A single character can change a reported concentration by a factor of 1,000. Nanomolar, micromolar and millimolar are all amount-concentration scales, but their prefixes are not interchangeable. A reliable comparison starts by expanding the notation before comparing the numbers.
Decode the symbols
IUPAC lists mol/L as a common amount-concentration unit, sometimes written M. BIPM defines milli, micro and nano as factors of 10⁻³, 10⁻⁶ and 10⁻⁹ respectively. Combining those conventions gives mM, µM and nM.IUPAC Gold Book — Amount concentration (opens in a new tab)BIPM — SI prefixes (opens in a new tab)
| Name | Notation | Equivalent in mol/L |
|---|---|---|
| Millimolar | mM | 10⁻³ mol/L |
| Micromolar | µM | 10⁻⁶ mol/L |
| Nanomolar | nM | 10⁻⁹ mol/L |
The lower-case prefix and upper-case M both matter. In a concentration expression, mM is not the same as mm, the symbol commonly encountered for millimetres. Read the quantity named in the heading rather than relying on the characters alone.
Use one scale for a comparison
For an illustrative comparison, take values of 0.025 µM, 40 nM and 0.00003 mM. Expressed in nanomolar units, they are 25 nM, 40 nM and 30 nM. The smallest-looking decimal in the original list was not the smallest concentration.
The operation follows the unit size. A micromolar unit contains 1,000 nanomolar units, so the numerical value grows by 1,000 when changing from µM to nM. Moving the other way divides the number by 1,000. Between mM and nM, there are two such steps.
Watch for lost micro symbols
PDF extraction, spreadsheets and fonts can turn the micro symbol into a lookalike or plain-text substitute. Some sources write uM when their system cannot display µM. Confirm that convention from the source context instead of treating every unfamiliar letter as a new unit.
Keep an image or stable link to the original table when a transcription is uncertain. A one-character error may be invisible in a polished summary yet change the scale dramatically. Resolving it against the original is better than selecting the unit that makes the result seem plausible.
When entering a literature table, store the original value and unit in separate fields, then add a converted value in a common unit. This makes the transformation auditable and prevents a later reader from confusing the source's notation with your standardised one.
A prefix conversion does not need molar mass
Changing nM to µM keeps the same kind of quantity and only changes the prefix. You do not need a peptide's molecular mass for that operation. Changing a mass concentration such as g/L into mol/L is a different task and does require the appropriate molar mass and amount basis.
This distinction is a useful diagnostic for a spreadsheet formula. If a simple nM-to-µM column depends on a molecular-weight cell, the formula may be doing more than the heading says. Trace the units through the calculation before using the output.
Converted concentrations also retain the original experiment's limits. A shared unit does not make different targets, exposure periods or response measurements directly comparable. It merely removes one source of numerical confusion.
Make concentration tables easy to check
- Define the concentration unit in the column heading.
- Preserve the source value and its prefix.
- Convert comparable values to one chosen scale.
- Check one row by reversing the conversion.
- Keep experimental context beside the numerical comparison.
Sources and further detail
- IUPAC Gold Book — Amount concentration (opens in a new tab)
Defines amount of a constituent divided by mixture volume and the common mol/L notation.
- BIPM — SI prefixes (opens in a new tab)
Official decimal factors and case-sensitive symbols.
Sources checked 19 September 2026. Worked examples are illustrative unless a supplied report is explicitly identified. This article has not undergone independent scientific peer review.