Serial dilution calculations

 

 

 

The above image depicts how to find the total dilution of each test tube. If you're using a serial dilution to find the CFU per ml in a solution of bacteria, then you have one more calculation to make:

#colonies on plate  x   dilution factor*   x   mL plated      = CFU/mL in original stock

*Note that here we're using the dilution factor, which is related to the total dilution, but we remove the negative from the exponent. Why? Think of it as moving backwards and making the original concentration from any of the tubes; we would have to concentrate instead of dilute and therefore use larger numbers.