Molarity Units and Conversions
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Molarity is commonly reported in mol/L and scaled with SI prefixes. Convert between M, mM, µM, nM, and pM by powers of 1,000.
| Unit | Symbol | Moles per liter | Relationship to M |
|---|---|---|---|
| molar | M | 1 | 1 M |
| millimolar | mM | 10^-3 | 0.001 M |
| micromolar | µM | 10^-6 | 0.000001 M |
| nanomolar | nM | 10^-9 | 0.000000001 M |
| picomolar | pM | 10^-12 | 0.000000000001 M |
Convert M to smaller units
Multiply by the appropriate power of ten:
- M to mM: multiply by 1,000.
- M to µM: multiply by 1,000,000.
- M to nM: multiply by 1,000,000,000.
- M to pM: multiply by 1,000,000,000,000.
For example:
0.00250 M × 1,000 = 2.50 mM
0.00250 M × 1,000,000 = 2,500 µM
Convert back to M
Divide by the same factor:
750 µM / 1,000,000 = 0.000750 M
Use the Molarity Calculator when the starting information is mass, molar mass, and final volume rather than an existing concentration value.
Convenient prefix relationships
When compatible prefixes appear in numerator and denominator, some values are numerically equal:
- mol/L equals mmol/mL numerically;
- mmol/L equals µmol/mL numerically;
- µmol/L equals nmol/mL numerically.
Keep units visible so a convenient equality is not mistaken for a universal cancellation rule.
Molarity is not mass concentration
mg/mL, g/L, and µg/mL are mass-per-volume units. They become molarity only after division by the correct molar mass:
molarity = mass concentration in g/L / molar mass in g/mol
A mass concentration of 1 g/L produces different molarities for sodium chloride and glucose because their molar masses differ.
Significant figures
Unit conversion does not create new measurement precision. Convert from the unrounded value and round the final displayed concentration to a precision supported by the inputs. Exact prefix factors such as 1,000 do not limit significant figures.
Common mistakes
- Confusing uppercase M with lowercase m used for molality.
- Treating mM and M as interchangeable.
- Converting mg/mL directly to mM without molar mass.
- Rounding after every prefix conversion.
- Replacing the micro symbol with the letter m; µM and mM differ by a factor of 1,000.
Frequently asked questions
How many mM are in 1 M?
There are 1,000 mM in 1 M.
How many µM are in 1 mM?
There are 1,000 µM in 1 mM.
Is mg/mL the same as mM?
No. Converting mass concentration to amount concentration requires the substance's molar mass.
Should I write micromolar as uM or µM?
The SI prefix symbol is µ. Plain-text interfaces sometimes use uM for compatibility; if the tool accepts it, explain that it means micromolar.
Sources
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