Beer Lambert Law Calculator

Calculator Inputs

Sample Name

Wavelength (nm)

Absorbance (A)

Molar Absorptivity (ε)

Path Length (l)

Concentration (c)

Transmittance (T)

Percent Transmittance (%T)

Concentration Unit

Path Unit

ε Unit

Enter any three core values among absorbance, molar absorptivity, path length, and concentration. You may also enter transmittance or percent transmittance to derive absorbance.

Beer-Lambert Plot

The chart shows how absorbance changes with concentration using the current ε and path length values.

Example Data Table

Sample	Wavelength (nm)	ε (L·mol⁻¹·cm⁻¹)	l (cm)	c (mol/L)	Predicted A	%T
Dye Solution 1	500	12500	1.0	0.000020	0.25	56.23
Dye Solution 2	500	12500	1.0	0.000040	0.50	31.62
Dye Solution 3	500	12500	1.0	0.000060	0.75	17.78
Dye Solution 4	500	12500	1.0	0.000080	1.00	10.00

Formula Used

Primary relation: A = εlc

A is absorbance, ε is molar absorptivity, l is optical path length, and c is concentration.

Transmittance relation: A = -log₁₀(T)

Percent transmittance: %T = T × 100

Rearranged forms:
ε = A / (lc)
l = A / (εc)
c = A / (εl)

How to Use This Calculator

Enter the sample name and wavelength if available.
Provide any three core Beer-Lambert variables.
Optionally enter transmittance or percent transmittance.
Click calculate to display the result above the form.
Review the table, derived values, and concentration plot.
Export the result using the CSV or PDF buttons.

Frequently Asked Questions

1. What does the Beer-Lambert law measure?

It relates light absorption to concentration, path length, and molar absorptivity. It is widely used in spectrophotometry for solution analysis.

2. Can I calculate concentration with this tool?

Yes. Enter absorbance, molar absorptivity, and path length. The calculator rearranges A = εlc to solve concentration.

3. What is a good path length value?

A standard cuvette often uses 1 cm. Some specialized instruments use shorter or longer optical paths depending on sample intensity.

4. What is molar absorptivity?

Molar absorptivity describes how strongly a substance absorbs light at a specific wavelength. It depends on the compound and measurement wavelength.

5. Can I use transmittance instead of absorbance?

Yes. If you provide transmittance or percent transmittance, the tool converts it into absorbance using A = -log10(T).

6. Why is wavelength important?

Absorption depends strongly on wavelength. For reliable results, use the wavelength associated with the compound’s calibration or peak absorbance.

7. Why do my results seem unrealistic?

Check units, decimal placement, path length, and molar absorptivity. Also confirm that transmittance is entered as either decimal T or percent %T correctly.

8. Can this calculator help with calibration curves?

Yes. The chart visualizes absorbance versus concentration, which helps when reviewing expected linear behavior for spectrophotometric calibration work.