How Big Do Bullfrogs Get? North America's Largest Frog

Typical size of an adult bullfrog

Frogs are measured from the tip of the snout to the vent, the opening at the rear of the body, because their legs fold up and are hard to measure. By that standard most adult American bullfrogs are 9–15 cm (3.6–6 in) long. That is about the length of a smartphone, but a bullfrog is far bulkier, with a broad head and thick thighs.

With the hind legs stretched out straight, the total length of a large bullfrog can be well over 30 cm (12 in). That is why a bullfrog can look surprisingly huge when it leaps from the bank, and why size stories often grow in the telling.

Weight varies a lot with age, season and food supply. A medium adult often weighs 200–400 g (7–14 oz), while large, well-fed frogs reach around 500 g (1.1 lb). Females are usually slightly larger than males.

How big can a bullfrog get? Record sizes

The biggest reliably measured American bullfrogs reach about 20 cm (8 in) snout to vent and roughly 800 g (1.8 lb). Such frogs are rare and are usually old females living in warm, food-rich ponds with few predators.

Reports of “two-pound” or “three-pound” wild bullfrogs do turn up, but they are rarely backed by a scale and a measured length. The giant frogs in some older photos are often goliath frogs or African bullfrogs, which are different species.

Giant tadpoles: why bullfrogs start big

Bullfrog tadpoles are famous for their size. Most frog tadpoles change into froglets within a few weeks or months, but bullfrog tadpoles often stay in the water for one to three years in the cooler northern part of the range, overwintering at the bottom of ponds. In the warm south they can change within a few months.

During that long larval life they keep feeding on algae and tiny particles in the water and growing, commonly reaching 10–15 cm (4–6 in) including the tail. Big tadpoles turn into big froglets, which gives young bullfrogs a head start over smaller native frogs. Fish often avoid them because they taste unpleasant.

What makes bullfrogs grow so large?

Several factors decide how big a bullfrog gets:

• Temperature. Bullfrogs grow fastest in warm water and warm summers, so southern frogs reach adult size sooner.
• Food. A varied diet that includes crayfish, fish, other frogs and occasionally small birds or mice provides plenty of energy. See What do bullfrogs eat? for the full menu.
• Age. Frogs keep growing slowly after they mature, so the largest bullfrogs are usually the oldest ones.
• Competition and predators. Where herons, otters and fish take many young frogs, few survive long enough to become giants.

In some introduced populations, with warm water and few natural enemies, bullfrogs grow very well, one reason they are such a problem outside their native range.

How bullfrogs compare with other large frogs

The American bullfrog is big for a North American frog, but it is not the world’s largest. The goliath frog (Conraua goliath) of Cameroon and Equatorial Guinea holds that title: the heaviest verified specimen weighed 3.3 kg (7.3 lb) and measured 34 cm (13.4 in) snout to vent.

Compared with Europe’s familiar common frog, an adult bullfrog is roughly twice as long and many times heavier. In North America, other native true frogs such as the green frog are noticeably smaller; only the pig frog of the southeastern US comes close.

Telling a big bullfrog from look-alikes

Size alone is not a safe way to identify a bullfrog, because young bullfrogs are small and large green frogs can be confused with them. The key feature is the skin ridge: a bullfrog has a fold that curves around the eardrum but no ridge running down each side of the back, while the green frog has two such ridges. A male bullfrog’s eardrum is also much larger than its eye.