What CFU/mL is a 0.5 McFarland standard?

About 1.5 × 10^8 CFU/mL for typical bacteria such as E. coli, which is the value used with CLSI disk diffusion and dilution methods. The WHO manual rounds it to 1 × 10^8 cells/mL. It is a turbidity equivalence, not a guaranteed viable count; organism size and clumping change it.

How do you prepare a 0.5 McFarland standard?

Add 0.5 mL of 0.048 M BaCl₂ (1.175% w/v BaCl₂·2H₂O) to 99.5 mL of 0.18 M H₂SO₄ (1% v/v) with constant stirring, or 0.05 mL to 9.95 mL for a single tube. Check the absorbance at 625 nm (0.08–0.13 in a 1 cm cuvette), dispense into tubes like those used for the inoculum, seal and store in the dark at room temperature, and vortex before each use.

How do you make McFarland 1, 2, 3 or 4?

Use 0.1, 0.2, 0.3 or 0.4 mL of 1% BaCl₂ and make up to 10 mL with 1% H₂SO₄ (9.9, 9.8, 9.7 or 9.6 mL). In general, BaCl₂ volume = McFarland number × 1% of the total volume.

How do I dilute a 0.5 McFarland suspension to 5 × 10^5 CFU/mL?

From 1.5 × 10^8 CFU/mL, 5 × 10^5 CFU/mL is a 300-fold total dilution. For 10 mL, that is 33.3 µL of suspension plus 9.967 mL of broth; in practice labs make it in steps (for example 1:150 in broth, then 1:2 when the inoculum meets an equal volume of drug in the well). Follow your CLSI M07 or EUCAST procedure and confirm by colony count.

What OD600 is a 0.5 McFarland?

A barium sulfate 0.5 standard reads roughly 0.08–0.10 at 600 nm and 0.08–0.13 at 625 nm in a 1 cm cuvette. Latex standards read lower (about 0.06 at 600 nm). A bacterial suspension matched visually to the standard may give a different OD on your instrument, so build a calibration curve for quantitative work.

Can McFarland be converted exactly to CFU/mL?

No. Turbidity responds to particle size, shape, clumping and instrument geometry, whereas CFU/mL counts viable colony-forming units. Yeasts are larger: a 0.5 McFarland Candida suspension is roughly 1–5 × 10^6 CFU/mL. Use organism-specific calibration and plate counts when accuracy matters.

McFarland Standard Calculator

Pick a McFarland standard to get its approximate bacterial density (0.5 ≈ 1.5 × 10^8 CFU/mL), the barium sulfate recipe scaled to your volume, the absorbance check, and the dilution needed to reach a target CFU/mL.

A 0.5 McFarland standard corresponds to about 1.5 × 10^8 CFU/mL (each whole unit adds about 3 × 10^8).

McFarland Value

Standard (0.5 or a whole number 1–10)

Common standards

Prepare this standard

1.175% BaCl₂·2H₂O (0.048 M, = 1% anhydrous BaCl₂) into 1% v/v H₂SO₄ (0.18 M), stirring constantly.

Total volume (mL)

BaCl₂ 1.175%

H₂SO₄ 1%

Enter a McFarland value to see results

Dilute to a target inoculum

Starts from the selected standard's nominal density. 5 × 10^5 CFU/mL is the broth microdilution target (CLSI M07).

Target CFU/mL

Final volume (mL)

McFarland Standard Chart

Per 10 mL: 1% BaCl₂ + 1% H₂SO₄. A600 = classic barium sulfate values; latex standards read lower.

Std	CFU/mL	BaCl₂ mL	H₂SO₄ mL	A600
0.5	1.5 × 10⁸	0.05	9.95	0.08–0.10
1	3.0 × 10⁸	0.10	9.90	0.257
2	6.0 × 10⁸	0.20	9.80	0.451
3	9.0 × 10⁸	0.30	9.70	0.582
4	1.2 × 10⁹	0.40	9.60	0.669
5	1.5 × 10⁹	0.50	9.50	—
6	1.8 × 10⁹	0.60	9.40	—
7	2.1 × 10⁹	0.70	9.30	—
8	2.4 × 10⁹	0.80	9.20	—
9	2.7 × 10⁹	0.90	9.10	—
10	3.0 × 10⁹	1.00	9.00	—

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Quick answer

A 0.5 McFarland standard corresponds to about 1.5 × 10^8 CFU/mL (each whole unit adds about 3 × 10^8). It is made by adding 0.05 mL of 1.175% BaCl₂·2H₂O (0.048 M) to 9.95 mL of 1% H₂SO₄ (0.18 M), or 0.5 mL to 99.5 mL per CLSI, and should read 0.08–0.13 absorbance at 625 nm in a 1 cm cuvette. Turbidity is not a viable count, so confirm by plating.

Examples

McFarland	Approx. CFU/mL	BaCl₂ + H₂SO₄ per 10 mL	A600 (BaSO₄)
0.5	1.5 × 10^8	0.05 + 9.95 mL	0.08–0.10
1	3.0 × 10^8	0.1 + 9.9 mL	0.257
2	6.0 × 10^8	0.2 + 9.8 mL	0.451
3	9.0 × 10^8	0.3 + 9.7 mL	0.582
4	1.2 × 10^9	0.4 + 9.6 mL	0.669

Pick a standard to see CFU/mL, scale the recipe to your tube volume and work out the dilution to your target inoculum.

Summary

McFarland standards are barium sulfate (or latex) turbidity references used to standardize bacterial suspensions for susceptibility testing. This calculator gives the conventional approximate density for standards 0.5 and 1–10 (0.5 ≈ 1.5 × 10^8 CFU/mL; the WHO manual rounds it to 1 × 10^8), scales the 1.175% BaCl₂·2H₂O plus 1% H₂SO₄ recipe to any volume, shows the CLSI 625 nm absorbance check for the 0.5 standard and the classic 600 nm absorbance table, and calculates the dilution from a McFarland suspension to a target CFU/mL. Turbidity is not a viable count: species, cell size and instrument change the relationship, so confirm quantitative work by plate count.

How it works

Choose 0.5 or a whole-number standard from 1 through 10 (or click a row in the table).
Read the approximate density: 0.5 ≈ 1.5 × 10^8 CFU/mL and each whole McFarland unit ≈ 3 × 10^8 CFU/mL.
To prepare a standard, enter the total volume: BaCl₂ volume = volume × McFarland × 1%, the rest is 1% H₂SO₄.
To dilute, enter the target CFU/mL and final volume: dilution factor = McFarland density ÷ target, and the tool gives suspension and diluent volumes.
Verify a prepared 0.5 standard at 625 nm (0.08–0.13, 1 cm path) and confirm inoculum counts by plating.

Use cases

Preparing a 0.5 McFarland inoculum for Kirby-Bauer disk diffusion.
Diluting a 0.5 McFarland suspension to 5 × 10^5 CFU/mL for broth microdilution MIC testing.
Making barium sulfate McFarland standards in 10 mL tubes or a 100 mL batch.
Checking a standard with a spectrophotometer at 625 nm or 600 nm.
Teaching microbiology students how turbidity relates to cell density.
Distinguishing a McFarland turbidity target from OD600 and viable CFU/mL.

Frequently Asked Questions

Last updated: 2026-10-10 · Reviewed by Nham Vu