How to Calculate CFU per mL: Formula and Examples

After you count the colonies on a plate, you need to convert that raw number into the concentration of viable cells in the original sample. That concentration is expressed in CFU per millilitre.

The CFU/mL formula

CFU/mL = number of colonies ÷ (dilution factor × volume plated)

Each term matters:

• Number of colonies — the count from a plate inside the 30–300 range.
• Dilution factor — how much the sample was diluted before plating, written as a fraction such as 10⁻⁶ (one millionth).
• Volume plated — the volume of diluted sample spread on the plate, in millilitres (often 0.1 mL or 1 mL).

Understanding the dilution factor

Samples are usually diluted in a ten-fold series: 1 mL of sample into 9 mL of diluent, repeated several times. Each step multiplies the dilution by ten. After six steps the dilution factor is 10⁻⁶ — the plated suspension is one millionth the concentration of the original. You divide by this factor to scale the count back up to the original sample.

Worked example 1

Suppose you count 150 colonies on a plate from the 10⁻⁶ dilution, and you plated 0.1 mL:

• CFU/mL = 150 ÷ (10⁻⁶ × 0.1)
• CFU/mL = 150 ÷ (10⁻⁷)
• CFU/mL = 150 × 10⁷ = 1.5 × 10⁹ CFU/mL

So the original sample contained about 1.5 billion colony forming units per millilitre.

Worked example 2

Now suppose you count 62 colonies from the 10⁻⁴ dilution and plated a full 1 mL:

• CFU/mL = 62 ÷ (10⁻⁴ × 1)
• CFU/mL = 62 × 10⁴ = 6.2 × 10⁵ CFU/mL

Step-by-step method

1. Count the colonies on a plate in the 30–300 countable range.
2. Write down the dilution factor of that plate.
3. Note the volume you plated, in millilitres.
4. Divide the colony count by (dilution factor × volume plated) to get CFU/mL.
5. Report the result in scientific notation, for example 6.2 × 10⁵ CFU/mL.

Common mistakes

• Forgetting the plated volume: if you spread 0.1 mL but divide as if you plated 1 mL, your result is ten times too low.
• Using a plate outside the countable range: a count from a TNTC or TFTC plate makes the whole calculation unreliable.
• Mixing up dilution steps: double-check how many ten-fold steps were actually performed.

From count to CFU, faster

The arithmetic is simple once you have an accurate colony count — and an accurate count is the hard part on dense plates. Use the free online colony counter to count from a photo, then apply the formula above to report CFU/mL.