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Undecillion — Definition, Formula & Examples

Undecillion is the name for the number 1 followed by 36 zeros, or 103610^{36}. It is the 12th named power of a thousand after one (thousand, million, billion, ... undecillion).

In the short scale (used in the United States and most English-speaking countries), one undecillion equals 103610^{36}, i.e., 1,000,000,000,000,000,000,000,000,000,000,000,0001{,}000{,}000{,}000{,}000{,}000{,}000{,}000{,}000{,}000{,}000{,}000{,}000. It occupies the 12th position in the sequence where each named number is 10310^{3} times the previous one, starting from one thousand (10310^{3}).

Key Formula

1 undecillion=10361 \text{ undecillion} = 10^{36}
Where:
  • 103610^{36} = The number 1 followed by 36 zeros

How It Works

Each large number name in the short scale represents a group of three zeros added to a thousand. A million is 10610^{6}, a billion is 10910^{9}, a trillion is 101210^{12}, and so on. Following this pattern — quadrillion (101510^{15}), quintillion (101810^{18}), sextillion (102110^{21}), septillion (102410^{24}), octillion (102710^{27}), nonillion (103010^{30}), decillion (103310^{33}) — the next step is undecillion at 103610^{36}. The prefix "un-dec" comes from Latin for "one" and "ten," meaning eleven, since undecillion is the 11th "-illion" after million.

Worked Example

Problem: How many zeros does 5 undecillion have when written out in full?
Write in scientific notation: 5 undecillion means 5 times 10 to the 36th power.
5 undecillion=5×10365 \text{ undecillion} = 5 \times 10^{36}
Count the zeros: When you write this number in standard form, the digit 5 is followed by 36 zeros.
5,000,000,000,000,000,000,000,000,000,000,000,0005{,}000{,}000{,}000{,}000{,}000{,}000{,}000{,}000{,}000{,}000{,}000{,}000
Answer: 5 undecillion has 36 zeros after the leading digit, making it a 37-digit number.

Why It Matters

Numbers this large appear in combinatorics and computer science — for example, the number of possible unique 128-bit identifiers (UUIDs) is roughly 340 undecillion. Understanding place-value naming also reinforces how powers of ten and scientific notation work.

Common Mistakes

Mistake: Confusing short-scale and long-scale values. In parts of Europe, "undecillion" means 106610^{66}, not 103610^{36}.
Correction: In the U.S. short scale, each new "-illion" multiplies by 10310^{3}. Always check which scale is being used.