For any exponentially growing culture, take a measurement of the number of cells at an initial time (N_i), and a second measurement at some later time (N_L). If T is the length of time between the observations:

doubling time = T ⋅ ln(2) / ln(N_L / N_i) measured in units of T.

example:
at 4:00 pm on Monday, 3 x 10⁵ cells are plated. At 3:00 pm the following Friday, 2.5 x 10⁶ cells are counted.

• N_i = 3 x 10⁵
  
• N_L = 2.5 x 10⁶
  
• T = 95 hours
  

The doubling time for this culture would be:

• 95 ⋅ ln(2) / ln( 2.5 x 10⁶ / 3 x 10⁵) = 31.1 hours ≃ 1.3 days