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Beer-Lambert law calculator

Solve the Beer-Lambert law A = ε·c·l for any variable — absorbance, molar absorptivity, concentration or path length. Central to spectrophotometry, it links how much light a solution absorbs to how concentrated it is. Pick what to solve for, enter the rest, and read the answer.

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The Beer-Lambert law calculator turns Solve for, Absorbance A (if known), Molar absorptivity ε (M⁻¹cm⁻¹), Concentration c, Concentration unit, Path length l (cm) into Result, Unit of result, instantly and for free. For instance, with Solve for = Absorbance (A), Absorbance A (if known) = 0.622, Molar absorptivity ε (M⁻¹cm⁻¹) = 6,220, Concentration c = 100, Concentration unit = M and Path length l (cm) = 1 it returns Result = 622,000 and Unit of result = AU.

How to use it

  1. Enter your values: Solve for, Absorbance A (if known), Molar absorptivity ε (M⁻¹cm⁻¹), Concentration c, Concentration unit, Path length l (cm).
  2. Read the result instantly: Result, Unit of result.

Frequently asked questions

How does the Beer-Lambert law calculator work?

It takes Solve for, Absorbance A (if known), Molar absorptivity ε (M⁻¹cm⁻¹), Concentration c, Concentration unit and Path length l (cm) and derives Result and Unit of result from them. The calculation is live as you type, so the result updates on every change.

Which values does the calculator ask for?

6 values: Solve for, Absorbance A (if known), Molar absorptivity ε (M⁻¹cm⁻¹), Concentration c, Concentration unit and Path length l (cm). Nothing else is required — no account, no file upload.

What does a typical calculation look like?

With Solve for = Absorbance (A), Absorbance A (if known) = 0.622, Molar absorptivity ε (M⁻¹cm⁻¹) = 6,220, Concentration c = 100, Concentration unit = M and Path length l (cm) = 1, the calculator returns Result = 622,000 and Unit of result = AU. Those figures come from running this exact tool, so you can reproduce them by entering the same values.

How much does the result change with different inputs?

It moves a lot. Using Solve for = Concentration (c), Absorbance A (if known) = 1.244, Molar absorptivity ε (M⁻¹cm⁻¹) = 12,440, Concentration c = 200, Concentration unit = mM and Path length l (cm) = 2 instead, Result goes from 622,000 to 0.05 — which is why it is worth testing a few scenarios rather than trusting a single figure.

Which “Solve for” option should I choose?

You can pick between « Absorbance (A) », « Concentration (c) », « Molar absorptivity (ε) » and « Path length (l) ». Each one changes what the calculator works out, so switch and compare — the default is « Absorbance (A) ».

What does it give for smaller values?

Scaled down to Solve for = Absorbance (A), Absorbance A (if known) = 0.311, Molar absorptivity ε (M⁻¹cm⁻¹) = 3,110, Concentration c = 50, Concentration unit = M and Path length l (cm) = 0.5, Result comes out at 77,750. The relationship is worth checking at both ends before you rely on a single result.

When would I actually use this?

Preparing a solution at a stated concentration, working out how much of a reagent a reaction needs, and converting between mass, moles and volume.

What is the most common mistake?

Confusing molarity with molality. One is per litre of solution and the other per kilogram of solvent; they diverge as concentration rises, and only one of them is temperature-independent.

What is the difference between the Beer-Lambert law calculator and the Ideal gas law calculator?

This one returns Result and Unit of result; the Ideal gas law calculator returns Pressure (atm). That is the whole difference — open the one whose figure you need.

Is there a tool for the next step?

Molality calculator is the closest one after this: Compute molality — moles of solute per kilogram of solvent — the concentration unit that, unlike molarity, does not change with temperature. Enter moles directly or a mass with its molar mass, plus the solvent mass, and it returns the molality in mol/kg.

Further reading

All guides →
ExplainerAn Absorbance of Two Means One Per Cent of the Light Gets Through6,220 M⁻¹cm⁻¹ at 100 µM through 1 cm gives A = 0.622, and 23.9 % of the light emerges. Double the path length and A doubles to 1.244 — the scale is logarithmic, and that is where it breaks.ExplainerOsmotic Pressure Counts Particles, Not Molecules0.15 M of a salt that splits in two and 0.30 M of one that does not give the identical 7.64 atmospheres. Only the particle count reaches the membrane.ExplainerSalting Pasta Water Raises the Boiling Point by Two Tenths of a DegreeFifty grams of salt in five litres is 0.171 mol/kg and buys +0.175 °C. Reaching a full degree would take 285 grams — most of a box.ExplainerThe Ideal Gas Law Explained: PV = nRT, R in Every Unit, and Where It BreaksPV = nRT holds when the gas is dilute and far from condensing. The value of R depends entirely on the units you feed it, and the temperature is never in degrees Celsius.ExplainerThe Same Burette Reading Gives Two Different Concentrations24.80 mL of 0.1 M titrant into 25.00 mL of analyte gives 0.0992 M — or 0.0496 M if the analyte releases two protons. The stoichiometry is typed, not measured.ExplainerWhy Salt Beats Sugar on an Icy PathOne mole per kilogram of a salt that splits in two lowers water's freezing point by 3.72 °C. The same amount of something that stays whole manages 1.86 °C — exactly half.