1 percent compound interest on $100 will cause the amount to double to $200 in about 70 years. That is called the rule of 73 which can be used to figure out how fast something grows. The rule can be used to figure out how fast anything that always grows exponentially at the same rate will double in size. Suppose a kangaroo grows 10 percent a year; dividing 10 into 70 gives 7, so the kangaroo will double in size in 7 years. If the kangaroo grew 7 percent a year it would take 10 years for its size to double. If it only grew 2 percent it would take 35 years for its size to double.