Transmittance and Absorbance Calculator

Convert between transmittance T and absorbance A using A = -log(T). Calculate percent transmittance from absorbance or find absorbance from measured transmittance readings.

🔬 Spectroscopy📐 A = -log10(T/100)🧪 Chemistry

Absorbance A

OR Transmittance %T

Please enter valid values.

Formula & Reference

Variable	Symbol	Formula	Units
Transmittance and Absorbance Calculator	—	A = -log10(T/100)	dimensionless

Step-by-Step Examples

Example 1

A to T

Convert A = 0.500 to %T.

T = 10^(-0.500) x 100%
T = 0.3162 x 100% = 31.62%

✓ T = 31.62%

Example 2

T to A

Convert T = 50.0% to absorbance.

A = -log10(50.0/100) = -log10(0.500)
A = -(-0.301) = 0.301

✓ A = 0.301

Example 3

Near Zero Transmittance

A = 3.000. What %T?

T = 10^(-3.000) x 100% = 0.1%
Only 0.1% of light passes through

✓ T = 0.100% (essentially opaque sample

Real-World Applications

🔬

UV-Vis Spectroscopy

Spectrophotometers report A or %T; conversion needed depending on software.

📊

Calibration

Calibration curves plot A (more linear) vs c rather than %T.

💊

Clinical Analysis

Spectrophotometric clinical assays report results in both A and %T.

🧬

Colorimetry

Color comparison methods in food and water analysis use %T readings converted to A for Beer-Lambert.

Common Mistakes to Avoid

⚠️

A uses log base 10 not ln

A = -log10(T), not -ln(T). log10 gives the conventional Beer-Lambert absorbance.

⚠️

T as fraction vs percentage

T as fraction: A = -log10(T). T as percentage: A = -log10(%T/100). Always check which form your instrument uses.

⚠️

Negative A not meaningful

Absorbance should be >= 0. Negative A means sample transmits more than blank (subtract blank carefully).

Frequently Asked Questions

What is %T? ▾

Percent transmittance: %T = (I/I0) x 100. I = intensity through sample, I0 = intensity through blank (solvent only).

Why is absorbance preferred for Beer-Lambert? ▾

A is directly proportional to c and l (linear relationship). %T is logarithmically related. Linear relationships are easier to use for calibration curves.

What does A = 1 mean physically? ▾

A = 1 means T = 10%. Only 10% of light passes through; 90% is absorbed. A = 2: 1% passes. A = 3: 0.1% passes.

What is optical density? ▾

OD is synonymous with absorbance A in biology. OD600 = absorbance at 600 nm, used to measure bacterial cell density.

What is the dynamic range of UV-Vis? ▾

Typically A = 0.001 to 2.5 (modern instruments). Below 0.001: noise dominates. Above 2.5: nonlinear detector response.

Related Chemistry Calculators

Formula Explorer connections

Interpretation: This analytical relationship converts an instrument signal, separation measure or optical response into concentration, identity or performance. Assumption: Calibration, blank correction, linear range, path length, matrix effects and instrument settings must match the sample and method.

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