Mice are breeding machines. They have a gestating period of 19 to 21 days. A female mouse gets pregnant about 5 to 10 times each year and can give birth to a litter of 3 to 14 pups. On average, the litter size ranges from 6 to 8 pups. On average, you are looking at least 32 to 56 pups each year from one female mouse.

The mice reproduce each year, and the mother can mate immediately after giving birth. So you could be looking at a new litter within 25 days. To make matters even more complicated, the female baby mouse is capable of reproduction at 6 weeks. So, they can start producing pups of their own and have about 10 litters of their own per year.

In a theoretical situation, two mice that sneak into your home could give birth to 60 in a year. Of these new mice, about 21 to 30 are female mice capable of having their babies within a month, which can theoretically lead to 5,082 mice in just one year. One mathematician calculated it to be 5 million mice per year, that is if it were a perfect world for mice. //

The average lifespan of a mouse is about 12 months in the outdoors. But when they take refuge in your house, they can live up to 2 or even 3 years.