What is the world's largest flower by diameter?

The world's largest flower by diameter is Rafflesia arnoldii. Its enormous bloom can reach roughly 1 metre across, making it the largest individual flower known in standard botanical descriptions.

Rafflesia arnoldii is a parasitic plant native to the rainforests of Sumatra and Borneo. It has no visible roots, stems, leaves, or chlorophyll; instead, it lives inside vines of the genus Tetrastigma and emerges only when it produces a flower.

The bloom is famous for its reddish-brown petals and pale spots. Its smell resembles rotting meat, which attracts carrion flies that pollinate the flower. The flower is also short-lived, usually lasting only several days.

A common mix-up is Amorphophallus titanum, or titan arum. Titan arum produces the world's largest unbranched inflorescence—a structure made of many tiny flowers—whereas Rafflesia produces one giant individual flower.