Use this Optical Density (OD) Calculator to convert between intensities (I0 and I), transmittance (T), percent transmittance (%T), and optical density/absorbance (OD or A). You can also optionally use Beer-Lambert law to solve for concentration, path length, or molar absorptivity.
Advanced options
How to use our Optical Density (OD) Calculator
- Choose what you want to start from (I0 and I, T, %T, or OD/A).
- If you choose intensities, enter incident intensity I0 and transmitted intensity I in the same units (for example, two meter readings).
- If you choose transmittance, enter T as a fraction from 0 to 1 (example: 0.25).
- If you choose percent transmittance, enter %T from 0 to 100 (example: 25).
- If you choose OD/A, enter optical density (OD) or absorbance (A) using base-10 logs (example: 0.60).
- Open Advanced options only if you want Beer-Lambert calculations or want to change number formatting.
- To use Beer-Lambert, turn it on, pick what to solve for, and enter the other two values (epsilon, path length, concentration) with the units shown.
- Click Calculate.
- Read the results: T, %T, OD, absorbance A, and the intensity ratio I0/I are all consistent conversions of the same measurement.
- If you see an error, change the input it points to (common fixes: I0 must be greater than 0, and log inputs must be greater than 0).
Definitions
Incident intensity (I0): The light intensity before the sample (or the detector reading with a blank/reference).
Transmitted intensity (I): The light intensity after the sample.
Transmittance (T): The fraction of light that gets through, defined as T = I/I0 (usually 0 to 1 for a passive sample). [1]
Percent transmittance (%T): Transmittance written as a percent: %T = 100 times T. [4]
Optical density (OD): A base-10 (decadic) log measure of attenuation: OD = log10(I0/I) = -log10(T). Each +1 OD means 10x less transmitted intensity. [2]
Absorbance (A): In many lab/spectrophotometry contexts, absorbance A is the same number as OD (same base-10 definition). [1]
Beer-Lambert law: A linear model that connects absorbance to concentration: A = epsilon times l times c, for clear (non-scattering) samples in the linear range. [1]
Methodology
Step 1: Convert your chosen input into transmittance (T)
If you start from intensities:
T = I / I0
If you start from percent transmittance:
T = %T / 100
If you start from OD (or A):
T = 10^(-OD)
Step 2: Convert between T, %T, OD, absorbance, and intensity ratio
%T = 100 * T
OD = -log10(T) = log10(1/T)
A = OD
I0/I = 1/T = 10^(OD)
These are the standard base-10 (decadic) definitions used in absorbance and optical density conversions. [1] [4]
Beer-Lambert (optional)
When enabled, the calculator uses absorbance A equal to the computed OD value.
A = epsilon * l * c
c = A / (epsilon * l)
epsilon = A / (l * c)
l = A / (epsilon * c)
Beer-Lambert is most reliable for clear solutions where scattering is small and the instrument is in its linear range. [1]
Validation and edge-case rules
Blocked errors (no results): I0 must be greater than 0; any log10 input must be greater than 0 (so T must be greater than 0 and %T must be greater than 0). Also, epsilon, l, and c must be greater than 0 when they are used in a Beer-Lambert solve.
I = 0 in intensity mode: T becomes 0, and OD = log10(I0/I) is undefined (infinite). The calculator shows OD as N/A and explains why, while still showing T = 0 and %T = 0.
Warnings (results still compute): If T is greater than 1 (or %T greater than 100), the math gives a negative OD. That can happen from measurement baselines, reference mismatch, or gain, so the calculator flags it as potentially not physically meaningful for passive samples. [3]