How Many Zeros in a Septillion?
A septillion has 24 zeros: 1,000,000,000,000,000,000,000,000. In scientific notation this is 1024. On the short scale — the standard used in the United States and modern scientific literature — a septillion equals one thousand sextillion, or equivalently, a thousand trillion trillion. The prefix septi- means seven, reflecting the seventh power of one thousand beyond one (1,0008 = 1024). In the older long-scale system used in some European countries, a septillion represents 1042 — a vastly larger value. The 1024 short-scale definition is the one you will encounter in physics, chemistry, and computing. Related: Zeros in an undecillion.
A septillion has
24
zeros
- Written Form
- 1 followed by 24 zeros
- Scientific
- 10²⁴
How Do You Write One Septillion in Zeros?
One septillion is written as the digit 1 followed by twenty-four zeros. Breaking it into groups of three makes it easier to parse: Learn more about quindecillion zeros.
1,000,000,000,000,000,000,000,000
Reading it aloud, you simply say "one sep-TILL-ee-on." Each of the eight digit groups (including the leading 1) corresponds to a place-value name. In scientific work the number almost always appears as 1024 or, with a coefficient, something like 6.02 × 1023 (Avogadro's number, which is close to one septillion and represents the number of atoms or molecules in one mole of a substance).
The SI (International System of Units) prefix for 1024 is yotta-, as in yottabyte. So one yottabyte = one septillion bytes. This prefix was officially added to the SI system precisely because quantities at this scale had become relevant in data storage and particle physics.
How Many Zeros Are in Octillion vs Septillion?
Septillion sits between sextillion (1021) and octillion (1027), following the short-scale rule of adding three zeros per step:
| Name | Zeros | Scientific Notation |
|---|---|---|
| Sextillion | 21 | 1021 |
| Septillion | 24 | 1024 |
| Octillion | 27 | 1027 |
At this scale, quantities appear in physical chemistry and astronomy. The number of atoms in a cubic centimeter of a dense metal is around a few times 1023, squarely in septillion territory. The estimated mass of Earth's atmosphere — expressed in kilograms — is roughly 5 × 1018 kg, while at higher scales the mass of our galaxy expressed in solar masses approaches 1012, but individual molecular counts in large samples push well into septillion range and beyond. Learn more about how many zeros does a trillion have.