How this calculator works
It runs your retirement month by month: each month your balance earns one month of investment return, then your withdrawal comes out. If you enter an inflation rate, the withdrawal gets a raise every 12 months, because the $2,500 that covers your bills today won't cover them in 2040. The answer is the moment the balance hits zero, or a happy notice that it never does.
That second outcome is real: if your balance earns more each month than you take out, the portfolio grows forever and the honest answer isn't a number of years; it's "indefinitely."
The formula
R is your annual return as a decimal and r is its monthly equivalent. The withdrawal is multiplied by (1 + inflation) at the end of each year. There's no closed-form answer once inflation is involved, which is exactly why this is a simulation and not a one-line formula. If you'd rather solve for a different unknown (say, the monthly payment a balance can support), the time value of money calculator handles the same math from any angle.
Worked example
$500,000 saved, withdrawing $2,500/month, earning 5% a year, with withdrawals rising 3% a year for inflation:
The money lasts about 20 years, 7 months. After 5 years you'd still have about $458,643; after 10 years, $377,270; after 15, $240,273. The decline accelerates as withdrawals grow and the balance shrinks. Ignore inflation and the same money appears to last 34 years, 7 months. That 14-year gap is why the inflation field matters.
How long will $500K, $750K, or $1 million last?
The most-searched version of this question comes with real numbers attached, so here are three, all run through this calculator at a $4,000/month withdrawal, 5% annual return, and 3% inflation:
| Starting balance | How long it lasts | Left after 10 years |
|---|---|---|
| $500,000 | About 11 years, 11 months | ~$114,964 |
| $750,000 | About 19 years | ~$522,187 |
| $1,000,000 | About 27 years, 3 months | ~$929,411 |
Notice the answer isn't linear: $1 million lasts well over twice as long as $500,000 at the same withdrawal. That's because $1 million earning 5% throws off about $4,167/month (very nearly the whole $4,000 withdrawal), so in the early years the balance barely moves, and only the inflation raises eventually tip it into decline. The $500,000 portfolio never gets that grace period; it's shrinking from month one.
The flip side of that math: if you're still a decade out, growing the pile is the highest-leverage move you have. The compound interest calculator shows what steady contributions do to a balance, and the S&P 500 investment calculator runs the same idea against historical market returns.
The 4% rule and its fine print
The classic guideline says: withdraw 4% of your starting balance in year one, raise the dollar amount with inflation each year, and a diversified portfolio has historically survived 30 years. On $500,000 that's about $1,667/month; on $1 million, about $3,333/month. It's a genuinely useful starting point, but it's worth knowing where it came from.
Financial planner William Bengen proposed it in 1994 after backtesting every US retirement start year since 1926 and finding that a 4% initial withdrawal survived even the worst 30-year stretch. The 1998 Trinity study (three Trinity University professors) broadened the test across portfolio mixes and withdrawal rates and reported success percentages instead of a single rule, which is how "4%" hardened into folklore.
Why do 2026 retirees still argue about it? Three reasons. It was built for a 30-year horizon, so anyone retiring at 50 needs a lower rate. It assumes 20th-century US market returns and a roughly half-stock portfolio, and some researchers (Morningstar's annual retirement-income studies among them) have pegged the safe starting rate closer to 3.7% to 4% in recent years, while flexible-spending advocates argue 4.5%+ works fine if you can cut back in bad markets. And it ignores taxes and fees entirely. Treat it as a sanity check, not a guarantee.
You can test it yourself right here: enter your balance, set the withdrawal to 4% of it divided by 12, use a conservative return, and add 3% inflation. At a steady 5% return, $1 million withdrawing $3,333/month lasts about 34 years, 11 months: the rule holding, with little to spare. (You'll get the same answer for $500,000 at $1,667/month: the duration depends on the withdrawal rate, not the size of the pile.)
Sequence-of-returns risk: same average, different retirements
This calculator assumes a smooth, identical return every year. Real markets deliver lumpy ones, and for a portfolio being drawn down, the order of returns matters as much as the average. Picture two retirees with identical balances, identical withdrawals, and identical 20-year average returns. Retiree A catches a deep bear market in years one and two, then a long recovery. Retiree B gets the good years first and the crash near the end. Retiree B coasts through: by the time the bad years arrive, decades of growth have built a cushion. Retiree A spends the crash selling shares at depressed prices to fund the same withdrawals, and the shares sold cheap are gone for good; the recovery compounds on a permanently smaller base. Same average, very different endings.
Here's the non-obvious part: if neither retiree withdrew anything, the order wouldn't matter at all: multiplying returns is commutative, and both would end with identical balances. Withdrawals are what break the symmetry. That's why retiring into a bear market is more dangerous than living through one mid-retirement, and why common defenses all attack the early years: keeping one to two years of spending in cash, trimming withdrawals after down years, or covering baseline expenses with guaranteed income (the annuity payout calculator shows what that trade looks like). Treat this calculator's steady-return answer as a midpoint, not a floor.
What this calculator leaves out
Three things, deliberately. Taxes: withdrawals from a traditional 401(k) or IRA are ordinary income, so if you need $4,000/month to spend and pay an effective 15% tax rate, you actually need to withdraw about $4,700; enter the gross number here. Roth withdrawals are tax-free, which is why the same balance goes further in a Roth. Required minimum distributions: starting at age 73, the IRS forces withdrawals from traditional accounts whether you need the money or not; the RMD calculator computes yours (you can always reinvest the excess in a taxable account, since RMDs are a tax event, not a spending mandate). Social Security: your benefit reduces what the portfolio must cover, so subtract it from your monthly spending and enter only the gap as your withdrawal. A $4,000 budget with $2,200 of Social Security is an $1,800 withdrawal problem, a dramatically easier one.