Dilution factors without the mix-ups
Keep the measured solution and original solution separate in your calculation.
Define which concentration you need
On this site, the dilution factor D is original concentration divided by measured-solution concentration. D = 1 means no dilution. After calculating the concentration of the solution that was measured, multiply by D to estimate the original concentration.
Make the volume statement explicit
If 1.00 mL of a solution is diluted to a final volume of 10.00 mL, D = 10.00 / 1.00 = 10, assuming the analyte amount is conserved. “Add 10 mL” describes a different preparation from “make up to 10 mL”. Use the actual final volume.
For sequential dilutions, multiply the factors: a 5-fold dilution followed by a 2-fold dilution has total D = 10.
A worked result
With illustrative ε = 20,000 L·mol⁻¹·cm⁻¹ and l = 1 cm, a measured A = 0.5 gives 25 µM in the measured solution. If D = 10, the original concentration is 250 µM. The measured solution remains 25 µM.
When predicting absorbance, enter the concentration of the solution in the cell. Do not multiply that predicted absorbance by D. Check units, blank fields and the direction of the dilution before interpreting a result.
Sources & further reading
Original educational material. Content review records a source and consistency check; it is not specialist peer review or laboratory method validation.