When counting colonies too numerous to count, the percentage of plate area covered is multiplied by what factor?

• A. 10,000
• B. 1,000
• C. 100
• D. 1,000,000

Answer: (A)

When colonies are too numerous to count, you estimate how many are present by looking at how much of the plate surface is covered and converting that area coverage into a colony count. The standard conversion uses a factor of 10,000, so you multiply the percent area covered by 10,000 to estimate the number of colony-forming units on the plate. This works because a small fraction of the plate covered by many tiny colonies roughly translates to a large number of colonies overall; for example, 1% coverage corresponds to about 100 CFU (1 × 10,000 = 100). So, if 5% of the plate is covered, you’d estimate about 50,000 CFU on that plate. The 10,000 factor reflects typical plate size and colony size, giving a practical, though approximate, conversion when counting individual colonies isn’t feasible.