# Absorbance↔Transmittance Converter (A = −log₁₀T) Convert A ↔ %T ↔ T with A = −log₁₀(T) and %T = 100·T; returns all three representations for spectrophotometry work. > Canonical page: https://elysiatools.com/en/tools/absorbance-transmittance-converter - **Category:** Education - **Keywords:** absorbance, transmittance, optical density, spectrophotometry, beer lambert law, absorbance calculator, percent transmittance, od600, uv vis, spectroscopy converter ## Overview The Absorbance↔Transmittance Converter quickly switches between absorbance (A), percent transmittance (%T), and fractional transmittance (T) using the standard relation A = −log₁₀(T). Designed for spectrophotometry and optical density workflows, it outputs all three corresponding values simultaneously with customizable decimal precision. ## Inputs - **Value** (number): Value of the selected input quantity. - **Input Quantity** (select): Whether the entered value is absorbance (A), percent transmittance (%T), or the 0–1 transmittance fraction (T). - **Decimal Places** (number) ## When to use - Converting instrument readings from percent transmittance (%T) into absorbance (A) for Beer-Lambert law calculations. - Evaluating how much light actually reaches the detector from measured optical density or absorbance values. - Checking neutral-density filters or spectrophotometer baseline calibration standards. ## How it works - Enter the numerical measurement into the Value field. - Select the input format: Absorbance (A), Transmittance (%T), or Transmittance (T, 0–1). - Specify the desired rounding precision using the Decimal Places option. - Receive a JSON result displaying the synchronized absorbance, fractional transmittance, and percent transmittance values. ## Use cases - Microbiology cell density measurements (OD600 to transmitted light ratio). - Analytical chemistry quantitative analysis using UV-Vis spectrophotometers. - Optical filter transmission verification and attenuation factor analysis. ## Frequently asked questions ### What mathematical formula is used for conversion? The tool uses A = −log₁₀(T), T = 10⁻ᴬ, and %T = 100 · T (or A = 2 − log₁₀(%T)). ### What does an absorbance value of 1.0 mean in terms of light transmission? An absorbance (A) of 1.0 corresponds to a transmittance fraction of 0.1, meaning exactly 10% of incident light passes through. ### Why does transmittance only accept values between 0 and 100%? In standard absorption spectroscopy, transmitted light cannot exceed incident light (100% or T = 1.0), and negative transmittance is physically impossible. ### Can I convert decimal fractional transmittance directly? Yes, choose 'Transmittance (T, 0–1)' as your input quantity to enter fractions such as 0.5 directly. ### How many decimal places can I configure? 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