,000,000. When a protocol gives a stock in mM and a working concentration in µM, convert both to the same unit before comparing them. The stock-to-working step itself (how much stock to add to reach the target) uses C1V1 = C2V2, covered in molarity and solution calculations for the lab and serial dilution technique.
How to compute it in Excel, R and Python
Excel or Google Sheets (value in mM in cell A2):
=A2/1000 (mM to M)
=A2*1000 (M to mM)
=A2/1000000 (µM to M)
=A2/1000 (µM to mM)R:
mM_to_M <- function(x) x / 1e3
uM_to_M <- function(x) x / 1e6
mM_to_M(100) # 0.1
uM_to_M(5) # 5e-06Python:
def mm_to_m(x): return x / 1e3
def um_to_m(x): return x / 1e6
print(mm_to_m(100)) # 0.1
print(um_to_m(5)) # 5e-06Floating-point output can show tiny trailing digits for some inputs; this is display behaviour, not a different answer, and you can round to the significant figures of your measurement.
Common mistakes
- Off by 1,000 between mM and M. A stock written “1 M” is 1,000 mM, and “1 mM” is 0.001 M. Writing the units on every line of the calculation catches this.
- Calling M “micromolar”. Capital M alone is molar. Micromolar is µM and millimolar is mM; the lowercase m and the mu are not interchangeable.
- Mixing up m and µ. Milli (m, 10-3) and micro (µ, 10-6) differ by a factor of 1,000. The NIST SI prefixes page notes that symbols for SI submultiples are lowercase, and that the prefix is attached to the unit without a space.
- “uM” in plain text. Many spreadsheets, databases and file names cannot hold the µ character, so “uM” is a common ASCII stand-in for µM. State it once in a protocol so a reader does not take it for a different unit.
- Using a prefix on a prefix. NIST SP 811 does not permit compound prefixes, so write 1 nM rather than “1 millimicromolar”.
Safety note
Converting a concentration does not change what the chemical is. Before you weigh, dilute or handle any solute, read its Safety Data Sheet (SDS) for hazards, protective equipment and disposal; see how to read a safety data sheet, and OSHA SDS requirements for the employer-side obligations. Concentrated stocks are usually more hazardous than the dilute working solutions made from them, so check the stock’s SDS, not just the final concentration. Have a second person check the arithmetic on any solution that will be used in a regulated or clinical-adjacent workflow, and keep the calculation in the lab notebook.
Frequently asked questions
How do you convert mM to M?
Divide the millimolar value by 1,000. For example, 250 mM ÷ 1,000 = 0.25 M.
How do you convert µM to M?
Divide the micromolar value by 1,000,000. For example, 20 µM ÷ 1,000,000 = 0.00002 M, or 2 × 10-5 M.
Is millimolar the same as millimolarity?
In everyday lab use, yes: millimolar (mM) is the unit and “millimolarity” is a loose way of naming the concentration expressed in it. Molarity is the concentration quantity, and molar (M) is its unit.
Related guides
- Molarity and solution calculations for the lab
- Buffer and solution preparation
- Serial dilution technique and calculation
- Pipetting technique for accuracy and precision
Sources
- NIST, Metric (SI) Prefixes, accessed 2026-10-29.
- BIPM, SI Prefixes, accessed 2026-10-29.
- NIST Special Publication 811, Chapter 6: Rules and Style Conventions for Printing and Using Units, accessed 2026-10-29.
