Beer-Lambert Law Calculator: A = epsilon l c

How much light a solution absorbs, from its concentration and the length of the light path.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Choose which quantity you want to find.

Enter the other values in the units shown.

The calculator gives the answer with the formula, the substituted numbers, and the answer in other units.

Formulas used

  • A = \varepsilon l c
  • c = \frac{A}{\varepsilon l}

Worked example

With the default numbers in the calculator above:

  1. The formula, arranged to find the absorbance: absorbance = molar absorptivity x path length x concentration
  2. Put the values in (every value in its SI unit): absorbance = 15000 x 1 x 5.0e-5
  3. The answer: absorbance: 0.75

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: What do you want to find? Concentration; Absorbance (leave it as it is if this is what you are finding) 0.75; Molar absorptivity (leave it as it is if this is what you are finding) 15000 L/mol cm; Path length (leave it as it is if this is what you are finding) 1 cm.

  1. The formula, arranged to find the concentration: concentration = (absorbance) / (molar absorptivity x path length)
  2. Put the values in (every value in its SI unit): concentration = (0.75) / (15000 x 1)
  3. The answer: concentration: 5.0e-5 mol/L
  4. The same answer in other units: 0.05 mmol/L; 50 umol/L

Common questions

What is the Beer-Lambert law?

The absorbance of a solution is proportional to the concentration of the absorbing substance and to the length of the light path through it.

How do I find the transmittance from the absorbance?

Transmittance = 10 to the power of minus A. An absorbance of 0.75 means 17.8 percent of the light gets through.

Why does the law fail at high concentration?

Molecules interact with each other and stray light in the instrument becomes important, so the graph of absorbance against concentration curves.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Any chemistry text (spectrophotometry, the Beer-Lambert law).

Need a calculator we do not have?

Tell us what you would like to calculate. We read every request and build the most asked-for ones.

We store your request (and your email if you give it) to decide what to build. We do not store the numbers you type into calculators.

Embed this calculator on your own page

Copy this code into your page. The calculator runs inside the frame and links back to Drafting Central.

Need this drawn, designed or checked?

Drafting Central is a mechanical engineering and drafting business working with customers across Australia and around the world. A calculator gives you the numbers; we can turn them into drawings and designs.

Need help with a real project?

Talk to Drafting Central →

← All engineering calculators