If you file an extension, do you pay interest on what you owe? Yes. Form 4868 gives you six more months to send in your return, but it does not give you more time to pay. Interest starts running on any unpaid federal tax the day after the original April deadline and keeps compounding daily until the balance is gone.1Internal Revenue Service. Topic No. 304 – Extensions of Time to File Your Tax Return In most cases a separate failure-to-pay penalty runs alongside it.
The Extension Moves Your Paperwork, Not Your Payment
Federal law requires interest on any tax not paid by the last date prescribed for payment, and that date is the original April filing deadline regardless of any extension you file.2Office of the Law Revision Counsel. 26 USC 6601 – Interest on Underpayment, Nonpayment, or Extensions of Time for Payment, of Tax The extension only pushes back when your return is due.
The IRS treats this interest as compensation for the time value of money owed to the Treasury, not as a penalty. That matters because interest, unlike penalties, cannot be waived for reasonable cause. The Internal Revenue Manual states that reasonable cause is never a basis for abating interest.3Internal Revenue Service. Abatement and Suspension of Underpayment Interest Owing $3,000 on April 16 means interest starts that day and does not stop until the balance is paid, no matter how legitimate your reason for waiting.
How Much Interest You’ll Owe
The IRS resets the underpayment interest rate each quarter using the federal short-term rate plus three percentage points.4Office of the Law Revision Counsel. 26 USC 6621 – Determination of Rate of Interest For the quarter starting April 1, 2026, individual taxpayers pay 6% annual interest, down from 7% in the first quarter.5Internal Revenue Service. Internal Revenue Bulletin 2026-8 Because the rate can change every three months, a balance carried through a full six-month extension may be charged at two different rates.
Federal law also requires tax interest to compound daily, so each day’s interest is added to the balance before the next day’s interest is calculated.6Office of the Law Revision Counsel. 26 USC 6622 – Interest Compounded Daily On a $5,000 unpaid balance at 6%, that starts at roughly $0.82 per day and creeps up as the base grows. Over a full six-month extension, daily compounding at 6% adds about $152 in interest compared with roughly $150 under simple interest. The gap widens on larger balances.
The Failure-to-Pay Penalty Runs at the Same Time
Interest is not the only thing that accrues. The failure-to-pay penalty is 0.5% of unpaid taxes for each month the balance is outstanding, capped at 25%. Filing an extension does not prevent this penalty; it starts the day after the April deadline and runs alongside the daily interest.7Internal Revenue Service. Failure to Pay Penalty
There is a second, larger charge the extension does spare you from. The failure-to-file penalty is 5% of unpaid taxes per month, up to 25%, and filing Form 4868 on time eliminates it entirely for the extension period. That is the whole point of requesting an extension when you can’t file.1Internal Revenue Service. Topic No. 304 – Extensions of Time to File Your Tax Return
One detail catches many taxpayers off guard: the IRS charges interest on penalties too. Once the failure-to-pay penalty is assessed, interest begins accruing on that penalty amount, not just on the original tax.8Internal Revenue Service. Interest Delay makes the bill grow on more than one front.
How to Keep the Bill Small
The single most effective move is paying as much as you can when you file Form 4868. You don’t need an exact figure. Estimate your total tax, subtract what you’ve already paid through withholding or estimated payments, and send the difference with the extension request. Interest and the failure-to-pay penalty are calculated only on the unpaid portion, so every dollar paid upfront reduces both charges.9Internal Revenue Service. Taxpayers Who Need More Time to File a Federal Tax Return Should Request an Extension
Deliberately overpaying is a reasonable strategy when your estimate is uncertain. Any overpayment is refunded once your final return is processed, and the cost of tying up a few hundred dollars briefly is small compared with months of compounding on an underpayment.
The 90% Threshold
Federal law provides that a taxpayer who makes a good-faith effort to determine their liability and pays at least 90% of the tax shown on the final return is treated as having met the payment requirement for the extension period.10Office of the Law Revision Counsel. 26 USC 6651 – Failure to File Tax Return or to Pay Tax Interest and the 0.5% failure-to-pay penalty still apply to whatever remains unpaid, but hitting 90% protects you from bigger problems if anything goes wrong.
If You Can’t Pay the Full Amount
Being short on cash is not a reason to skip the extension. Filing Form 4868 without a payment still eliminates the 5% monthly failure-to-file penalty, which is ten times the failure-to-pay penalty. Filing nothing and paying nothing is always the worst option.
For extra time, the IRS offers a short-term payment plan of up to 180 days with no setup fee, available through the Online Payment Agreement tool.11Internal Revenue Service. Tax Payment Options Interest and the failure-to-pay penalty continue during the plan, but the arrangement keeps more aggressive collection off the table.
Two programs can reduce the failure-to-pay penalty, though neither touches interest:
- First-time abatement: if you filed on time and had no penalties in the prior three tax years, the IRS may waive the failure-to-pay penalty as a one-time courtesy. You request it by phone or letter.12Internal Revenue Service. Administrative Penalty Relief
- Reasonable cause: circumstances such as serious illness, natural disaster, or unavoidable absence can support penalty relief when documented. The IRS reviews these case by case. Lack of funds alone does not qualify, though it may be considered with other facts.13Internal Revenue Service. Penalty Relief for Reasonable Cause
Because interest cannot be reduced through either program, paying down the balance quickly remains the only real way to limit the total cost.
Your State Extension Is a Separate Question
A federal extension does not automatically extend your state income tax deadline. Some states honor the federal extension without a separate form; others require their own extension request. State interest rates and late-payment penalties vary, and many states start charging interest on the same April date the IRS uses. If you live in a state with an income tax, check the state tax agency’s rules before assuming your federal filing covers you.