Convert between McFarland turbidity standards and estimated bacterial cell concentration (CFU/mL) using standard or custom conversion factors. Use this tool when preparing inocula for antimicrobial susceptibility testing — a 0.5 McFarland standard corresponds to approximately 1.5 × 10⁸ CFU/mL for most bacteria. Accurate standardization is critical because over- or under-inoculation directly affects MIC and disk diffusion results, potentially leading to incorrect susceptible, intermediate, or resistant classifications.
About McFarland Standards
The McFarland turbidity standard is used in microbiology to standardize bacterial cell concentrations for antimicrobial susceptibility testing and other assays. A 0.5 McFarland standard corresponds to approximately 1.5 × 10⁸ CFU/mL for most bacteria. The relationship is linear: doubling the McFarland value doubles the cell concentration.
