CFU/cm2 ??? HOW - (Sep/30/2010 )

How is the number of CFU/ cm² calculated?
A plate left in a room (6m²) in the open air, grew on a 6mm agar plate 16 colonies after 24 hr incubation.

What is the CFU/cm2 or CFU/m2

maryjo on Thu Sep 30 20:25:38 2010 said:

Hola, I think that you have a mistake in the measure of the plate, 6mm is very small.Well, supose that the plate is of 6 cm of diameter => 3.14x3x3= 28cm2. 6m2= 60000cm2, 60000/28=2.143 times the surface of a plate in the total room surface. 16colonies x 2143= 34,288 col in 6 m2 <> 5714cfu/m2 <> 57.1/dm2 <>0.571/cm2.
If the plate was different repeat he same procedure. Good luck

maryjo on Thu Sep 30 20:25:38 2010 said:

CFU/ cm² is the number of colonies formed per cm² of surface, so first work out the surface area of your plate.

I will assume you meant 6cm diameter plate rather than 6mm diameter, as 6mm is very small.

Area=pi*radius²

radius is 1/2 of the diameter, so pi*3² which is about 28cm²

if you had 16 colonies on that, then the CFU is 16 colonies per 28cm²

CFU/cm² would me 16/28 = 0.57CFU/cm²


I am not sure what the size of the room has to do with anything, but I suppose you could calculate the total number of CFU in the whole room if you wanted to.

Thanks "protolder" and "philman" it makes alot more sense now