What Is CFU? Colony Forming Units Explained

CFU stands for colony forming unit. It is the unit used to express how many viable microorganisms are in a sample — that is, how many cells are alive and able to multiply. Rather than trying to see individual bacteria, microbiologists let them grow into visible colonies on an agar plate and count those colonies. Each colony is counted as one colony forming unit.

Why “colony forming unit” and not “cell”?

The wording is deliberate. A single visible colony might have grown from one isolated cell, but it might also have grown from a small clump of cells that were stuck together when they landed on the agar. Either way it produces one colony. Because you cannot tell which happened, the colony is counted as one colony forming unit — one or more viable cells that gave rise to a single colony — rather than claiming it represents exactly one cell.

CFU measures viability, not total cells

The key feature of a CFU count is that it only counts cells that are alive and able to reproduce. Dead cells do not form colonies, so they are not counted. This is exactly what you want for most practical questions: whether drinking water is safe, whether a food product meets a spec, or how many live bacteria are in a culture. A direct count under a microscope, by contrast, counts living and dead cells alike and so overstates the viable population.

How CFU is reported

A bare colony count is not very useful on its own — it depends on how much sample was plated and how far it was diluted. So results are normalised to a concentration:

• CFU/mL — colony forming units per millilitre, for liquid samples such as water or broth cultures.
• CFU/g — colony forming units per gram, for solid samples such as food or soil.

Because microbial concentrations span huge ranges, CFU values are usually written in scientific notation, for example 2.5 × 10⁶ CFU/mL. To get from a plate count to one of these numbers you apply the dilution factor and plated volume — see how to calculate CFU per mL.

From plate to CFU

In practice, a CFU result comes from a careful plate count. You choose a plate in the 30–300 countable range, count the colonies following a consistent method, and convert the count into CFU/mL. The whole workflow is covered in how to count bacterial colonies on agar plates, and you can do the counting step with the free online colony counter.