How big do rats grow? - briefly
Domestic rats usually measure 9–11 inches (23–28 cm) in body length, with tails of comparable size; wild Norway rats can reach 12–14 inches (30–36 cm) body length, plus a tail of similar length. Female rats tend to be slightly smaller than males.
How big do rats grow? - in detail
Rats exhibit a wide range of dimensions depending on species, environment, and nutrition. The common laboratory or pet rat (Rattus norvegicus) typically reaches a body length of 18–20 cm (7–8 in) excluding the tail, which adds another 20–25 cm (8–10 in). Adult weight usually falls between 300 and 500 g (10.5–17.5 oz). Wild brown rats can be slightly larger, with body lengths up to 25 cm (10 in) and tail lengths of 30 cm (12 in), and weights occasionally exceeding 600 g (21 oz).
Smaller species, such as the black rat (Rattus rattus), average 15 cm (6 in) body length and 150–250 g (5–9 oz) weight. Dwarf varieties bred for pets may remain under 150 g (5 oz) throughout adulthood. Large laboratory strains selected for research can approach 600 g (21 oz) when fully mature.
Growth follows a predictable pattern:
- Birth: 5–7 g, hairless, eyes closed.
- Weaning (3–4 weeks): 30–50 g, fur present, capable of solid food.
- Juvenile (5–8 weeks): rapid increase, reaching 150–250 g.
- Sexual maturity (10–12 weeks): size stabilizes, further gain limited to fat accumulation.
Factors influencing final size include genetic line, diet quality, housing conditions, and health status. High‑protein diets and ample space promote optimal growth, while chronic stress or disease suppress weight gain.
In captivity, regular weighing and measurement of nose‑to‑base length provide reliable monitoring of development. Wild populations display greater variability due to fluctuating food availability and predation pressure, resulting in occasional individuals markedly larger or smaller than average.
Overall, rat dimensions span from under 100 g and 10 cm body length in miniature breeds to over 600 g and 25 cm in robust wild or laboratory strains, with tail length typically matching or exceeding body length.