How Social Security and Pension Change the Coast Fire Calculation
Social Security and defined-benefit pension income reduce the amount your investment portfolio needs to generate in retirement. This directly shrinks your FIRE number — and therefore your coast fire number.
The key principle: Social Security and pension income are spending offsets, not invested assets. They reduce how much your portfolio needs to provide each year, which reduces your FIRE target, which reduces the coast fire number needed today. They should never be counted in your Current Invested Assets field — only in the Annual Spending adjustment.
How to Use Coast Fire Calculator with Social Security
Before benefits begin, it separately values the bridge years. After claiming begins, it subtracts the modeled benefit from spending and discounts that later portfolio need back to retirement.
Use the benefit estimate from your my Social Security account rather than a generic national average.
Change any live input to update the comparison automatically. The generic homepage field is limited to income beginning at retirement; this dedicated calculator is the appropriate tool when Social Security begins later.
The Impact on Your Coast Fire Number
The illustration below shows a simplified age-35 comparison at a 7% real return when income starts at retirement. For a later start, use the live bridge model above.
| Annual Spending | SS Benefit | Net Spending | FIRE Number | Coast Fire Number (Age 35) |
|---|---|---|---|---|
| $60,000 | $0 (excluded) | $60,000 | $1,500,000 | $197,051 |
| $60,000 | $12,000 | $48,000 | $1,200,000 | $157,641 |
| $60,000 | $14,000 | $46,000 | $1,150,000 | $151,072 |
| $60,000 | $20,000 | $40,000 | $1,000,000 | $131,367 |
Under those assumptions, a $14,000 annual benefit beginning at retirement reduces the modeled coast number by $45,979. A later claim age produces a smaller reduction because the portfolio must first fund the bridge period.
How to Use Coast Fire Calculator with Pension
The method for a coast fire calculator with pension is identical to the Social Security approach:
Example: $70,000 planned spending − $22,000 pension = $48,000 adjusted spending
Enter $48,000 as Annual Spending in the coast fire calculator.
Important: Enter only liquid invested assets (401k, IRA, brokerage) in Current Assets. Do not include pension capitalized value — pension income is a spending offset only.
For defined benefit pensions with a lump-sum commutation option: if you plan to take a lump sum at retirement and invest it, that amount can be included in your projected retirement portfolio — but should not be counted in your Current Invested Assets today (it's not yet invested and compounding).
How Conservative to Be with SS and Pension Estimates
The core risk with relying on Social Security or pension income in your coast fire calculation is that these income streams can change. Key considerations:
- 2026 OASI stress scenario — The 2026 Trustees Report projects OASI reserve depletion in Q4 2032, with continuing income sufficient for 78% of scheduled OASI benefits at that time. The combined OASDI projection is Q3 2034 with 83% payable. The calculator's 78% option is a dated stress test, not a prediction of your individual benefit.
- Benefit age and cohort matter — The calculator derives full retirement age from birth year and applies SSA's monthly early-claiming reductions and delayed-retirement credits. Your SSA statement remains the best input because survivor, disability, earnings-test, and other rules can differ.
- WEP/GPO repeal — The Social Security Fairness Act, signed January 5, 2025, repealed WEP and GPO for benefits payable after December 2023. This calculator does not reduce future benefits for either provision.
- Pension stability — Private sector pensions can be reduced if the plan becomes underfunded. Government pensions are generally more stable but not completely risk-free.
- Planning horizon length — The longer your planning horizon, the less certainty you have about these income streams. For a 35-year-old, a 30-year horizon increases the uncertainty of both SS and pension income.
For comparison, run both the scheduled-benefit and excluded-benefit scenarios above. The difference shows sensitivity to this assumption; it is not a recommendation about which result to adopt.
RSU and Non-Standard Income
For workers with RSU (Restricted Stock Unit) compensation: unvested RSUs should not be counted in Current Invested Assets. Once RSUs vest, the resulting shares can be included if held as invested assets. For the coast fire calculator pension RSU scenario: handle them separately — RSU income that will vest before retirement can be modelled as a one-time future contribution rather than a standing asset.