Im assuming this is for experimental data derived from some test using spectrophotometric detection like ORAC.

To calculate IC50 for antioxidants divide spectrophotometric data of your tested supstance with a control sample containing everithing other then your test substance. Also background should be measuerd and results should be corrected accordingly. Measure multiple concentrations of your test substance, plot a diagram and see where your LDR is. Pick data that is in the LDR (input data in Excel for this) and use FORECAST command. x=50, y (concentrations of tested substance), x (absorbance). To do it manually, plot a diagram, derive an equasion of LDR (a slope and y-axis crosspoint). Use y=50 to get your x (concentration that inhibits 50%)

Dear Wijdan Al-Tamimi ,

Please see my comments on this link https://www.researchgate.net/post/How_can_I_calculate_IC50_value_from_percent_inhibition_graph_for_antioxidant_activity4

Regards,

Egbuna