Absorbance to Transmittance Calculator
Convert between absorbance (A) and percent transmittance (%T), the two ways spectrophotometers report how much light a sample lets through.
Absorbance vs. transmittance
Spectrophotometers measure how much light of a chosen wavelength passes through a sample. That can be reported as transmittance (the fraction or percent of light that gets through) or as absorbance (a logarithmic scale that is directly proportional to concentration via the Beer-Lambert law).
The formula
The two are related by A = 2 − log10(%T), which comes from A = −log10(T), where T is transmittance expressed as a fraction (T = %T ÷ 100). This calculator converts in either direction: give it an absorbance to get %T, or a %T to get absorbance.
Note: Absorbance values above about 2 (meaning less than 1% of light transmitted) are often unreliable on standard instruments due to stray light, so very high inputs should be treated cautiously. For general lab and educational use.