Einstein is often credited for inventing the "Rule of 72', which is an estimating tool to figure out how long it would take your money to double at a specific compound interest rate.  For example, 12% will double your money in about 6 years (72 divided by 12 is 6).  The Rule of 72 has actually been around at least since the 1500s, according to Wikipedia.  Einstein's actual quote was “Compound interest is the eighth wonder of the world. He who understands it, earns it ... he who doesn't ... pays it.”