What sizes can rats be? - briefly
Typical laboratory or pet rats measure 5–10 inches (13–25 cm) in body length, with tails of similar length; larger breeds such as the giant or «fancy» rat can reach 12–18 inches (30–45 cm) including tail. Weight ranges from about 150 g for small individuals to over 600 g for the largest specimens.
What sizes can rats be? - in detail
Rats exhibit a broad spectrum of dimensions, determined by species, age, sex, and breeding selection.
The most widespread species, the brown rat (Rattus norvegicus), typically reaches a head‑body length of 20–25 cm, with a tail of comparable length. Adult weight ranges from 250 g to 500 g, males generally exceeding females by 10–20 %. Growth accelerates during the first eight weeks, after which weight stabilises near the adult average.
The black rat (Rattus rattus) presents a slightly smaller profile: head‑body length 16–20 cm, tail length 18–25 cm, and adult weight between 150 g and 300 g. Sexual dimorphism is less pronounced than in the brown rat, with males averaging 5–10 % greater mass.
Domesticated varieties, often referred to as fancy rats, encompass numerous coat‑color and body‑type lines. Standard pet rats align with the brown rat’s dimensions, but selective breeding yields dwarf lines, whose head‑body length falls to 12–15 cm and weight to 80–150 g. Giant lines, such as the “Hooded” and “Dutch” types, may surpass 30 cm in body length and attain 600–800 g.
Juvenile rats display rapid proportional changes. Newborns measure 3–4 cm in length and weigh 5–7 g. By four weeks, length doubles and weight reaches approximately one‑third of adult mass.
Geographic variations influence size modestly; populations inhabiting colder climates tend toward larger bodies, reflecting Bergmann’s rule. Conversely, island dwarfism can reduce dimensions in isolated groups.
In summary, rat size ranges from under 10 cm and 50 g in extreme dwarf forms to over 30 cm and 800 g in giant pet lines, with the most common wild species occupying the intermediate 20–25 cm and 250–500 g brackets.