20 g0.71 oz
An adult house mouse weighs about 20 g (0.7 oz, or 0.044 lb). Wild adults usually range from 11 to 30 g, depending on age and how much food they find. Laboratory mice, the same species, often grow heavier, with well-fed males passing 35 g by six months. The giant wild mice of Gough Island average about 35 g too.
| Lab mouse, male, 24 weeks oldC57BL/6J average at The Jackson Laboratory | 35.3 g1.25 oz |
|---|---|
| Gough Island mouse, averageAbout twice as heavy as mainland relatives | 35 g1.23 oz |
| Lab mouse, female, 24 weeks oldC57BL/6J average | 25.7 g0.91 oz |
| Typical adult house mouseMost weigh 11 to 30 g | 20 g0.71 oz |
| Pup at weaningAbout 3 weeks old | 10.1 g0.36 oz |
| Newborn pupAbout 0.75 to 2 g | 1 g0.035 oz |
Most adult house mice weigh between 11 and 30 g (0.4 to 1 oz), and 20 g is a fair middle value. Sources word it a little differently: Animal Diversity Web gives 12 to 30 g, the University of California’s pest guide says about ½ ounce (14 g), and veterinary reference tables use about 20 g for a typical adult.
The body is 7.5 to 10 cm (3 to 4 in) long from nose to the base of the tail, and the tail adds another 5 to 10 cm (2 to 4 in). In wild mice, males and females are about the same size. At 20 g each, it takes about 23 mice to make a pound.
A mouse is born after a pregnancy of 19 to 21 days, weighing only about 1 g (0.75 to 2 g). A litter is usually 5 or 6 pups, and a female can have 5 to 10 litters a year.
Growth is fast. The Jackson Laboratory weighs its C57BL/6J mice, the most common lab strain, every week: pups weigh about 10 g at weaning at three weeks, females reach about 19.5 g and males 24.7 g at eight weeks, and by 24 weeks females average 25.7 g and males 35.3 g. Lab and pet mice belong to the same species as wild house mice, but with steady food and no need to forage they usually end up heavier, and in the lab males clearly outweigh females.
The heaviest wild house mice live on Gough Island, a remote island in the South Atlantic. Seal hunters brought mice to the island between 130 and 200 years ago, and the mice now weigh about twice as much as their mainland relatives. National Geographic puts their average at 35 g (1.2 oz).
A 2015 study in the journal Genetics found that they get so big mainly by growing faster in the six weeks after birth, and that many genes each add a little to the extra size. Food is part of the story: when seeds and insects run short in winter, Gough Island mice attack seabird chicks, including albatross chicks hundreds of times their own weight.
A 20 g body is the reason mice are so hard to keep out of buildings. The University of California’s pest guide says house mice can squeeze under gaps ¼ inch (about 6 mm) tall and through openings ⅜ inch (about 1 cm) wide, about the width of a pencil.
They eat up to 20% of their body weight every day, so a 20 g mouse needs only about 3 to 4 g of food, taken in around 200 small meals a night. They can get by without drinking if their food holds enough moisture.
A typical house mouse weighs about 0.044 lb (0.7 oz), and even a large one at 30 g is only about 0.066 lb (1 oz). It takes roughly 23 average mice to make a pound.
A newborn mouse weighs about 1 g, usually between 0.75 and 2 g. By the time it is weaned at about three weeks old, it weighs around 10 g.
House mice can squeeze under a gap about ¼ inch (6 mm) tall and through an opening about ⅜ inch (1 cm) wide, roughly the width of a pencil.
Up to about 20% of its body weight, which for a 20 g mouse is only 3 to 4 g of food. It eats in many small meals, around 200 a night.
The wild house mice of Gough Island in the South Atlantic are the largest known, averaging about 35 g (1.2 oz), roughly twice the weight of mainland mice. Well-fed lab males can reach a similar weight by six months old.
Last checked October 2, 2026.
