990-PF Estimated Tax Payments: Due Dates, Methods, Underpayment

A private foundation that expects to owe $500 or more in Section 4940 excise tax must submit four estimated tax payments during the year, and 990-PF estimated tax payments are due on the 15th day of the 5th, 6th, 9th, and 12th months of the foundation’s tax year. Miss a deadline or underpay an installment and the IRS assesses an interest-based penalty on the shortfall for each quarter it stayed unpaid.

Who Has To Make Estimated Payments

Every domestic private foundation exempt under IRC Section 501(a) owes an annual excise tax of 1.39% on its net investment income.¹ The flat rate replaced the older two-tiered structure for tax years beginning after December 20, 2019.²

If projected excise tax for the year hits $500, quarterly deposits begin.³ Below that threshold, the foundation skips estimated payments entirely and pays any balance when it files Form 990-PF.

The tax base is net investment income: gross investment income (interest, dividends, rents, royalties, securities-loan payments) plus net capital gains from investment property, minus expenses allocable to producing that income.⁴ Capital losses can offset gains but cannot create a net loss, and there are no carrybacks or carryovers.¹

The Four Due Dates

Private foundations sit on a different clock than ordinary corporations. Where a corporate filer’s first installment is due the 15th day of the fourth month, a foundation’s first installment falls a month later, on the 15th day of the fifth month. The remaining installments are due on the 15th day of the 6th, 9th, and 12th months.³

For a calendar-year foundation, that translates to:

1st installment: May 15
2nd installment: June 15
3rd installment: September 15
4th installment: December 15

Any date landing on a Saturday, Sunday, or legal holiday moves to the next business day.⁵ The gap between the first and second payments is only about 30 days, which trips up foundations expecting a normal quarterly rhythm.

One trap worth flagging: an extension on Form 8868 gives the foundation more time to file Form 990-PF, but it does not extend the deadline for the estimated tax itself. The full balance still has to be deposited by the original return due date to avoid interest and penalties.⁶

How Much To Pay Each Quarter

Three methods are available for sizing each installment. The right one depends on how predictable the foundation’s investment income is and whether it counts as a large organization.

Current-Year Method

The default. Forecast full-year NII, multiply by 1.39%, divide by four, and pay that amount each quarter.⁵ If a large gain or unexpected income event shifts the projection during the year, the foundation should recalculate and adjust the remaining installments upward.

Prior-Year Safe Harbor

Paying 100% of the prior year’s excise tax across the four installments is a complete defense against the underpayment penalty, no matter how high the current year’s actual tax turns out.³ Two conditions apply: the prior year must have been a full 12-month tax year, and the prior return must have shown a tax liability greater than zero. First-year foundations and those with no tax last year cannot use the safe harbor and must estimate the current year’s tax instead.

Annualized Income Installment Method

Foundations with uneven investment income (a large capital gain expected late in the year, for example) can compute each installment based on income actually received through that point. For each period, the foundation totals NII through a specified number of months, annualizes it, and computes the tax on the annualized figure.

Default measurement periods are:

1st installment: first 3 months
2nd installment: first 3 months
3rd installment: first 6 months
4th installment: first 9 months

Alternative periods can be elected on Form 8842: 2, 4, 7, and 10 months, or 3, 5, 8, and 11 months.⁷ For each installment, the required payment is the lesser of the standard current-year amount or the annualized amount. A foundation using this method attaches Form 2220 to Form 990-PF to show how the amounts were derived.⁵

The Large-Organization Restriction

A foundation is a “large organization” if its net investment income reached $1 million or more in any of the three preceding tax years.⁸ The threshold is measured in income, not tax, so $1 million in NII produces only about $13,900 in excise tax. Loss carrybacks and carryovers are excluded from the test.⁹

Large organizations lose the prior-year safe harbor for every installment after the first. The first payment can still track last year’s tax, but any shortfall between that amount and the current year’s projected requirement gets rolled into the second installment. From the second installment forward, payments must reflect 100% of the current year’s projected tax.³ For large foundations with hard-to-predict income, the annualized method usually becomes the practical penalty-avoidance tool.

How To Submit Payments

Estimated tax deposits must go through electronic funds transfer.⁵ The standard channel is EFTPS, the Electronic Federal Tax Payment System, which is free and run by the Treasury.¹⁰

New foundations need lead time. After enrollment, the IRS validates the foundation’s information and mails a PIN to the address on file, which takes five to seven business days. Payments themselves must be scheduled by 8 p.m. Eastern the day before the due date to count as timely.¹¹ Foundations that cannot use EFTPS directly can arrange ACH credit or same-day wire payments through a bank, or route deposits through a tax professional or payroll provider. Keep the confirmations. They are the primary evidence of timely payment if the IRS questions a deposit later.

Form 990-W (titled Estimated Tax on Unrelated Business Taxable Income for Tax-Exempt Organizations) is not a payment voucher. It is an optional worksheet for calculating the estimated tax amount, kept in the foundation’s records and not sent to the IRS.⁵

What Happens If You Underpay

Missing an installment or paying too little triggers an addition to tax. The charge is essentially interest on the shortfall for the period it went unpaid, computed independently for each quarter, so a foundation that gets three payments right and misses one still owes a penalty for that quarter.

Three inputs drive the amount: the size of the underpayment, the length of the underpayment period (from the installment due date to the earlier of the payment date or the Form 990-PF due date), and the IRS’s quarterly interest rate. The rate is the federal short-term rate plus three points, reset each quarter. For 2026, the underpayment rate is 7% for the first quarter and 6% for the second.¹²

Three defenses stop the penalty. The prior-year safe harbor blocks it entirely when 100% of last year’s tax was paid across the installments (unavailable to large organizations after the first installment). The annualized income method reduces what was “required” for earlier installments when income was concentrated in later months, provided the foundation attaches a completed Form 2220 showing the calculation. And no penalty applies if total tax on the return is under $500.³

When a penalty is owed, the foundation calculates it on Form 2220, reports it on Part V, line 8 of Form 990-PF, and checks the box on that line if Form 2220 is attached.⁸ Leaving line 8 blank leaves the calculation to the IRS, which usually means a later bill and a slower resolution.

If You Overpay

A foundation that overpaid its estimated tax can file Form 4466 for a quick refund instead of waiting for the return to be processed. The overpayment must be at least $500.¹³ Form 4466 must be filed after the last day of the tax year but before the 15th day of the third month after year-end, or before Form 990-PF is filed, whichever comes first.¹⁴ For a calendar-year foundation, that window generally runs from January 1 to March 15. The IRS acts on the application within 45 days, though applications with material errors or omissions can be denied.

Two Situations Handled Differently

If the foundation also has unrelated business taxable income, it makes separate estimated tax payments for the Section 511 tax on UBTI, using a separate Form 990-W worksheet.¹⁵ The $500 threshold applies independently to each tax. Both use the same quarterly due dates and penalty rules but are tracked and deposited as separate liabilities.

Foreign private foundations are outside this system. Instead of paying the 1.39% excise tax through quarterly deposits, a foreign foundation is generally subject to a 4% withholding tax on its U.S.-source gross investment income, collected by the payer. Claiming the 4% rate (rather than the default 30% withholding) requires giving the payer Form W-8 EXP.¹⁶ The estimated payment process described above applies to domestic exempt private foundations.

1
Office of the Law Revision Counsel. 26 USC Ch. 42 – Private Foundations and Certain Other Tax-Exempt Organizations
2
Internal Revenue Service. Tax on Net Investment Income
3
Office of the Law Revision Counsel. 26 USC 6655 – Failure by Corporation To Pay Estimated Income Tax
4
eCFR. 26 CFR 53.4940-1 – Excise Tax on Net Investment Income
5
Internal Revenue Service. Instructions for Form 990-PF (2025)
6
Internal Revenue Service. Instructions for Form 8868 (01/2026)
7
eCFR. 26 CFR 1.6655-2 – Annualized Income Installment Method
8
Internal Revenue Service. 2025 Instructions for Form 990-PF
9
Internal Revenue Service. 2025 Instructions for Form 2220 – Underpayment of Estimated Tax by Corporations
10
Internal Revenue Service. EFTPS: The Electronic Federal Tax Payment System
11
Electronic Federal Tax Payment System. Welcome to EFTPS Online
12
Internal Revenue Service. Quarterly Interest Rates
13
Internal Revenue Service. Instructions for Form 4466
14
eCFR. 26 CFR 1.6425-1 – Adjustment of Overpayment of Estimated Income Tax by Corporation
15
Internal Revenue Service. Estimated Tax: Unrelated Business Income
16
Internal Revenue Service. Foreign Governments and Certain Other Foreign Organizations