To estimate the population of a liquid bacterial culture, serial dilutions are made.

A sample is then plated on semisolid medium in a petri dish. Each bacterium grows into one visible colony, so the original concentration can be found by multiplying the number of colonies by the dilution factor. (Note: when high concentrations of cells are plated, the colonies can fuse into a "lawn" of cells.)

Each of the plates above is diluted by a factor of 10. The dish on the right contains 15 colonies.