The way I understand this is that you must pay at least 90% of what you paid for tax from the previous year, regardless of how much money you are making this year, in order to avoid late payment penalty. Since this is the case, do people just pay as little as possible by 4/15, 6/15, and 9/15 (let's say 10% of what you paid last year on these dates for a total of 30%), then pay at least 60% or more by 1/15 of the following year in order to avoid the penalty? I basically want to pay as little as possible to earn interests or use that money to trade until January 15, but still end up paying at least 90% of previous year's tax liability. Or do people just pay 25% of previous year's tax liability on each quarterly due date? Thanks.