Start at 1 and double it. Then double the result. Then double that. The quiz shows the current number and you type its double. The first wrong answer ends the run.
There is no time limit. The challenge is the size of the numbers: after 10 doublings you are past 1,000, after 20 past a million, and after 30 past a billion.
How to play
The clock starts at your first keystroke and counts up. There is no time limit.
The card shows a number. Type its double, then press Enter. The answer commits only on Enter.
The first wrong answer ends the run. The quiz shows the correct number first.
Your score is the number of doublings in a row. Reaching 30 clears the streak.
A full 30 ranks by time: the fastest player is first.
Give up ends the run early and shows your results.
Select "Review answers" on the result screen to see each doubling you played, with its answer.
What counts in this quiz
The numbers are the powers of two, from 2 to 2³⁰ = 1,073,741,824.
The card groups the digits in threes with spaces, so a long number can be read. You can type the answer with or without spaces or commas.
What's not included, and why
No calculator. The point of the quiz is to double large numbers in your head, one step at a time.
Good to know
2³⁰ is 1,073,741,824. That is why a gigabyte was long taken to be 1,073,741,824 bytes, not a round billion.
Fold a sheet of paper in half 42 times and its thickness passes the distance to the Moon. Doubling grows faster than intuition expects.