How to use (3 steps)
- Pick the mode: simple A = ε·l·c or the calibration curve regression.
- Fill the known values. Choose one unknown in the simple mode, or enter 2+ standards for the calibration.
- Tap Compute to see the solved value, transmittance, regression line, residuals, and steps. Copy URL shares the exact setup.
The example is preloaded and calculated automatically so you can see the output at a glance.
Inputs
Keep units consistent so A = ε·l·c holds. The unknown field is disabled because it is solved automatically.
Standard solutions (concentration and absorbance)
| Standard # | Concentration | Absorbance | Remove |
|---|---|---|---|
| 1 | |||
| 2 | |||
| 3 | |||
| 4 | |||
| 5 |
Results
How it's calculated
Beer-Lambert law for calibration curves and lab reports
This page fits the common workflow of estimating concentration from absorbance, checking molar absorptivity assumptions, and documenting a calibration curve for a lab report. Keep the instrument blank, path length, and concentration units fixed before you compare any fitted values.
Concentration from absorbance
Use simple mode when ε and path length are already known for your wavelength and solvent. Use calibration mode when you built standards experimentally and need the unknown concentration from the fitted line instead of from a literature ε value.
Molar absorptivity and blank correction
If you are solving for ε, report the wavelength, solvent, path length, and concentration units together. In calibration mode, blank subtraction matters just as much as R²: a drifting blank or baseline offset can create a plausible-looking line with a chemically implausible intercept.
When to force the fit through the origin
Only force the line through the origin when your method justifies A = 0 at c = 0 after blank correction and the residuals stay well behaved. If the free-intercept fit shows a meaningful intercept, treat that as a clue to investigate sample prep, baseline correction, or standard quality before you write up the result.
What to include in a lab report
Most lab reports need the standards used, fitted equation, R², whether the intercept was fixed, the unknown absorbance, and the final concentration with units. Add a short note when the unknown falls outside the standard range or when the residual pattern suggests the linear range is being stretched.
Need the interpretation guide first?
Open the Beer-Lambert topic guide when the real question is blank correction, fit choice, calibration-curve reading, or what belongs in the lab report, not just the final number.
FAQ
Which units should I use?
Use any consistent units so that A = ε·l·c is valid. A typical set is ε in L·mol⁻¹·cm⁻¹, l in cm, and c in mol/L. This calculator does not convert units automatically.
How many standard solutions are enough?
Two points define a line, but to average out noise you usually measure 4–6 standards. Check R² and the residuals to ensure no outliers dominate the fit.
When should I force the line through the origin?
Ideally A is zero when c is zero, so the line crosses the origin. Instrument offsets or blanks can shift the intercept, so compare both models to see which matches your data better.
What should I check if the unknown concentration becomes negative?
Review the blank correction, concentration units, and whether the unknown absorbance sits below the reliable calibration range. It also helps to compare the free-intercept fit against the origin-forced fit before reporting the result.
Is R² enough to judge a calibration curve?
No. A high R² can still hide curvature or one influential outlier. Read the residuals, confirm that the standards bracket the unknown, and make sure the fitted intercept is chemically plausible for your setup.
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