Tax Extension Calculator (Form 4868)
Missed the April 15, 2026 deadline? Estimate IRS failure-to-file and failure-to-pay penalties plus daily-compounded interest through your actual filing date — with or without a Form 4868 extension. See how much more you owe for every month you delay.
Your Tax Situation
How IRS Extension Penalties Work in 2026
If you miss the April 15, 2026 federal tax deadline without filing Form 4868, the IRS charges two separate penalties on top of interest. The failure-to-file penalty is 5% of unpaid tax per month (or part of a month), capped at 25% after five months. The failure-to-pay penalty is 0.5% per month, also capped at 25%. When both apply in the same month, the failure-to-file penalty is reduced by the 0.5% failure-to-pay amount — so the combined monthly rate is 5%, not 5.5%. On top of penalties, the IRS charges interest at the federal short-term rate plus 3% — currently 8% annualized for Q2 2026 — compounded daily on the unpaid balance including accrued penalties. Last updated: April 2026.
Why Filing Form 4868 Cuts Penalties 10x
Filing Form 4868 by April 15 gives you until October 15, 2026 to file your return. The extension does NOT give you extra time to pay — but it slashes the failure-to-file penalty from 5% per month to zero. You still owe the 0.5% failure-to-pay penalty on any unpaid balance, plus daily interest. That is a 10x reduction in monthly penalties: without an extension, six months of delay costs 25% in penalties alone; with an extension, the same six months cost just 3%. Filing the extension takes five minutes on IRS Free File and is approved automatically — there is almost no reason not to file it if you think you will miss the deadline.
Tax Extension Calculator vs IRS Notice CP14
After you file late, the IRS sends a CP14 notice showing the exact penalty and interest amounts. This calculator estimates those numbers in advance so you can decide whether to pay now or wait. It applies the same monthly penalty caps (25% max failure-to-file, 25% max failure-to-pay) and the current 8% annual interest rate compounded daily. Actual IRS figures may differ slightly because interest rates can change quarterly and partial-month periods always round up to a full month for penalty calculations. If you qualify for first-time penalty abatement — which applies to taxpayers with a clean three-year compliance record — the IRS may waive the failure-to-file and failure-to-pay penalties entirely on request.
Tips to Minimize Extension Costs
Pay as much as you can by April 15, even if you cannot pay in full — each dollar paid stops the 0.5% monthly penalty and 8% interest on that amount immediately. File Form 4868 online through IRS Free File, TurboTax, or IRS Direct Pay (the payment itself counts as an extension request if you check the box). If you cannot pay the full amount, set up an installment agreement on Form 9465 — the failure-to-pay rate drops to 0.25% per month once the agreement is active. Members of the military in combat zones and taxpayers in federally declared disaster areas get automatic extensions with no penalties. Always file the return by October 15 even if you cannot pay — the failure-to-file penalty is 10x the failure-to-pay penalty, so filing is always the priority.