Learn English – How to say 0.0001782%

It's:

one hundred quintillion

or:

a hundred quintillion

The words for very large numbers

If you're wondering how to form other huge numbers like this, here's the pattern:

A thousand thousands is a million: 1,000,000.

A thousand millions is a billion: 1,000,000,000.

A thousand billions is a trillion: 1,000,000,000,000.

A thousand trillions is a quadrillion: 1,000,000,000,000,000.

A thousand quadrillions is a quintillion: 1,000,000,000,000,000,000.

A thousand quintillions is a sextillion: 1,000,000,000,000,000,000,000.

And so on. The part before -illion is the Latin prefix for the number of times you went through the process of multiplying by a thousand. So, you can continue to septillion, octillion, nonillion, decillion, undecillion, duodecillion, and so on forever.

Within the scale defined by one of these huge units, you multiply by a number from 1 to 999 in the usual manner, by putting the multiplier ahead of the unit, and you add smaller numbers by putting them after the unit, in the same manner as for thousands:

215,002 is "two hundred fifteen thousand and two".

215,000,000,000,000,000,002 is "two hundred fifteen quintillion and two".

The usual customs for "hundred" apply:

123,456,100,000,000,000,000 is "a hundred and twenty-three quintillion four hundred and fifty-six quadrillion one hundred trillion" or "one hundred twenty-three quintillion four hundred fifty-six quadrillion one hundred trillion", or other variations, the same as for hundreds of thousands.

Exponents

When you work with these numbers on a daily basis like I do,* you soon find that they become rather unwieldy, at least until you get up to a centillion pengős.^† In the physical sciences, if not in economics, one normally writes and pronounces these numbers using powers of ten. A quintillion is 10¹⁸, which you pronounce like this:

Ten to the eighteenth power.

Ten to the eighteenth. [for short]

Ten to the eighteen. [even shorter]

In scientific notation, you always choose an exponent large enough so the multiplier has one digit to the left of the decimal point, like this: 2.15 ⨉ 10¹⁷. That's pronounced:

215 quadrillion is "two point one five times ten to the seventeenth."

If the multiplier is exactly 1, you can omit it in speech. So:

100,000,000,000,000,000,000 is a hundred quintillion, or ten to the twentieth power.

Does anybody really say “quintillion”?

“Quintillion” is an obscure word, though not much more obscure than “quadrillion”, which often gets totted out when government budgets and monetary inflation make news. A fluent speaker can guess it from the pattern of “billion”, “trillion”, etc. Here are a few samples to illustrate typical contexts where people really use it to communicate (that is, not just to talk about words for huge numbers, which might be its most frequent use):

Government budgets: “For instance, the expected state income for oil and gas was reduced from 99,591 quintillion rupiah (about 9 billion euro) to 72,930 quintillion rupiah.”

Pop science: “The quantum simulation of the 69 electrons must specify all possible 600 quintillion states simultaneously.”

Bizarre religious tracts: “When this universe collapses in 70–100 billion years from now Jesus has given Kush a Quintillion universes like the one we live in as its territory forever. That is our promised land.”

Very low probabilities resulting from calculations: “Using FBI statistics, Schoon calculated that the DNA profile at issue would be found in 1 in 2.7 quintillion African-Americans, 1 in 52 quintillion Caucasians and 1 in 260 quintillion Hispanic unrelated individuals.” (This is from a U.S. appellate court opinion.)

Often when “quintillion” appears in print, it’s accompanied with an explanation. Usually when I’ve seen it used without explanation, it’s been in the context of economics. Presumably that crowd is well accustomed to talking about vast sums of money.

Long scale and short scale

Notice that in the Indonesian budget described above, “quintillion” occurs with multipliers greater than 999. That suggests that they're following the “long scale” system, in which each successive ‑illion is a million times greater than the previous one. That's an older usage, now nonstandard in English in all countries, but some people still use it, especially in countries like Indonesia where the dominant language follows the long-scale system. See kasperd's answer for more about that.

*_{Just kidding.}

^†_{When you get up to a centillion pengős, you're talkin’ real money.}

one-hundredth
the second decimal digit in a percentage

In our case, one (1) is the second decimal digit. So it should be

one one-hundredth of a percent

This is exactly what he says.