The math behind this calculator (click to expand)
Accumulation uses monthly contributions. The nominal annual effective return r is converted to m = (1 + r)^(1/12) - 1, then each month applies balance_next = balance * (1 + m) + monthly_contribution. That conversion makes a 7% input compound to 7% over a full year rather than slightly more.
Drawdown uses the same monthly-rate conversion for a fixed 5% nominal return and 2.5% inflation/COLA assumption. Each month applies growth, then subtracts spending minus that month's Social Security payment. The Social Security offset is zero before your claim age; the entered benefit is the first payment at claim age and receives the modeled COLA from then onward, including the years before retirement when claim age is earlier. The "years covered" output is the number of years until the balance hits zero or age 90. 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. In the cited 50/50 stock/bond case, a 4% initial withdrawal followed by annual inflation adjustments had a 95% success rate over 30 years. That is strong historical evidence, not a promise that a portfolio can never run out.
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 $345,000, which works out to roughly $1,150/month over 25 years and excludes long-term care. 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
SSA estimates the average retired-worker benefit at $2,071/month for January 2026, but an individual's benefit depends on covered earnings and claiming age. For people born in 1960 or later, claiming at 62 can reduce the worker benefit by 30% versus claiming at the full retirement age of 67. Delayed retirement credits stop at age 70.
The 2026 Social Security wage base is $184,500, and SSA lists a maximum 2026 benefit of $4,152/month for a worker retiring at full retirement age. That maximum requires a high covered-earnings history; use your own SSA estimate rather than treating it as a default.
The 2026 Trustees Report projects OASI reserve depletion in the fourth quarter of 2032; continuing income would cover 78% of scheduled OASI benefits then absent legislation. The hypothetical combined OASDI projection is depletion in 2034 with 83% payable. Testing the calculator at both the scheduled benefit and a reduced percentage makes the uncertainty visible without assuming Congress's response.
Contribution Limits and Tax-Advantaged Growth
The 2026 contribution limits: $24,500 for 401(k)/403(b) plans, $7,500 for IRAs, plus an $8,000 catch-up for 401(k) participants over 50 and $1,100 for IRA participants over 50 (IRS Notice 2025-67). If you max out a 401(k) and an IRA, that's $32,000/year ($2,667/month) growing tax-deferred.
The difference between tax-sheltered and taxable compounding is substantial over decades. Consider $2,000/month invested at a 7% annual effective return for 30 years. Under this calculator's monthly-contribution convention, that grows to approximately $2,338,900. If annual tax drag reduces the effective return to 6.7%, the same contributions grow to approximately $2,213,500. That roughly $125,400 gap comes purely from the lower compounding rate. Actual taxable-account drag depends on yield, turnover, tax rates, and the timing of gains.
The mega backdoor Roth - contributing after-tax dollars to a 401(k) above the $24,500 limit and converting to Roth - allows up to $72,000 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
First, Social Security: the June 2026 Trustees Report projects OASI reserve depletion in the fourth quarter of 2032, with 78% of scheduled OASI benefits payable from continuing income then absent legislation. On the hypothetical combined OASDI basis, the projection is 2034 and 83%. Legislation could change taxes, benefits, or both before those dates, so keep a reduced-benefit scenario in the plan.
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.