McFarland Standard Calculator

Summary

Enter a McFarland standard value and instantly get the approximate bacterial cell concentration in CFU/mL.

How it works

1. Select or type a McFarland standard value (e.g., 0.5, 1, 2, 3, 4, or 0.5–10).
2. The calculator looks up the CLSI-referenced CFU/mL range for that standard.
3. The midpoint concentration and optical density at 625 nm (OD625) are displayed.
4. Use the result to dilute or concentrate your bacterial suspension accordingly.
5. Validate visually by comparing your suspension turbidity to a printed McFarland standard card.

Use cases

• Preparing bacterial inocula for disk diffusion (Kirby-Bauer) susceptibility testing.
• Standardizing cultures before MIC (minimum inhibitory concentration) assays.
• Teaching microbiology students how turbidity relates to cell density.
• Checking suspension density before inoculating growth media.
• Converting spectrophotometric OD readings to estimated CFU/mL.
• Quality control for clinical microbiology laboratories.
• Preparing bacterial suspensions for biofilm experiments.
• Estimating cell counts without a hemocytometer or flow cytometer.

Frequently Asked Questions