What Is Actuarial Funding? Methods, Minimums, and PBGC Premiums

Actuarial funding is how a defined benefit pension plan figures out, in today’s dollars, how much money it needs on hand to pay the retirement benefits it has promised to deliver years or decades from now. The calculation pulls together participant data, investment return expectations, salary projections, and mortality estimates into a single number the sponsor is legally required to fund. Federal law backs the math with minimum contribution rules, benefit restrictions tied to funded status, insurance premiums owed to the Pension Benefit Guaranty Corporation, and annual filings that carry stiff penalties when they slip.

What Goes Into the Calculation

The actuary begins with a census of every participant: age, service, salary history, and benefit elections. That data feeds two sets of assumptions.

Economic assumptions deal with money. The most consequential is the discount rate, which reflects what the plan expects to earn on its invested assets. Set it too high and future obligations look cheaper than they really are, creating a real funding gap later. The actuary also projects salary growth and inflation, since many pension formulas tie the final benefit to pay near retirement.

Demographic assumptions deal with people. Mortality tables predict how long retirees will collect checks. Disability and turnover rates estimate who leaves early and who quits before vesting. Together these forecasts determine how many people the plan will pay, for how long, and starting when.

Federal law requires each assumption to be individually reasonable based on the plan’s own experience and current expectations, and the full set must represent the actuary’s best estimate of what the plan will actually face.¹ Unrealistic assumptions can lead to severe underfunding, excise taxes, and plan collapse.

How the Discount Rate Is Set

The discount rate is not a free choice. Single-employer plans must use three segment rates derived from corporate bond yields published by the IRS, each covering a different time horizon of expected benefit payments.² Because market rates can swing dramatically year to year, Congress added a stabilization mechanism. Plans compare the current segment rates to a 25-year historical average and apply a corridor that limits how far the valuation rate can deviate from that average.

For plan years beginning in 2026, the corridor holds rates within 95% to 105% of the 25-year average, and any segment whose 25-year average falls below 5% is treated as 5%.¹ When market rates drop sharply, stabilization keeps the valuation rate from cratering and causing a sudden spike in required contributions. The tradeoff is that smoothing can also mask genuine underfunding during long low-rate stretches.

Cost Methods That Shape the Yearly Bill

Once the assumptions are set, the actuary needs a framework for spreading the plan’s total cost across individual years. Two outputs matter most: the normal cost (the price of benefits being earned in the current year) and the actuarial accrued liability (the value of benefits already earned for past service).

The entry age normal method is the most common approach for single-employer plans. It calculates a level percentage of pay that, if contributed every year from hire to retirement, would exactly fund each participant’s projected benefit. Contributions stay stable and predictable even as the workforce ages.

The projected unit credit method takes a different angle, focusing on the specific benefit a participant earns in a single year, projected to retirement using estimated final salary. Costs start low for younger workers and climb as employees approach retirement. The method tracks the legal right to benefits more precisely but creates a contribution pattern that gets more expensive as the workforce ages. The total lifetime cost of the plan is the same under either method; only the timing changes.

Minimum Required Contributions

Federal law does not leave contribution amounts to the sponsor’s discretion. IRC Section 430 and ERISA Section 302 set minimum funding standards for single-employer defined benefit plans.¹ The core calculation compares the funding target (the present value of benefits earned to date) to the current value of plan assets. If assets fall short, the sponsor must contribute more.

The minimum required contribution has two pieces. The target normal cost covers benefits expected to be earned during the current year plus anticipated expenses. If the plan is underfunded, the sponsor must also make shortfall amortization installments to close the gap. For plan years beginning after 2021, shortfall amortization bases are paid off over 15 years in level annual installments.¹

The plan’s funded status is expressed as the funding target attainment percentage, or FTAP, which equals plan assets divided by the funding target.¹ That single number drives nearly every downstream consequence: benefit restrictions, at-risk loading, premium costs, and reporting obligations.

Quarterly Installments for Underfunded Plans

Plans that had a funding shortfall in the prior year cannot wait until year-end to pay. They must make quarterly installments on April 15, July 15, October 15, and January 15 of the following year. Each installment equals 25% of the required annual payment, generally the lesser of 90% of the current year’s minimum required contribution or 100% of the prior year’s contribution.¹

Missing an installment triggers an interest penalty equal to the normal underpayment rate plus five percentage points, which makes catching up expensive and rewards staying current even when cash flow is tight.

Consequences of Being Underfunded

When a plan’s adjusted funding target attainment percentage (AFTAP) drops below set thresholds, IRC Section 436 automatically restricts what the plan can pay and how it can be amended.

Below 80% AFTAP, the plan cannot adopt any amendment that increases liabilities, such as raising benefit levels or accelerating vesting.³
Between 60% and 80% AFTAP, lump-sum distributions and other accelerated payouts are partially restricted. The plan can pay the lesser of 50% of what the participant would otherwise receive or the present value of the PBGC maximum guarantee.³
Below 60% AFTAP, the plan cannot pay lump sums or other accelerated distributions, cannot pay shutdown or other unpredictable contingent event benefits, and must freeze all future benefit accruals.³

Sponsors near the 60% cliff often make additional voluntary contributions specifically to push the AFTAP above that line and avoid a freeze.

At-Risk Status

A plan is considered at-risk when the prior year’s FTAP was below 80% and the at-risk FTAP was below 70%.⁴ Plans with 500 or fewer participants are exempt.

At-risk status changes the funding target. The actuary must assume every participant eligible to retire early will do so at the earliest possible date and elect the most expensive form of benefit available. The at-risk funding target also carries a loading factor of $700 per participant plus 4% of the standard funding target, and the target normal cost gets a 4% loading as well.⁴

PBGC Premiums

Every covered defined benefit plan pays annual premiums to the Pension Benefit Guaranty Corporation. For 2026, single-employer plans owe a flat-rate premium of $111 per participant, and multiemployer plans owe $40 per participant.⁵

Underfunded single-employer plans also owe a variable-rate premium of $52 for every $1,000 of unfunded vested benefits, capped at $751 per participant for 2026.⁵ For a large, poorly funded plan, the variable-rate premium alone can run into the millions, which creates its own financial incentive to improve funded status.

Premium filings for calendar-year plans are due October 15, 2026, the 15th day of the 10th full calendar month of the plan year.⁵

Annual Reporting and Missed Contribution Notices

Plan sponsors document compliance annually on Form 5500, filed jointly with the Department of Labor, IRS, and PBGC.⁶ The actuarial details live on Schedule SB for single-employer plans and Schedule MB for multiemployer plans.⁷

Form 5500 is due the last day of the seventh month after the plan year ends, which is July 31 for calendar-year plans. An extension of up to two and a half months is available by filing Form 5558 before the original deadline, and all filings run through the EFAST2 electronic system.⁶

An Enrolled Actuary must sign Schedule SB or MB, certifying that the assumptions are individually reasonable, that in combination they represent the actuary’s best estimate, and that the calculations comply with applicable law.⁷ That signature carries personal professional liability.

Separately, when a sponsor misses a required contribution and the cumulative unpaid balance with interest exceeds $1 million, the plan must notify the PBGC on Form 200 within 10 days of the missed due date.⁸ The notice applies to the contributing sponsor and any controlled group member. Curing the missed contribution within 30 days of the due date waives the broader reportable event notice but does not waive the Form 200 itself.

Penalties for Funding and Filing Failures

An employer that fails to make the minimum required contribution faces an excise tax of 10% of the unpaid amount. If the deficiency is still uncorrected by the end of the taxable period, the tax jumps to 100% of the unpaid amount.⁹

On the filing side, the IRS imposes a penalty of $250 per day for late Form 5500 filings, up to a maximum of $150,000.¹⁰ The Department of Labor imposes a separate penalty that currently runs over $2,800 per day with no cap. Both penalties apply at the same time.

Surplus Reversions on Plan Termination

When a fully funded plan terminates and assets remain after all benefits are paid, any surplus that reverts to the employer is hit with a 20% excise tax. Without a qualified replacement plan or specified benefit increases to participants, the rate climbs to 50%.¹¹ The reversion is also taxable as ordinary income on top of the excise tax, so the combined effective rate can consume most of the surplus. Employers in bankruptcy liquidation are exempt from the 50% rate.

1
Office of the Law Revision Counsel. 26 USC 430 – Minimum Funding Standards for Single-Employer Defined Benefit Pension Plans
2
Internal Revenue Service. Pension Plan Funding Segment Rates
3
Office of the Law Revision Counsel. 26 USC 436 – Limits on Benefits and Benefit Accruals Under Single-Employer Defined Benefit Plans
4
eCFR. 26 CFR 1.430(i)-1 – Special Rules for Plans in At-Risk Status
5
Pension Benefit Guaranty Corporation. Comprehensive Premium Filing Instructions for 2026 Plan Years
6
U.S. Department of Labor. Form 5500 Series
7
U.S. Department of Labor. Schedule SB (Form 5500) – Single-Employer Defined Benefit Plan Actuarial Information
8
eCFR. 29 CFR 4043.81 – PBGC Form 200, Notice of Failure to Make Required Minimum Funding Payment
9
Office of the Law Revision Counsel. 26 USC 4971 – Taxes on Failure to Meet Minimum Funding Standards
10
Internal Revenue Service. 401(k) Plan Fix-It Guide – You Haven’t Filed a Form 5500 This Year
11
Office of the Law Revision Counsel. 26 USC 4980 – Tax on Reversion of Qualified Plan Assets to Employer