CFU GUIDE

CFU/mL vs CFU/g: formulas, units, and sample preparation

When to report CFU/mL and when to report CFU/g, how homogenization volume and sample mass enter the formula, and how to plan dilutions for solid samples.

The unit you report follows the basis of your sample: per millilitre for liquids, per gram for solids. The colony count itself never changes between the two — what changes is the preparation that produced the aliquot you diluted and plated, and the two extra numbers that preparation contributes to the formula.

Liquid samples: CFU/mL

For a well-mixed liquid, the standard formula applies directly:

CFU/mL = colonies ÷ (dilution fraction × plated volume)

Nothing extra is needed, because the sample you dilute is already the sample you want to report on. Every dilution in the chain is a deliberate, measured reduction of that liquid.

Solid samples: CFU/g

A solid cannot be diluted directly. It is first homogenized into a known volume of diluent, and that preparation — not the original solid — is what your pipette touches for the rest of the series:

CFU/g = colonies × homogenization volume ÷ (sample mass × dilution fraction × plated volume)

Two terms join the liquid formula:

• Sample mass (m) — the mass of the solid in grams, weighed before any diluent was added.
• Homogenization volume (H) — the final volume in millilitres that the mass is dispersed into, including the volume the sample itself contributes.

Worked example

A 25 g sample is homogenized in 225 mL of diluent. From that preparation, a 10⁻² dilution tube is made, and 0.1 mL of it is plated, yielding 64 colonies.

1. The homogenate has a total volume of 250 mL (225 mL diluent + the 25 g sample), so 1 mL of homogenate represents 25 ÷ 250 = 0.1 g of the original solid.
2. Mass represented on the plate = sample mass × dilution fraction × (plated volume ÷ homogenization volume) = 25 × 0.01 × (0.1 ÷ 250) = 10⁻⁴ g.
3. CFU/g = 64 ÷ 10⁻⁴ = 6.4 × 10⁵ CFU/g.

The calculator performs this when you select the solid sample type and enter the mass, homogenization volume, and plate data — the example is prefilled so you can inspect each substituted term.

Homogenization ratios, and the 1 : 9 trap

“1 g in 9 mL” and “10 g in 90 mL” are the same 1 : 9 ratio, and both give a 10⁻¹ preparation relative to the original solid. A frequent error is to treat the added diluent volume as the homogenization volume while forgetting the sample’s own contribution, or vice versa. Decide on a convention — most protocols use the total final volume — state it in your report, and be consistent. The calculation methods page lists the formula exactly as the calculator implements it.

Planning dilutions from a homogenate

The homogenate is already a dilution. Plan the tube series from the preparation, not from the original solid: if the homogenate represents 10⁻¹ of the sample and you then make three 1:10 steps, the fourth tube is 10⁻⁴ of the original — do not apply the homogenization step a second time. The serial dilution planner lays out the tube series once you know the factor per step, and its cumulative tracking keeps the exponents honest.

Liquid and solid, side by side

The two bases compared on equal terms. A liquid sample: 0.1 mL of a 10⁻³ dilution yields 86 colonies → 86 ÷ (0.001 × 0.1) = 8.6 × 10⁵ CFU/mL. A solid sample: 10 g into 90 mL diluent (a 10⁻¹ homogenate), then a 10⁻² tube, plated at 0.1 mL, yields 86 colonies. The mass on the plate is 10 × 0.01 × (0.1 ÷ 100) = 10⁻⁵ g, so CFU/g = 86 ÷ 10⁻⁵ = 8.6 × 10⁶. Same colony count, same plated volume — two orders of magnitude apart, entirely because of what sat behind the dilution chain. When a result surprises you by ~10×, the homogenization step is the first term to re-check.

Reporting

State the basis with the number — CFU/mL or CFU/g — and record the preparation details alongside: sample mass, diluent volume, homogenization method, and any neutralizers used. If subsamples were taken from a larger batch, the result describes the homogenized batch as prepared, not necessarily the batch in its original state. For quantitative comparison between batches, keep the preparation ratio constant; changing from 1 : 9 to 1 : 10 shifts every result by 11% before biology enters the picture.

Limitations

Homogenization efficiency varies with the matrix. Tissues, powders, and viscous foods release organisms unevenly, and surfactants or blender homogenization change recovery in ways the arithmetic cannot model. A CFU/g result is an estimate of the viable count in the prepared homogenate under your incubation conditions — treat it as such, and follow the sample-preparation procedure of your validated method.

Open the CFU calculator to switch between liquid and solid inputs, or read what one CFU represents for the interpretation limits that apply to both units.

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