Colony Forming Units Calculator

Work out colony forming units instantly with clear inputs, formula shown and shareable results.

About Colony Forming Units Calculator

Multiplies the colony count by the dilution factor and divides by the volume plated to give viable counts per millilitre.

Formula

Viable count

CFU/mL = colonies counted x dilution factor / volume plated (mL)

Worked example

With the default values below, Colony Forming Units Calculator returns Colony forming units of 14,500,000. Change any input above and every figure here is recalculated in your browser.

Inputs used

Colonies countedaccepts 1–1,000: 145
Dilution factoraccepts 1–1,000,000,000,000: 10,000
Volume platedaccepts 0.001–10 mL: 0.1 mL

Results

Colony forming units: 14,500,000 CFU/mL
Log10 CFU/mL: 7.161
Colony forming units: 14,500,000,000 CFU/L
Plate validity: Within the countable 30 to 300 colony range

Step by step

1Values used
2Viable count
3Colony forming units= 14,500,000 CFU/mL
4Log10 CFU/mL= 7.161
5Colony forming units= 14,500,000,000 CFU/L
6Plate validity= Within the countable 30 to 300 colony range

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why 30 to 300 colonies?

Below 30 the Poisson error is large; above 300 colonies merge and are undercounted.

Why is it called colony forming units?

One colony may arise from a clump of cells, so the count measures viable units rather than individual cells.