The number quadrillion consists of one followed by fifteen zeros. It can be written as 1,000,000,000,000,000. 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. We write numbers with commas separating sets of three zeros so that it’s easier to read and understand the value. One quadrillion is also equal to 1,000 trillion. After a quadrillion, we have one quintillion, sextillion, septillion, and octillion, and so on until we reach Graham’s number, which is currently the largest known number.