How do you find the calculation of percent inhibition and IC50 of bacteria and fungi tested?
If you use GraphPad Prism for your plots and/or statistical analysis, the calculation will be performed for you with ease. However, I simply use the Hill equation for my IC50 determinations. You could look it up
Percentage of inhibition can be calculated as (mean colony diameter on amended medium/mean colony diameter on unamended medium) x 100.
To get an approximation of IC50 you can generate a dose-response plot of growth on y-axis and concentration on x-axis, you should be able to see where growth rate is halved (relative to 0 ppm) and check x-axis for concentration.
More accurately, to generate actual numbers you can use probit analysis to smooth the curve and output exact IC50 (or IC90, etc). SAS has PROC PROBIT function that can be used for this. I believe R also has packages like 'ic50' and others that can also perform these calculations.
Rayan Gourlie has mentioned right formula for calculating the inhibition: percentage of inhibition can be calculated as (mean colony diameter on amended medium/mean colony diameter on unamended medium) x 100.
However, to easily determine it is better to use statistical software or programme for LC50 or LC 90
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