There is actually no connection between the MIC value and the EC50 (or ECx for that matter). Anyhow if you determined the MIC values yourself, you probably recorded the dose response (i.e. a growth parameter in function of concentration). From that curve you can calculate the EC50/ECx by fitting the appropriate curve, most often a Gompertz curve or a sigmoidal with variable slope will do fine. Software you can use: SPSS, Graphpad Prism, Statistica, even excel if you can program the regression yourself.

Or maybe, you are searching LC50.. Thats is possible to estimate from MIC

There is no relation between MIC or EC50 you can calculate LC50,

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DOn't forget that calculating LCx or ECx from a MIC or MAC value, without fitting the complete dose response are merely estimations. If you want to know accurately, you need the dose response curve.

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