6621 Interest Calculator (IRC §6621 / §6622)

6621 interest calculator that computes IRS underpayment interest under IRC §6621 and §6622, commonly used for PFIC §1291 and Form 8621 Line 16.

Instructions: For IRC §1291, select Allocated Year and Return Year.
The tool auto-fills statutory dates and computes the Accrual Factor.
Enter Deferred Tax to view interest. Dates are adjustable for other calculation needs.

Allocated Year

Return Year

Deferred Tax ($)

Due Date

Payment Date

Accrual Factor

Total Deferred Interest

$0.00

Total Tax + Interest

$0.00

Period (Quarterly)	Rate	Days	Interest Added	Running Balance

About this Tool

How is IRS interest calculated under §6621?
IRS underpayment interest is calculated using daily compounding under IRC §6622, based on quarterly rates published under IRC §6621(a)(2).

This calculator implements the specific interest rules required for PFIC Excess Distributions under IRC §1291(c)(3).

IRC §6621(a)(2): Uses the federal short-term underpayment rate plus 3 percentage points.
IRC §6622: Interest is compounded daily.
IRC §7503: Automatically adjusts start/end dates if they fall on a weekend or legal holiday.
COVID-19 Postponements: Reflects the general due date postponements announced in IRS Notice 2020-23 and Notice 2021-21 where applicable. Users should independently verify applicability to their specific facts.

Note on Accuracy: This tool is designed to replicate the standard IRS interest computation methodology used for the "Interest Charge" reported on Form 8621, Line 16, based on published rates and statutory due dates. Actual filing results may vary depending on elections, extensions, or taxpayer-specific facts.

View Historical IRS §6621 Interest Rates (1987–Present)

👉 Rates shown are the quarterly short-term underpayment rates (IRC §6621(a)(2)).

Year	Q1 (Jan-Mar)	Q2 (Apr-Jun)	Q3 (Jul-Sep)	Q4 (Oct-Dec)

Frequently Asked Questions

How is IRS interest calculated under §6621?

IRS underpayment interest is calculated using daily compounding under IRC §6622, based on quarterly interest rates published under IRC §6621(a)(2). The rate generally equals the federal short-term rate plus three percentage points and applies from the statutory due date of the tax until the payment date.

Does IRS interest compound daily or annually?

IRS interest compounds daily, not annually. Daily compounding applies to underpayment interest, including interest charged on PFIC excess distributions reported on Form 8621.

How is interest calculated on PFIC excess distributions?

For PFIC excess distributions under IRC §1291, interest is calculated on the deferred tax amount allocated to each prior year. Interest accrues from the statutory due date of the return for that year (without extensions) through the due date of the current return, using IRS daily compounded rates.

Which part of Form 8621 includes the IRS interest charge?

IRS interest on PFIC excess distributions is reported on Form 8621, Line 16 as part of the “Interest Charge” calculation. The amount reflects interest computed under IRC §6621 and §6622 on the deferred tax.

Can this calculator be used to file Form 8621?

No. This calculator is provided for verification and informational purposes only. It does not prepare or file Form 8621, determine filing obligations, or replace professional tax advice.

This calculator is provided for informational and verification purposes only and does not constitute tax advice or a determination of filing obligations.