BioDeviceHub

McFarland Standard Calculator

Estimate the bacterial density of a suspension matched to a McFarland turbidity standard, and work out the dilution to reach a target inoculum such as 5 × 10⁵ CFU/mL.

Formula

DF=suspension densitytarget density\mathrm{DF} = \dfrac{\text{suspension density}}{\text{target density}}
0.5 McFarland≈1.5×108 CFU/mL(Gram-negative bacilli)0.5\ \text{McFarland} \approx 1.5\times 10^{8}\ \mathrm{CFU/mL}\quad(\text{Gram-negative bacilli})
McFarland number\text{McFarland number}
turbidity standard, made from barium sulfate
target density\text{target density}
the inoculum wanted, in CFU/mL

How it works

A McFarland standard is a suspension of barium sulfate whose turbidity matches that of a known density of bacteria. A bacterial suspension adjusted by eye or with a photometer to match the 0.5 standard is taken to contain about 1.5 × 10⁸ CFU/mL, which is then diluted to the working inoculum, for example 5 × 10⁵ CFU/mL for a broth microdilution test.

It is a convenient approximation. Turbidity depends on how much light cells scatter, which varies with the species, the cell size and shape and the instrument, so the CFU/mL for a given McFarland value is not constant. The figures here are for Gram-negative bacilli such as E. coli; for other organisms, or where an exact count matters, confirm by plate count.

Worked example

A 0.5 McFarland suspension diluted to 5 × 10⁵ CFU/mL, making 10 mL of inoculum.

  1. Dilution = 1.5 × 10⁸ / 5 × 10⁵ = 300.
  2. Suspension = 10 mL / 300 = 33.3 µL, with 9.967 mL of broth.

A 1:300 dilution: 33.3 µL of the 0.5 McFarland suspension in 9.967 mL of broth.

These are the values the calculator opens with, so you can check its output against this example.

Assumptions

  • The CFU/mL values are approximate for Gram-negative bacilli.
  • The suspension has been matched to the standard under the same lighting or on a calibrated photometer.

Common mistakes

  • Treating a McFarland value as an exact count.
  • Using the same conversion for yeasts, Gram-positive cocci or large organisms.
  • Letting the suspension stand before use, so that the cells settle or grow.

Related tools

OD600 to Cell Density CalculatorEstimate bacterial cell density from optical densityOpen calculatorCFU Calculator (Colony-Forming Units per mL)Viable count from a plate count and dilutionOpen calculatorColony Count Calculator (CFU/mL)CFU/mL from plate counts at several dilutions, weighted over twoOpen calculatorSerial Dilution CalculatorTransfer volumes and the concentration in every tubeOpen calculator

Related equipment

Service documentation, failure modes and parts for the instruments this calculation is used with.