Petri plates commonly used in microbiology labs are usually 90 mm in diameter, which is why they are often referred to as 90 mm plates. However, smaller sizes, like 55 mm, or larger sizes, like 150 mm, can also be used depending on the requirements of the experiment.
RODAC (Replicate Organism Detection and Counting) plates are typically smaller than standard petri plates, with a common diameter of 55 mm. These plates are designed for surface sampling and monitoring microbial contamination on flat surfaces.