What is the relationship between absorbance and transmittance?

Absorbance is the negative base-10 logarithm of transmittance: A = −log₁₀(T), where T is the fraction of light passing through the sample. Equivalently T = 10^(−A). Percent transmittance is simply T × 100.

How do I convert percent transmittance to absorbance?

Divide %T by 100 to get T as a fraction, then take A = −log₁₀(T). For example 50 %T is T = 0.5, so A = −log₁₀(0.5) ≈ 0.301. A reading of 10 %T gives A = 1.0.

Why can't transmittance be zero?

A transmittance of exactly zero would mean infinite absorbance, which the logarithm cannot represent. In practice readings approach but never reach zero. The converter treats 0 %T as effectively infinite absorbance and flags it.

What absorbance range is reliable?

Most spectrophotometers are accurate roughly between A = 0.1 and A = 1.0, corresponding to about 80 %T down to 10 %T. Outside this band, small transmittance errors translate into large absorbance errors, so dilute concentrated samples to bring readings into range.

Is absorbance the same as optical density?

For a clear absorbing solution, optical density (OD) and absorbance are used interchangeably and follow the same A = −log₁₀(T) relationship. The term OD is more common for turbid samples like bacterial cultures, where scattering also contributes.

Absorbance ↔ Transmittance / % Transmittance Converter

Spectrophotometers report readings as absorbance, transmittance, or percent transmittance depending on the instrument and protocol. This converter moves between all three instantly using the single defining relationship, with proper handling of the edge cases that trip people up.

How it works

Absorbance and transmittance are linked by a base-10 logarithm:

A   = −log₁₀(T)
T   = 10^(−A)
%T  = T × 100

Edit any field and the value is converted to transmittance internally, then back out to the other two. Because the logarithm of zero is undefined, a transmittance of 0 corresponds to infinite absorbance and is flagged rather than shown as a number. Transmittance above 1 (or %T above 100) and negative absorbance are physically impossible for a normal absorbing sample and are also flagged.

Example and tips

A reading of 25 %T is T = 0.25 and A = −log₁₀(0.25) ≈ 0.602. The convenient anchor points are A = 0 at 100 %T (no absorption), A = 1 at 10 %T, and A = 2 at 1 %T. Keep working samples in the reliable A = 0.1–1.0 window; if a reading sits above A = 1, dilute and re-measure, since the relationship is logarithmic and high absorbances amplify small transmittance errors.