Penalty and interest calculators

Individual tax penalty and interest calculator

Work out the cost of a late or unpaid Form 1040, or of missed estimated payments, with the IRS rates for each quarter. The state tabs load each state's published rules for you to adjust.

Estimates for 2026. Written by Prem Tax and Accounting Corp, a CPA firm. The calculators follow the published federal rules and the state rules we checked, and nothing you type leaves your browser. The IRS or state notice is the official amount, and relief such as first-time penalty abatement or reasonable cause can lower it. This is general information, not tax advice.

Prepared by Prem Tax and Accounting Corp. Last checked against official sources on November 11, 2026.

An estimate, not an IRS bill. The figures follow the federal rules and rates explained on this page. Relief such as first-time penalty abatement or reasonable cause can reduce penalties, and the notice you receive is the official amount.

What this calculator figures

Federal late filing and late payment: both penalties and daily interest on a balance due, with each payment you made applied to the tax first and then to interest.
Federal estimated tax: the Form 2210 penalty by the regular method, with the 90% and last-year safe harbors, the $1,000 threshold and your payment dates.
State return and state estimated tax: pick a state to load its late-filing, late-payment and interest rules, then change any number the state has updated.

The 2026 IRS interest rates

Quarter of 2026	Underpayment rate
January to March	7%
April to June	6%
July to September	7%
October to December	7%

Rates for 2027 are not published yet, so later dates use the October 2026 rate of 7%. A return due in 2026 and filed more than 60 days late carries a minimum late-filing penalty of the smaller of $525 or 100% of the tax.

Common questions

Does an extension stop the penalties?

An extension gives more time to file, not more time to pay. Interest and the late-payment penalty run from the original due date. If you paid at least 90% of the tax by then and pay the rest by the extended due date, the late-payment penalty does not apply.

Why does the penalty keep growing after I file?

The late-payment penalty and interest keep accruing until the tax is paid. Paying part of the balance now lowers both, because they are charged on the tax unpaid at the start of each month.

How reliable are the state results?

Each state tab shows the rates it used, the date they were checked and a link to the source. Treat the result as a good estimate and use the state notice for the final figure.

Official sources

Got a notice or a penalty you do not understand? We check the math, look for relief and deal with the IRS or the state for you. Book a 10-minute call or send us a message.

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