The Iowa Department of Revenue today announced that the annual interest rate for overdue taxes and refunds in 2018 will be 6 percent. The annual rate has not changed since 2010, when it was set at 5 percent. The department has also determined the tax brackets and standard deduction amounts for the 2018 tax year, for income taxes due and payable in calendar year 2019. The tax year 2018 standard deduction for single taxpayers or married taxpayers filing separately will be $2,030. The tax year 2018 standard deduction for married filing jointly taxpayers will be $5,000.