Path length and concentration units
Get centimetres, millimetres, M, mM and µM onto the same scale.
Match the coefficient’s units
The Beer–Lambert calculator uses ε in L·mol⁻¹·cm⁻¹. It therefore converts path length to centimetres and concentration to mol/L before multiplying. The units cancel, leaving dimensionless absorbance.
Path length is the distance light travels through the sample. 10 mm = 1 cm; a 2 mm path is 0.2 cm.
Read the prefixes
M means mol/L here. One mM is 10⁻³ mol/L, and one µM is 10⁻⁶ mol/L. Thus 25 µM = 0.025 mM = 0.000025 M. Scientific notation expresses that last value as 2.5e-5.
Changing the unit of an existing value should change its displayed number while preserving the physical quantity. A result of 25 µM should become 0.025 mM, not 25 mM.
Mass concentration is a different quantity
mg/L measures mass per volume; mol/L measures amount of substance per volume. A molecular mass is required to convert between them. This release supports molar concentration only, so do not relabel an mg/L measurement as µM.
For an imported calibration table, select the unit that already belongs to its concentration column. The initial selection labels the numbers; later unit changes rescale them.
Sources & further reading
Original educational material. Content review records a source and consistency check; it is not specialist peer review or laboratory method validation.