Can I solve for different variables?

Yes! This calculator supports reverse solving. You can solve for: Absorbance, Concentration, Molar absorption coefficient, Path length, Transmittance.

Beer-Lambert law

Solve beer-lambert law relationships quickly using core chemistry formulas.

Formula shown4 inputs definedUpdated Nov 2025By Automated Chemistry GeneratorFree, no sign-up

Quick Summary

Use this when you need:

Convert spectrophotometer absorbance readings into solution concentration
Verify that cuvette path length or ε tables give the expected absorbance
Relate percent transmittance specs to absorbance for instrument settings

You’ll need:Concentration, Molar absorption coefficient, Path length, Transmittance

You’ll get:Absorbance

How this is calculated

Formula:

A = ε * b * c and A = -log10(T)

In plain language:

Beer–Lambert states absorbance equals molar absorptivity ε times path length b times concentration c, and equivalently A is the negative log base 10 of transmittance T.

The formula is always visible so you can verify the math and explain your numbers to anyone who asks.

What these terms mean

Absorbance

Absorbance used in the calculation.

Concentration

Concentration used in the calculation.

Molar absorption coefficient

Molar absorption coefficient used in the calculation.

Path length

Path length used in the calculation.

Transmittance

Transmittance used in the calculation.

When to use this calculator

Convert spectrophotometer absorbance readings into solution concentration
Verify that cuvette path length or ε tables give the expected absorbance
Relate percent transmittance specs to absorbance for instrument settings

Last updated: November 19, 2025

Beer-Lambert law calculator explained

Spectrophotometer readings become actionable only when you can connect absorbance to concentration. This calculator exposes the Beer-Lambert relationships among {absorbance}, {conc}, {molar_abs_coeff}, {path_length}, and {transmittance}, so you can move between molar absorptivity data and instrument outputs instantly.

Use it to derive unknown concentrations from absorbance, confirm whether a cuvette path length is correct, or convert percent transmittance specifications into absorbance during method development.

How the conversion works

The Beer-Lambert expression is:

A = \\varepsilon b c

where $\\varepsilon$ is molar absorptivity (L mol $^{-1}$ cm $^{-1}$ ), $b$ is path length (cm), and $c$ is concentration (mol/L). Absorbance also links to transmittance $T$ :

A = -\\log_{10} T, \\qquad T = \\frac{I}{I_0}

The calculator keeps both forms synchronized so you can enter whichever data your instrument provides.

Units and conversions

Quantity	Symbol	Units	Notes
Absorbance	$A$	dimensionless	Valid for 0.1-1.5 A in most photometers.
Molar absorptivity	$\\varepsilon$	L mol $^{-1}$ cm $^{-1}$	Pull from literature or calibration curves.
Path length	$b$	cm	Standard cuvettes use 1.00 cm.
Concentration	$c$	mol/L	Can convert from mg/L using molar mass.
Transmittance	$T$	fraction or %	$T = 10^{-A}$ .

Worked examples

Determine concentration from absorbance
A dye has $\\varepsilon = 2.15 \\times 10^{4}\\ \\text{L}\\,\\text{mol}^{-1}\\,\\text{cm}^{-1}$ . You measure $A = 0.650$ in a 1.00 cm cuvette.

c = \\frac{A}{\\varepsilon b} = \\frac{0.650}{2.15 \\times 10^{4} \\times 1.00} = 3.02 \\times 10^{-5}\\ \\text{mol}\\,\\text{L}^{-1}

Convert to mass concentration by multiplying by molar mass if needed.

Translate transmittance to absorbance
An instrument spec lists $T = 35\\% = 0.35$ .

A = -\\log_{10}(0.35) = 0.456

Plug this $A$ back into the first equation to confirm whether the concentration falls inside the linear range.

Tips and pitfalls

Dilute highly absorbing samples to keep absorbance below about 1.5 for linearity.
Verify that $\\varepsilon$ applies to the wavelength and solvent you are using.
Always blank the instrument with the same solvent to remove baseline offsets.
If a cuvette is scratched or its path differs from 1.00 cm, update {path_length} so calculations stay accurate.

References and further reading

Frequently Asked Questions

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Calculator info

Version:: v1.0.0
Last reviewed:: Nov 15, 2025
Sensitivity:: low
References:: 1

Last updated: November 19, 2025

Beer-Lambert law calculator explained

Use it to derive unknown concentrations from absorbance, confirm whether a cuvette path length is correct, or convert percent transmittance specifications into absorbance during method development.

How the conversion works

The Beer-Lambert expression is:

A = \\varepsilon b c

where $\\varepsilon$ is molar absorptivity (L mol $^{-1}$ cm $^{-1}$ ), $b$ is path length (cm), and $c$ is concentration (mol/L). Absorbance also links to transmittance $T$ :

A = -\\log_{10} T, \\qquad T = \\frac{I}{I_0}

The calculator keeps both forms synchronized so you can enter whichever data your instrument provides.

Units and conversions

Quantity	Symbol	Units	Notes
Absorbance	$A$	dimensionless	Valid for 0.1-1.5 A in most photometers.
Molar absorptivity	$\\varepsilon$	L mol $^{-1}$ cm $^{-1}$	Pull from literature or calibration curves.
Path length	$b$	cm	Standard cuvettes use 1.00 cm.
Concentration	$c$	mol/L	Can convert from mg/L using molar mass.
Transmittance	$T$	fraction or %	$T = 10^{-A}$ .

Worked examples

Determine concentration from absorbance
A dye has $\\varepsilon = 2.15 \\times 10^{4}\\ \\text{L}\\,\\text{mol}^{-1}\\,\\text{cm}^{-1}$ . You measure $A = 0.650$ in a 1.00 cm cuvette.

c = \\frac{A}{\\varepsilon b} = \\frac{0.650}{2.15 \\times 10^{4} \\times 1.00} = 3.02 \\times 10^{-5}\\ \\text{mol}\\,\\text{L}^{-1}

Convert to mass concentration by multiplying by molar mass if needed.

Translate transmittance to absorbance
An instrument spec lists $T = 35\\% = 0.35$ .

A = -\\log_{10}(0.35) = 0.456

Plug this $A$ back into the first equation to confirm whether the concentration falls inside the linear range.

Tips and pitfalls

Dilute highly absorbing samples to keep absorbance below about 1.5 for linearity.
Verify that $\\varepsilon$ applies to the wavelength and solvent you are using.
Always blank the instrument with the same solvent to remove baseline offsets.
If a cuvette is scratched or its path differs from 1.00 cm, update {path_length} so calculations stay accurate.

Quick Summary

Calculator Tool

How this is calculated

What these terms mean

Absorbance

Concentration

Molar absorption coefficient

Path length

Transmittance

When to use this calculator

Beer-Lambert law calculator explained

How the conversion works

Units and conversions

Worked examples

Tips and pitfalls

References and further reading

Frequently Asked Questions

Related Calculators

pKa

Molarity

Mole

Neutralization

Normality

Reconstitution

Was this calculator helpful?

Found an error?

Calculator info

Beer-Lambert law calculator explained

How the conversion works

Units and conversions

Worked examples

Tips and pitfalls

References and further reading

Frequently Asked Questions

Related Calculators

pKa

Molarity

Mole

Neutralization

Normality

Reconstitution

Was this calculator helpful?

Found an error?

Calculator info