When a named analyte has a suitable molar absorptivity at the measurement wavelength, Beer–Lambert’s relationship converts blank-corrected absorbance to molarity. Supply its molar mass to express the same concentration in mg/mL.

Formula: corrected A = sample A − blank A; mol/L = corrected A ÷ (ε in L/(mol·cm) × path cm); mg/mL = mol/L × molar mass g/mol

Worked example

A sample absorbance of 0.50 minus a 0.05 blank gives 0.45. With ε = 1,000 L/(mol·cm), a 1 cm path and 100 g/mol molar mass, concentration is 0.00045 mol/L, or 0.045 mg/mL.

Use the right analyte and wavelength

Molar absorptivity must apply to the specified analyte at the measurement wavelength and conditions. OpenStax expresses absorbance as ε times molar concentration times path length. This page accepts ε rather than trying to infer it from color or a product label.

Blank correction is an explicit measurement

Subtract the blank reading entered for the same instrument, solvent and path setup. A sample below the blank is rejected as a nonnegative-concentration result. This calculation does not determine whether the instrument response is linear or the sample is free of interferences.

Mass concentration is not a dose

The molar mass links analyte moles to analyte grams; 1 g/L equals 1 mg/mL numerically. This is laboratory concentration arithmetic only. It does not identify a substance, validate a clinical measurement, recommend a dose or determine a volume to administer.

Explore the related calculation

For a different starting point, try the standard curve concentration calculator or return to the main calculator. Browse the complete tool directory for this site.

Reference and corrections

The definitions and assumptions above follow these sources; the examples and calculator arithmetic are original to this page.

openstax.org reference

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Beer–Lambert mg/mL calculator

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