Enter percent transmittance (%T) to calculate absorbance (A), or enter absorbance to determine %T. Absorbance scales linearly with concentration via Beer-Lambert law (A = εlc), making it the standard unit for quantitative spectrophotometry rather than %T.
Related Calculators
- Concentration from Absorbance Calculator
- Peak Area Calculator
- Mg/Ml To Mol/L Calculator
- M To Mg/Ml Calculator
- All Chemistry Calculators
Percent Transmittance to Absorbance Formula
The following formula converts percent transmittance to absorbance:
A = -log10(T / 100)
- A is the absorbance (unitless)
- T is the percent transmittance (%)
Dividing %T by 100 converts to fractional transmittance. The negative base-10 logarithm yields absorbance. The inverse formula recovers %T: T% = 100 x 10^(-A).
| Percent Transmittance (%) | Absorbance (A) |
|---|---|
| 0.1 | 3.000 |
| 0.2 | 2.699 |
| 0.5 | 2.301 |
| 1 | 2.000 |
| 2 | 1.699 |
| 3 | 1.523 |
| 5 | 1.301 |
| 10 | 1.000 |
| 20 | 0.699 |
| 30 | 0.523 |
| 40 | 0.398 |
| 50 | 0.301 |
| 60 | 0.222 |
| 70 | 0.155 |
| 80 | 0.097 |
| 90 | 0.046 |
| 95 | 0.022 |
| 99 | 0.004 |
| Formula: A = -log10(T/100). Absorbance is unitless. | |
Why Absorbance Over Percent Transmittance
Percent transmittance has an exponential relationship with concentration, producing nonlinear calibration curves. Absorbance is directly proportional to concentration via Beer-Lambert law (A = εlc, where ε is the molar absorptivity in L/mol/cm, l is path length in cm, and c is concentration in mol/L), enabling linear calibration. Each additional absorbance unit represents a 10-fold reduction in transmitted light: 1.0 A transmits 10%, 2.0 A transmits 1%, 3.0 A transmits 0.1%.
| Absorbance (A) | %T | Light Absorbed | Measurement Zone |
|---|---|---|---|
| 0.05 | 89.1% | 10.9% | Poor (noise dominated) |
| 0.1 | 79.4% | 20.6% | Acceptable lower limit |
| 0.2 | 63.1% | 36.9% | Optimal range start |
| 0.3 | 50.1% | 49.9% | Optimal |
| 0.5 | 31.6% | 68.4% | Optimal |
| 1.0 | 10.0% | 90.0% | Acceptable upper limit |
| 1.5 | 3.16% | 96.8% | Stray light interference begins |
| 2.0 | 1.00% | 99.0% | Poor (stray light dominant) |
| 3.0 | 0.10% | 99.9% | Beyond most instrument limits |
| Highlighted rows indicate the optimal working range (0.2 to 0.5 A) for UV-Vis spectrophotometry. | |||
Instrument Limits and Stray Light
The reliable upper absorbance limit of any spectrophotometer is set by its stray light specification, not its display range. Stray light reaches the detector without passing through the sample, creating a false floor on %T readings and causing measured absorbance to be lower than the true value (negative deviation from Beer-Lambert law). At high absorbance, the true transmitted signal approaches the stray light level, making the conversion from %T to absorbance unreliable.
| Stray Light (%T) | Max Reliable A | Typical Instrument Class |
|---|---|---|
| 1.0 %T | ~2.0 A | Basic educational spectrophotometer |
| 0.1 %T | ~3.0 A | Standard laboratory UV-Vis |
| 0.01 %T | ~4.0 A | Research-grade double-beam UV-Vis |
| 0.001 %T | ~5.0 A | High-performance UV-Vis |
| Above these limits, the %T-to-absorbance conversion is unreliable. Dilute samples to bring readings into the 0.2 to 0.5 A range. | ||
Common Analyte Wavelengths
Measurements are taken at the wavelength of maximum absorbance (lambda-max) to maximize sensitivity. The table below lists standard analytical wavelengths for common laboratory analytes where percent transmittance is regularly converted to absorbance for quantification.
| Analyte | Wavelength (nm) | Optimal A Range | Notes |
|---|---|---|---|
| dsDNA | 260 | 0.1 to 1.0 | A260/A280 ratio greater than 1.8 indicates pure DNA |
| Protein (aromatic residues) | 280 | 0.1 to 1.0 | Tryptophan and tyrosine absorbance |
| NADH / NADPH | 340 | 0.05 to 0.8 | Low molar absorptivity; may require concentrated samples |
| Hemoglobin (oxyhemoglobin) | 415, 541, 577 | 0.1 to 1.0 | Soret band at 415 nm has highest absorptivity |
| p-Nitrophenol | 405 | 0.1 to 1.0 | Common enzyme assay reporter at pH greater than 8 |
| Bradford assay (protein) | 595 | 0.1 to 0.7 | Coomassie brilliant blue G-250 dye complex |
| BCA assay (protein) | 562 | 0.05 to 0.6 | Preferred for detergent-containing samples |
| Molar absorptivity (epsilon) varies by analyte and wavelength. Use Beer-Lambert law (A = epsilon x l x c) to convert absorbance to concentration. | |||
What is Absorbance?
Absorbance (A) quantifies how much light a sample removes from a beam at a given wavelength. It is dimensionless and defined as the negative base-10 logarithm of transmittance: A = -log10(I/I0), where I0 is incident light intensity and I is transmitted intensity. Because absorbance is linear with concentration and path length (Beer-Lambert law), it is the standard output unit in quantitative UV-Vis, microplate reader, and flow cytometry measurements. Values above approximately 1.0 A begin to deviate from Beer-Lambert linearity due to stray light, molecular interactions at high concentration, and detector saturation.
How to Calculate Absorbance from Percent Transmittance
- Measure or obtain the percent transmittance (T) from the spectrophotometer readout.
- Apply the formula: A = -log10(T / 100).
- Verify the absorbance falls between 0.1 and 1.0 A. If the value exceeds 1.0 A, dilute the sample and re-measure to ensure Beer-Lambert linearity.
Example Problem:
Percent Transmittance (T) = 35%
A = -log10(35 / 100) = -log10(0.35) = 0.4559
At 0.456 A, this sample falls within the optimal 0.2 to 0.5 A range, indicating a reliable measurement with minimal stray light and detector noise influence.
