Cockroach population calculator

How the calculator works

The model uses two figures from NC State Extension's German cockroach biology page: a female lays about 200 to 250 eggs in her life (5 to 8 egg cases), and it takes 70 to 100 days to go from egg to an adult that lays eggs.

• Generations = days in your time span ÷ days per generation, rounded down. A month counts as 30.4 days.
• Young in a generation = adult females × eggs per female × survival rate.
• Females for the next generation = half of those young. NC State says infestations run roughly 50/50 male and female.

Worked example

Start with 1 female, 200 eggs and no deaths. Generation 1 is 200 young. Half are female, so 100 females each produce 200 young: generation 2 is 20,000. At 85 days per generation, that second generation arrives around day 170, under 6 months. With 250 eggs per female, generation 2 is about 31,000, which lines up with the "over 30,000 in a year" figure in UC IPM's cockroach Pest Notes.

What the model leaves out

• A female lays her egg cases over months, not all at once, so real growth is smoother than the generation steps shown here. Run the calculator past 6 months and the totals climb far beyond the published yearly estimates, because the model never runs out of food or space.
• Many nymphs die from lack of food, water or shelter. Lower the survival rate to see how much that matters.
• The figures are for German cockroaches, the species most often found breeding indoors. American cockroaches have far fewer young per egg case.

For the species comparison and a month-by-month timeline, read our guide on how fast cockroaches multiply. To find and remove egg cases before they hatch, see what cockroach eggs look like and where they hide. If roaches are still showing up after you treat, our guide to how long after treatment roaches die explains why new nymphs keep appearing.