菌落形成单位（CFU，colony-forming unit）是计算细菌数量的一种方法，其值越高表示样品中所含的细菌越多。菌落形成单位的计量方式与一般的计数方式不同，一般直接在显微镜下计算细菌数量会将活与死的细菌全部算入，但是CFU只计算活的细菌。计算的方式是将一份样本平均地抹在洋菜培养基上，等待菌落生成，再计算形成的菌落数目。菌落形成单位并不是一个非常精准的计算方式。

In microbiology, a colony-forming unit (CFU) is a unit used to estimate the number of viable bacteria or fungal cells in a sample. Viable is defined as the ability to multiply via binary fission under the controlled conditions. Counting with colony-forming units requires culturing the microbes and counts only viable cells, in contrast with microscopic examination which counts all cells, living or dead. The visual appearance of a colony in a cell culture requires significant growth, and when counting colonies it is uncertain if the colony arose from one cell or a group of cells. Expressing results as colony-forming units reflects this uncertainty.