How big does a rat grow? - briefly
Domestic rats usually grow to a body length of 23–28 cm (9–11 in) with tails of an additional 18–23 cm (7–9 in), and they typically weigh between 300 and 500 g. Larger breeds, such as the giant rat, can exceed 45 cm (18 in) body length and reach up to 1 kg.
How big does a rat grow? - in detail
Rats reach maturity within a few months, but their final dimensions vary with species, sex, diet, and environment.
Newborn pups measure 3–4 cm in body length (excluding the tail) and weigh 5–7 g. By the third week they double in length and gain roughly 15 g. Growth accelerates until the sixth week, when most individuals approach adult size.
Typical adult measurements:
- Brown (Norwegian) rat, Rattus norvegicus
- Body length: 20–25 cm
- Tail length: 18–25 cm
- Weight: 300–500 g (large males may exceed 600 g)
- Black rat, Rattus rattus
- Body length: 16–20 cm
- Tail length: 19–25 cm (tail often longer than body)
- Weight: 150–250 g
- Pet fancy breeds (e.g., Dumbo, Rex)
- Body length: 15–20 cm
- Weight: 250–350 g, with some individuals reaching 500 g under optimal care
Growth factors
- Nutrition – high‑protein diets shorten the time to maturity and increase final mass.
- Housing – spacious cages and enrichment promote normal skeletal development; overcrowding can stunt growth.
- Genetics – selective breeding for size produces larger specimens, while wild populations tend to be leaner.
- Health – chronic disease or parasitic load reduces weight gain and may limit length.
Maturation timeline
- 0–2 weeks: rapid increase in body mass; eyes open, fur appears.
- 3–4 weeks: weaning; body length reaches ~10 cm.
- 5–6 weeks: sexual maturity approached; weight 150–250 g for brown rats.
- 8–12 weeks: adult size attained; further growth minimal except for muscle development.
Maximum recorded dimensions exceed 30 cm total length (including tail) and 800 g in exceptionally large brown rats kept in laboratory settings. Wild specimens rarely surpass 500 g due to limited food availability and predation pressure.