The IRS announced on August 21, 2026 (IR-2026-98) that interest rates are unchanged for the quarter beginning October 1, 2026. The underpayment rate stays at 7%. The announcement is routine. The timing is not — the September 15 estimated tax deadline falls three weeks from now, and the rate the IRS just locked in is the one that governs what happens if you short it.
What happened
Per the IRS, the rates effective October 1, 2026 are:
- 7% for overpayments (individuals and other non-corporate taxpayers)
- 6% for corporate overpayments
- 4.5% for the portion of a corporate overpayment exceeding $10,000
- 7% for underpayments
- 9% for large corporate underpayments
The rates are computed from the federal short-term rate determined during July 2026, and appear in Revenue Ruling 2026-15, scheduled for Internal Revenue Bulletin 2026-36 on August 31, 2026.
For context, the IRS quarterly rate table shows the same 7% underpayment rate ran in Q3 2026 and in Q4 2025. Apart from a brief dip to 6% in Q2, the underpayment rate has sat at 7% for a year.
Why it matters
Business owners routinely treat a light estimated tax payment as a free cash-flow lever. Money is tight in September, the payment is large, so you pay part of it and square up in April.
Price that decision honestly and it stops looking free.
The underpayment charge is 7% annualized, compounded daily, and — critically — not deductible as a business expense for individual taxpayers. A 7% non-deductible cost is not comparable to 7% on a business line of credit whose interest you write off. Depending on your bracket, you would need something closer to a 9–10% deductible rate to be equivalent. That is worse than most secured credit lines available to a profitable small business today.
There is a second cost that does not show up in the rate. The estimated tax penalty is computed per quarter, not on your annual balance. Shorting the September 15 installment starts the clock on that installment immediately — paying extra in January does not retroactively cure it. This is why owners who “caught up in April” are still surprised by a penalty line on the return.
The flip side deserves a mention too: the IRS pays 7% on overpayments. Sitting on a large refund is a 7% taxable return you cannot access. Neither direction is a good place to park working capital.
What this means for your business
- Do the September 15 math against your actual year-to-date numbers, not last year’s. If your 2026 income is running well below 2025, the safe-harbor payment based on last year’s tax may be materially overfunding the IRS at 7% you cannot touch until you file.
- Know which safe harbor you are using. Paying 100% of last year’s tax (110% if prior-year AGI exceeded $150,000) protects you from the penalty regardless of how this year lands. Paying 90% of current-year tax requires that your current-year estimate is actually right. Pick one deliberately.
- If cash is genuinely short, compare rates before you short the IRS. A drawn line of credit at 8% deductible can beat a 7% non-deductible underpayment charge. Run that comparison rather than defaulting to the IRS as the lender of last resort.
- Use the annualized income installment method if your revenue is lumpy. Seasonal and project-based businesses often owe far less on the September installment than an even-quarters split implies — but only if you compute it that way and file Form 2210 to show it.
- Reconcile what you have actually paid in. Estimated payments get misapplied to the wrong year or the wrong entity more often than owners expect. Confirm the posted total before you size the September check — and note that payments take about two weeks to appear in your IRS account.
“7% for underpayments (taxes owed but not fully paid)… 9% for large corporate underpayments.” — IRS, IR-2026-98, August 21, 2026
The bottom line
A rate that does not change is easy to ignore, which is exactly why it is worth flagging three weeks before a payment deadline. The IRS has now confirmed it will charge 7%, non-deductible and compounded daily, straight through the end of the year. That is a known number — which means the September 15 decision is a pricing decision, and one you can make on purpose.
