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.

What is the Absorbance ↔ Transmittance / % Transmittance Converter?

Apply A = −log₁₀(T) to interconvert absorbance, transmittance (0–1), and percent transmittance from any spectrophotometer reading. Edit any one field and the others update instantly, with edge-case handling for zero and saturated readings. For analytical and clinical chemists. It runs free in your browser on Gera Tools, with nothing uploaded.

Absorbance ↔ Transmittance / % Transmittance Converter

Name: Absorbance ↔ Transmittance / % Transmittance Converter
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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.

Reference conversion table

The most useful anchor points for quick mental checks:

Absorbance (A)	Transmittance (T)	%Transmittance (%T)
0.000	1.000	100 % (no absorption)
0.097	0.800	80 %
0.155	0.700	70 %
0.222	0.600	60 %
0.301	0.500	50 %
0.398	0.400	40 %
0.523	0.300	30 %
0.699	0.200	20 %
1.000	0.100	10 %
2.000	0.010	1 %

The convenient anchors to memorise: A = 0.301 at 50 %T (T = 0.5), A = 1 at 10 %T, and A = 2 at 1 %T.

Why the reliable working range matters

Most single-beam spectrophotometers produce reliable readings roughly in the A = 0.1 to 1.0 range (about 10–80 %T). Below A = 0.1 the signal difference between sample and blank is small and noise dominates. Above A = 1.0 only a tenth of the incident light reaches the detector, so tiny stray-light contributions become a significant fraction of the measured signal and readings deviate from linearity (Beer-Lambert law breaks down). For quantitative work, dilute concentrated samples until the reading falls comfortably in this window.

Absorbance versus optical density

For clear absorbing solutions, absorbance (A) and optical density (OD) are used interchangeably — both follow A = −log₁₀(T). In microbiology, OD at 600 nm (OD600) is the standard measure of bacterial culture density, where scattering rather than true absorption contributes significantly to the signal. This means OD600 does not strictly follow Beer-Lambert, but the same conversion formula is used for instrument readings regardless.

Example

A reading of 25 %T is T = 0.25 and A = −log₁₀(0.25) ≈ 0.602. If your method requires working at A < 0.5, dilute the sample roughly 3-fold to bring the absorbance down to around 0.2 and re-measure.