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Beer–Lambert

Absorbance A = ε · b · c. Fill any three fields and leave the one you want blank — it solves for it.

How to use: Fill any three of A, ε, b and c, and leave the one you want blank — it solves for it. The lower panel converts %transmittance ↔ absorbance.
Solves for whichever single field is left blank. If ε is unknown but you have A, b and c, it returns ε.

%Transmittance ↔ Absorbance

A = 2 − log₁₀(%T)  ·  %T = 100 × 10⁻ᵃ

Advisory. Valid in the linear (dilute) regime; deviations at high absorbance, stray light, or chemical/instrumental non-idealities are on you to check.