The math behind this calculator (click to expand)
Accumulation runs the same compounding loop as the compound interest calculator: balance_next = balance * (1 + r) + annual_contribution, year by year from your current age to retirement age, where r is the annual return rate.
Drawdown then iterates forward: each year subtracts the inflation-adjusted withdrawal (less expected Social Security) from the balance, then grows the remainder at the assumed return rate. balance_next = (balance - (withdrawal_year * (1 + i)^(year - retirement_age) - ss_annual)) * (1 + r), where i is the inflation rate. The "years covered" output is the number of years until the balance hits zero. The 4% rule monthly income estimate is simply retirement_balance * 0.04 / 12 using the Bengen/Trinity baseline.
Implementation by Michael.
The 4% Rule and Why It Matters
The 4% rule originates from William Bengen's 1994 research and the subsequent Trinity Study (1998), which analyzed rolling 30-year periods from 1926 to 1995. The finding: a retiree who withdrew 4% of their portfolio in year one, then adjusted that dollar amount for inflation annually, never ran out of money over any historical 30-year window with at least a 50% stock allocation.
The rule has attracted justified skepticism. The original research used a period with generally higher bond yields and strong equity returns. Morningstar's 2024 update suggested 3.7% as a safer starting withdrawal rate given current valuations. But the 4% rule was never meant as a precise prescription - it's a stress-tested baseline. This calculator uses it to generate your monthly income estimate, which you can then compare against your actual spending target. If your projected 4% income exceeds your spending needs by a comfortable margin, you have a meaningful cushion against adverse market conditions.
How Much Do You Actually Need?
The "multiply your annual spending by 25" shortcut is the 4% rule in reverse. If you expect to spend $6,500/month ($78,000/year) in retirement, you need roughly $1,950,000 in invested assets. But that number shifts dramatically with a few variables.
Social Security changes the math. If you receive $2,100/month from SSA, your portfolio only needs to cover $4,400/month ($52,800/year), dropping the target to $1,320,000 - a $630,000 reduction. A couple both claiming Social Security at $1,800 and $2,400/month needs their portfolio to cover even less.
Healthcare is the expense most people underestimate. Fidelity's 2025 Retiree Health Care Cost Estimate puts the average 65-year-old couple's lifetime healthcare spending at $351,000, which works out to roughly $1,170/month over 25 years. If your $6,500/month spending target doesn't explicitly include healthcare premiums and out-of-pocket costs, add $1,000-1,500/month and recalculate.
Social Security's Role in Your Plan
The average Social Security retirement benefit sits at approximately $1,920/month in early 2026, but your actual benefit depends on your 35 highest-earning years and claiming age. Claiming at 62 permanently reduces benefits by roughly 30% compared to your full retirement age (67 for those born after 1960). Waiting until 70 increases benefits by about 24% over the full retirement age amount.
For a high earner hitting the Social Security wage base ($176,100 in 2026), the maximum monthly benefit at full retirement age is approximately $4,018. At age 70, that climbs to about $4,982. These are meaningful income streams - a couple maximizing benefits could receive $8,000-10,000/month, covering a substantial portion of retirement spending and dramatically reducing the required portfolio size.
The program's funding uncertainty is real but manageable for planning. Even the worst-case scenario - trust fund depletion projected for 2033 - would result in roughly 79% of scheduled benefits paid from ongoing payroll taxes. If you're under 50, modeling 75-80% of your projected benefit is a pragmatic approach.
Contribution Limits and Tax-Advantaged Growth
The 2026 contribution limits: $23,500 for 401(k)/403(b) plans, $7,000 for IRAs, plus a $7,500 catch-up for 401(k) participants over 50 and $1,000 for IRA participants over 50. If you max out a 401(k) and an IRA, that's $30,500/year ($2,541/month) growing tax-deferred.
The difference between tax-sheltered and taxable compounding is substantial over decades. Consider $2,000/month invested at 7% for 30 years. In a tax-deferred account, this grows to approximately $2,440,000. In a taxable account with 15% annual tax on dividends (roughly 2% of the return), the effective return drops to about 6.7%, yielding approximately $2,330,000. That $110,000 gap comes purely from the annual tax drag on compounding. And that's before considering capital gains taxes on rebalancing or high-turnover funds.
The mega backdoor Roth - contributing after-tax dollars to a 401(k) above the $23,500 limit and converting to Roth - allows up to $70,000 total annual 401(k) contributions in 2026 (including employer match). Not every plan supports it, but if yours does, it's one of the most powerful accumulation strategies available for high earners already maxing standard limits.
Sequence of Returns Risk
A 7% average annual return doesn't mean 7% every year. If your portfolio drops 30% in your first year of retirement, the damage is disproportionate - you're withdrawing from a reduced base, leaving less to recover during eventual upswings. This "sequence of returns risk" is why the first 5-10 years of retirement are the most vulnerable period.
Concrete example: Two retirees start with $1,250,000 and withdraw $50,000/year (4%). Both average 7% over 25 years. Retiree A gets the bad years first (-15%, -10%, +5%, then strong growth). Retiree B gets the good years first. After 25 years, Retiree B has $1.8M remaining. Retiree A runs out in year 22. Same average return, opposite outcomes.
The standard hedge: keep 2-3 years of spending in cash or short-term bonds. When equities drop, spend from the cash buffer instead of selling stocks at a loss. This calculator uses a conservative 5% return during drawdown (versus your accumulation rate) to partially account for a more conservative retirement allocation, but real-world sequence risk requires a more dynamic strategy than any single-rate model can capture.
What might change in the next 24 months
Three pieces to watch on the retirement-planning side. First, Social Security: the SSA Trustees' 2024 report projects the OASI trust fund can pay full benefits through 2033, after which incoming payroll taxes would cover roughly 79% of scheduled benefits absent legislation. That's seven years out at the time of writing, with multiple proposals under discussion (raising or eliminating the wage base cap, adjusting full retirement age, modifying the bend points). Conservative planning under 40 typically uses 75 to 80% of the projected benefit.
Second, RMD ages under SECURE 2.0: the required minimum distribution age is 73 for those born 1951-1959 and 75 for those born 1960 or later. That delay extends the runway for Roth conversions in the gap between retirement and the RMD start, where ordinary income is typically lower. The conversion window is one of the most consequential planning levers SECURE 2.0 introduced.
Third, healthcare costs continue to outpace headline inflation. Fidelity's annual estimate has risen by an average of about 5% per year over the past decade, faster than the 3% long-run CPI baseline. Build the gap into your spending assumption rather than hoping CPI captures it.