What is a McFarland standard?

A McFarland standard is a turbidity reference made from barium chloride and sulfuric acid. The resulting barium sulfate precipitate scatters light to a degree that correlates with known bacterial cell densities, providing a quick visual or spectrophotometric benchmark for suspension density.

What CFU/mL does a 0.5 McFarland standard equal?

A commonly used nominal value is approximately 1.5 × 10^8 bacterial cells/mL, while some public-health manuals round it to 1 × 10^8. It is a turbidity equivalence, not a guaranteed viable count; organism and method matter.

Can McFarland be converted exactly to CFU/mL?

No. Turbidity responds to particle size, shape, clumping and instrumentation, whereas CFU/mL counts viable colony-forming units. Use organism-specific calibration and plate counts when accuracy matters.

OD625 is absorbance measured at 625 nm. CDC guidance states that a prepared 0.5 McFarland reference standard should read 0.08–0.10 at 625 nm using a 1 cm light path. That quality-control interval must not be extrapolated as a universal OD table.

Can OD600 be converted to McFarland?

Not with one universal factor. OD600 and OD625 differ by wavelength, and readings also depend on cuvette path length and instrument geometry. Build or use a calibration curve for the organism and instrument in your protocol.

McFarland Standard Calculator

Look up the nominal bacterial density for McFarland 0.5–10 and see why OD600, OD625 and viable CFU/mL are not interchangeable.

McFarland Value

Standard (0.5 or a whole number 1–10)

Common standards

Enter a McFarland value to see results

McFarland Reference Table

Conventional nominal bacterial density; only 0.5 has the OD625 QC interval shown here

Standard	CFU/mL (approx.)	OD625

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Summary

Look up the nominal bacterial density for McFarland 0.5–10 and see why OD600, OD625 and viable CFU/mL are not interchangeable.

How it works

Choose 0.5 or a whole-number standard from 1 through 10.
The tool retrieves the conventional nominal bacterial-density value; it does not interpolate invented standards.
For McFarland 0.5 only, it also shows the CDC absorbance check for a prepared reference standard at 625 nm and a 1 cm light path.
Treat the density as an approximation, then follow the organism, instrument and susceptibility-test protocol used by your laboratory.
Use a viable plate count when an actual CFU/mL measurement is required.

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.
Distinguishing a McFarland turbidity target from OD600 and viable 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

Last updated: 2026-09-22 · Reviewed by Nham Vu