For the second quarter of 2019 (April 1 through June 30, 2019), the IRS interest rate on individual tax refunds and tax underpayments is remaining at 6% (Rev. Rul. 2019-5). This is the same rate that applied in the first quarter of 2019.
Keep in mind that if you are due a refund on your 2018 return, and you file by the April 15 due date, the IRS will not pay interest on a refund that it issues within 45 days; that is, by May 31. If you file after the April 15 due date, no interest is paid on a refund issued within 45 days of your filing date.
For corporations, the second quarter IRS interest rate on underpayments is generally 6% (same rate as for individuals), but for large corporate underpayments, the rate is 8%. The interest rate on a corporate overpayment is generally 5%, but on the portion of a corporate overpayment exceeding $10,000, the rate is limited to 3.5%.