A quadrillion is one followed by fifteen zeroes. If you're looking for a mnemonic, the root word, quad, "four" from the Latin quadri, refers to the number of groups of three zeroes that follow one thousand.

What is the meaning of Quadrillionth?

US : a number equal to 1 followed by 15 zeros — see Table of Numbers also, British : a number equal to 1 followed by 24 zeros — see Table of Numbers.

How many millions is a quadrillion?

A thousand million millions. We could also think of it as a thousand trillion or a million billion.