Micminimum Inhibitory Concentration Calculator
Compute micminimum inhibitory concentration using validated scientific equations. See step-by-step derivations, unit analysis, and reference values.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
MIC = Stock Concentration / (Dilution Factor ^ (Well Number - 1))
Where Stock Concentration is the initial antimicrobial concentration in mcg/mL, Dilution Factor is typically 2 for twofold dilutions, and Well Number is the position of the last well showing no visible microbial growth. The MIC represents the lowest concentration of antimicrobial that inhibits visible growth.
Worked Examples
Example 1: Standard Twofold Dilution MIC
Problem:You have a stock antibiotic solution at 256 mcg/mL and need to set up 8 wells with twofold dilutions in a 200 mcL format. Growth is inhibited starting at well 4. What is the MIC?
Solution:Well concentrations: 256, 128, 64, 32, 16, 8, 4, 2 mcg/mL\nWell 4 = 256 / 2^(4-1) = 256 / 8 = 32 mcg/mL\nMIC = 32 mcg/mL (lowest concentration inhibiting growth)
Result:MIC = 32 mcg/mL
Example 2: Extended Range MIC Determination
Problem:Starting concentration is 1024 mcg/mL with twofold dilutions across 10 wells. No growth is observed in wells 1-6, growth appears in well 7. What is the MIC?
Solution:Well 6 = 1024 / 2^(6-1) = 1024 / 32 = 32 mcg/mL\nWell 7 = 1024 / 2^(7-1) = 1024 / 64 = 16 mcg/mL\nMIC = well 6 concentration (last well with no growth) = 32 mcg/mL\nLog2(MIC) = log2(32) = 5
Result:MIC = 32 mcg/mL, Log2(MIC) = 5
Frequently Asked Questions
What is Minimum Inhibitory Concentration (MIC)?
The Minimum Inhibitory Concentration (MIC) is the lowest concentration of an antimicrobial agent that prevents visible growth of a microorganism after overnight incubation. It is the gold standard for determining the susceptibility of organisms to antimicrobials. MIC values are used by diagnostic laboratories to confirm resistance and to guide clinical treatment decisions. The MIC is typically expressed in micrograms per milliliter (mcg/mL) or milligrams per liter (mg/L). Lower MIC values indicate greater antimicrobial potency against the tested organism.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy