Absorbance (A) to Transmittance (T) Calculator
Enter absorbance (also called optical density, OD) to convert instantly to transmittance as both a fraction and percent.
Formula used: T = 10-A and %T = 100 × 10-A.
What this calculator does
This tool converts absorbance to transmittance using the standard spectrophotometry relationship. If you work with UV-Vis measurements, microbiology OD readings, water testing, or analytical chemistry, you often need to switch between these two ways of expressing light behavior through a sample.
Absorbance tells you how much light is absorbed. Transmittance tells you how much light passes through. They represent the same measurement from different angles.
Absorbance vs. Transmittance
Absorbance (A)
Absorbance is logarithmic. Larger absorbance values indicate stronger attenuation of light by the sample. In many instruments, absorbance is displayed directly because it behaves linearly with concentration under Beer-Lambert law conditions.
Transmittance (T)
Transmittance is the ratio of transmitted light intensity to incident light intensity: T = I / I0. It is usually shown as a decimal (0 to 1) or percentage (0% to 100%).
A = -log10(T)
T = 10-A
%T = 100 × 10-A
How to use the calculator
- Enter your absorbance value in the input box.
- Click Calculate.
- Read the transmittance as:
- Fractional transmittance (T)
- Percent transmittance (%T)
- Use Clear to reset the calculator.
Worked examples
Example 1: A = 1.00
T = 10-1.00 = 0.1, so %T = 10%. That means only 10% of incident light passes through the sample.
Example 2: A = 0.30
T = 10-0.30 ≈ 0.5012, so %T ≈ 50.12%. Roughly half the light is transmitted.
Quick reference table
| Absorbance (A) | Transmittance (T) | Percent Transmittance (%T) |
|---|---|---|
| 0.00 | 1.0000 | 100.00% |
| 0.30 | 0.5012 | 50.12% |
| 0.50 | 0.3162 | 31.62% |
| 1.00 | 0.1000 | 10.00% |
| 2.00 | 0.0100 | 1.00% |
Why this conversion matters in lab work
Different methods, instruments, and reports may ask for absorbance or transmittance. Being able to convert quickly helps with:
- Interpreting spectrophotometer output across different device settings
- Comparing legacy datasets where %T was recorded instead of A
- Preparing calibration curves and checking linearity ranges
- Teaching and documentation in chemistry and biology labs
Common mistakes to avoid
- Mixing logarithm bases: absorbance uses base-10 logarithms.
- Forgetting percent conversion: multiply T by 100 to get %T.
- Rounding too early: keep extra digits in intermediate steps for precision.
- Ignoring instrument limits: very high absorbance values can be less reliable experimentally.
FAQ
Can absorbance be negative?
In ideal conditions, absorbance is usually non-negative. Negative values can appear due to blanking issues, noise, or drift. The calculator will still compute the corresponding transmittance.
Is OD the same as absorbance?
In many practical contexts, yes. OD (optical density) is often used interchangeably with absorbance, especially in microbiology (for example OD600).
What if my transmittance is extremely small?
For large absorbance values, transmittance becomes very small and may be displayed in scientific notation. That is normal and mathematically correct.