What is molar absorptivity (ε)?

Molar absorptivity, also called the molar extinction coefficient, is a measure of how strongly a chemical species absorbs light at a given wavelength. Its units are L·mol⁻¹·cm⁻¹ (equivalently M⁻¹·cm⁻¹). A high ε value means the compound absorbs light very strongly; a low value means it absorbs weakly. For example, the dye p-nitroaniline has ε ≈ 16,000 M⁻¹·cm⁻¹ at 380 nm, while hemoglobin at 540 nm has ε ≈ 13,800 M⁻¹·cm⁻¹.

What units should I use for concentration?

Concentration must be in mol/L (molarity, M). If your concentration is in mg/mL or μg/mL, convert it first using the compound's molecular weight: mol/L = (g/L) / (MW in g/mol). If your concentration is in μM, divide by 1,000,000 before entering it, or use scientific notation (e.g., 5e-6 for 5 μM).

What path length should I enter?

Standard rectangular cuvettes have a path length of exactly 1 cm. Microvolume instruments (e.g., NanoDrop) often use a shorter fixed path length such as 0.1 mm (0.01 cm) or 1 mm (0.1 cm). Check your instrument's documentation. If you used a 1 cm cuvette, enter 1.

What absorbance values are reliable?

Most spectrophotometers give reliable readings between A = 0.1 and A = 1.5. Below 0.1, signal-to-noise becomes significant. Above about 1.5–2.0, stray light and detector saturation introduce errors. If your reading is outside this range, dilute the sample or use a shorter path length cuvette.

Does ε change with wavelength?

Yes. Molar absorptivity is wavelength-dependent. The value you calculate is valid only at the wavelength at which absorbance was measured. Always record the wavelength alongside ε. The maximum ε (ε_max) occurs at the wavelength of maximum absorbance (λ_max), which is also the wavelength where Beer-Lambert Law gives the most sensitive concentration measurements.

Why might my calculated ε deviate from literature values?

Common causes: (1) sample purity — impurities absorb and inflate A; (2) concentration error from weighing or pipetting; (3) cuvette contamination or scratches; (4) pH affecting the ionization state of chromophores (e.g., phenolphthalein, tyrosine); (5) high-concentration aggregation that violates Beer-Lambert linearity; (6) temperature — ε varies slightly with temperature.

Molar Absorptivity Calculator

Name: Molar Absorptivity Calculator
Availability: InStock
Author: Nham Vu

Enter absorbance, concentration, and path length to calculate molar absorptivity (ε) using the Beer-Lambert Law: ε = A / (c × l).

Beer-Lambert Inputs

ε = A / (c × l)

Absorbance (A) — dimensionless

Typical reliable range: 0.1 – 1.5

Concentration (c) — mol/L (M)

Use scientific notation: 2.5e-5 for 25 μM

Path Length (l) — cm

Standard cuvette = 1 cm

Wavelength (λ) — nm (optional)

Recorded in result for reference only

Common ε Reference Values

Compound	λ (nm)	ε (M⁻¹·cm⁻¹)
DNA (double-stranded)	260	~6,600 / cm·(mg/mL)
Tryptophan	280	5,690
Tyrosine	280	1,280
NADH	340	6,220
p-Nitroaniline	380	16,000

Enter absorbance, concentration, and path length, then click Calculate.

Summary