Every investment can double your money. However, if the returns are higher, you will be able to double your money faster. High-risk investments can double your money faster. For example, stock investments can double your money in 3-5 years and MFs in 5-6 years. On the other hand, low-risk investments take a longer time. PPF investments take about 8 years to double your money, and FDs around 8-9 years.