The quadrillion is a large number which has one of two values depending on how or where it is being used. Quadrillion comes after trillion.
Short scale usage
In short scale usage, a quadrillion is equal to a 1 with 15 zeroes, namely:
1,000,000,000,000,000 (1015)
It is a thousand trillion (short scale), i.e. a thousand million million. The SI prefix for this number is peta.
Long scale usage
In long scale usage, a quadrillion is equal to a 1 with 24 zeroes, namely:
1,000,000,000,000,000,000,000,000 (1024)
It is a million trillion (long scale), i.e. a million million million million. The SI prefix for this number is yotta.
While people in the British Isles, Australia and New Zealand traditionally employed the long scale usage, they have largely switched to the short scale usage.
See also