The failure to pay penalty is a penalty that the Internal Revenue Service assesses when a taxpayer does not pay a tax by the mandated due date. The fine is 0.5% of the unpaid tax for each month after the due date that the payment is late, not to exceed 25% of the unpaid tax. If a taxpayer is subject to penalties for both the failure to file and the failure to pay, the failure to file penalty is reduced by the amount of the failure to pay penalty. If the return is more than 60 days late, the mandated minimum penalty that may be charged is the lesser of $135 or 100% of the unpaid tax. A taxpayer does not have to pay the fine if it can be shown that the payment issue was due to reasonable cause and not to willful neglect.