Spectrophotometry Calculator

Work out spectrophotometry instantly with clear inputs, formula shown and shareable results.

About Spectrophotometry Calculator

Older instruments read percent transmittance, which must be logged before Beer-Lambert can be used: A = -log10(T). The concentration that follows is the diluted one actually in the cuvette, so multiplying by the dilution factor recovers the original sample. A 25% transmittance reading is absorbance 0.602, giving 4.0e-5 mol/L in the cell and 4.0e-4 mol/L in the undiluted sample at a tenfold dilution.

Formulas

Absorbance from transmittance

A = -log10(%T / 100)

Original concentration

c(original) = A x dilution factor / (epsilon x l)

Worked example

With the default values below, Spectrophotometry Calculator returns Concentration Before Dilution of 4.0137 × 10⁻⁴. Change any input above and every figure here is recalculated in your browser.

Inputs used

Percent transmittance read from the instrumentaccepts 0–100 %: 25 %
Dilution factor applied before readingminimum 0: 10
Molar absorptivityminimum 0 L/(mol cm): 15,000 L/(mol cm)
Cuvette path lengthminimum 0 cm: 1 cm

Results

Concentration Before Dilution: 4.0137 × 10⁻⁴ mol/L
Absorbance From The Transmittance Reading: 0.6021
Concentration In The Cuvette: 4.0137 × 10⁻⁵ mol/L
Light Absorbed: 75.00%

Step by step

1Values used
2Absorbance from transmittance
3Original concentration
4Concentration Before Dilution= 0.0004 mol/L
5Absorbance From The Transmittance Reading= 0.6021
6Concentration In The Cuvette= 0.0000 mol/L
7Light Absorbed= 75.00

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why not work in transmittance throughout?

Because transmittance is not proportional to concentration - it is exponential. Only absorbance gives a straight calibration line.

Where does the dilution factor come from?

It is the total volume divided by the sample volume: 1 mL made up to 10 mL is a factor of 10.