60-second understanding game · #1
Compound Interest Race
One question, no math allowed: how long until money doubles? Lock in your gut feeling — then meet the curve. The gap between the two is the most useful thing this page teaches.
You invest $10,000 at 8% a year, growth reinvested.
In how many years does it double to $20,000? Trust your gut — no math allowed.
Why we built a game instead of another paragraph
Everyone has read "compound interest is powerful". Reading doesn't recalibrate intuition — being measurably wrong does. Most players give compounding several extra years, because human guessing is linear while growth on growth accelerates. Feeling that gap once is worth ten articles: it's the visceral case for starting early, and the built-in skepticism against anyone promising to double your money suspiciously fast.
Fair play, by our rules: the game runs entirely in your browser, guesses aren't recorded anywhere, and the math is the same audited formula as our compound calculator — see how we verify.
The math your gut is racing
Compound growth follows FV = PV × (1 + r)t — each year's interest earns its own interest, so the curve bends upward while intuition draws a straight line. The quick mental shortcut is the Rule of 72: divide 72 by the annual rate to estimate doubling time. At 8% that's about 9 years; at 6%, about 12. Most first guesses land years late precisely because linear thinking undershoots every bend of the curve.
After the reveal, take your real numbers to the Compound Interest Calculator — same math, your money — or sanity-check any rate in seconds with the Rule of 72.