Calculate the fund needed to bridge an early retirement income gap or the monthly spending an available bridge fund can support until benefits begin.
Early Retirement Bridge Formula
A retirement bridge fund covers an income gap between leaving work and the start of Social Security, a pension, or another income source.
Convert nominal return to real return:
r_{real}=(1+r)/(1+i)-1
Present value of monthly withdrawals:
BridgeFund=MonthlyGap*[1-(1+r_m)^{-N}]/r_m
To solve for supported spending:
MonthlyGap=BridgeFund/AnnuityFactor
Variables:
- r is nominal annual return
- i is annual inflation
- rm is equivalent real monthly return
- N is bridge months
Fund-needed mode starts from the monthly gap. Supported-gap mode starts from a known bridge account.
Using a real rate keeps the spending input in today's purchasing power while allowing it to rise with inflation.
The fund is measured at the bridge start age, not at the current age when those dates differ.
Illustrative Bridge Needs
Approximate funds for a $3,000 real monthly gap at a 2.44 percent real annual return.
| Bridge length | Undiscounted gap | Approximate starting fund |
|---|---|---|
| 3 years | $108,000 | About $104,000 |
| 5 years | $180,000 | About $169,000 |
| 7 years | $252,000 | About $231,000 |
| 10 years | $360,000 | About $319,000 |
Example Problems
Example 1: Calculate the bridge fund.
A bridge from age 60 to 67 lasts 84 months. Discount $3,000 monthly withdrawals using the real monthly return.
Example 2: Calculate supported spending.
Divide a $200,000 fund by the 84-month annuity factor to estimate the monthly real gap it can cover.
Frequently Asked Questions
What income belongs in the gap?
Subtract reliable bridge-period income from planned monthly spending.
Should the return be conservative?
Yes. A short withdrawal period is exposed to sequence risk, so test lower returns and a cash reserve.
Does the bridge include taxes?
Only if the monthly gap already includes taxes or withdrawals are entered on an after-tax basis.
