Can’t pay your taxes by April 18? Even if you don’t pay, file your return on time. Failure to pay tax penalty is 1/2% a month plus interest (4% annualized right now). Failure to file penalty is 5% a month up to maximum of 25%. These penalties are based on tax, so if you are due a refund, there are no penalties (the IRS already has your money.) If you owe, as I said in a previous post, you can set up an installment plan with the IRS to make monthly payments of your taxes (and penalties and interest).