Decision workflow
Project your savings to retirement, compare them with what your lifestyle actually requires, and get a concrete monthly number to close any gap.
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Last reviewed
July 10, 2026
Content update
Auto-updated on Jun 28, 2026
Scope: This workflow projects savings to retirement age, computes the corpus needed to fund inflation-growing expenses through retirement, and reports the gap. Social Security, account wrappers (401(k)/IRA), and taxes are not modelled; returns are your post-tax assumptions.
Primary references
How to fill this quickly
When the paychecks stop.
Plan long; outliving the money is the failure mode.
Only expenses that continue into retirement.
All earmarked retirement accounts combined.
Total going toward retirement each month.
How much you raise the monthly saving each year.
Long-term post-tax expectation, not a peak year.
Use a conservative, lower-risk portfolio return.
Most US retirement rules of thumb compress a hard question into one number — 25× your spending, the 4% rule, “a million dollars.” They hide the two things that actually decide your outcome: inflation between now and retirement, and how long the corpus must keep paying you afterward. At 2.5% inflation, a $4,000-a-month lifestyle at age 30 costs about $9,493 a month at 65 — and that number keeps growing through 25 years of retirement.
This workflow answers the question directly instead. It computes the corpus that funds your inflation-growing expenses from retirement to your planning age, projects what your current savings and monthly contributions will actually become, and reduces the comparison to one readiness score plus a concrete monthly amount that closes any gap. It is deliberately simpler than a financial plan: Social Security is left out (treat your ssa.gov estimate as a buffer, or subtract part of it from the expense input), and 401(k)/IRA wrappers are treated as one pool — enter combined balances and use post-tax return assumptions.
Against the familiar 25× rule, this method is more precise in both directions: it inflates your spending to retirement age first (the step most people skip), and then sizes the corpus off your actual post-retirement return and time horizon rather than a fixed multiple. For the default profile it needs $2.17 million — less than the $2.85 million a naive 25× of retirement-age spending implies, because the corpus keeps earning above inflation while paying out.
The default profile: 30 years old, retiring at 65, planning to 90, spending $4,000 a month today, with $30,000 saved and $800 a month going in, stepped up 3% a year. Assumptions: 2.5% inflation, 8% return before retirement, 5% after. Required corpus at 65: $2,165,053. Projected corpus: $443,560 from the existing savings plus $2,532,984 from contributions — $2,976,545, a 137% readiness score, enough to sustain about $5,499 a month in today’s money.
The two levers that dominate: without the 3% annual step-up the same saver lands at 105% — still funded, but with the safety margin gone. Start the identical plan at 45 instead of 30 and readiness collapses to 48.6%, a $768,609 shortfall that costs $1,305 in extra monthly savings to close. Time in the market is doing most of the work; the earlier dollars are simply worth more.
Required corpus is the present value of a growing annuity-due: first-year retirement expenses (today’s expenses inflated to retirement age), withdrawn at the start of each year, growing at inflation g while the remaining corpus earns the post-retirement return r — corpus = E × (1 − xⁿ)/(1 − x) with x = (1+g)/(1+r). Projected corpus = current savings compounded to retirement plus the future value of monthly contributions with the annual step-up applied.
Simplifications to know: Social Security and pensions are excluded (a deliberate buffer, or subtract them from expenses); taxes and account wrappers are not modelled, so use post-tax return assumptions; and the plan assumes spending grows exactly with inflation, while real retirees often spend more early and less late.
It depends on spending, retirement age, and planning horizon — not a universal figure. For a 30-year-old spending $4,000 a month who retires at 65 and plans to 90 (2.5% inflation, 5% post-retirement return), the requirement is about $2.17 million at retirement. The biggest sensitivity is the expense input: every $500 a month of retirement lifestyle moves the requirement by roughly $270,000.
For someone retiring decades from now, usually not: for the default 30-year-old profile, $1 million at 65 sustains about $1,848 a month in today’s purchasing power through 25 years. For someone retiring soon with a paid-off home and Social Security on top, it can be — the calculator lets you test your own numbers instead of arguing about the folklore.
The 4% rule (withdraw 4% of the corpus in year one, then adjust for inflation) is a backward-tested heuristic that implies a 25× corpus. This workflow computes the corpus directly from your own post-retirement return, inflation, and horizon — for the defaults it implies an initial withdrawal rate of about 5.3%, higher than 4% because the horizon is a defined 25 years rather than open-ended. If you want 4%-rule conservatism, lower the post-retirement return input.
It is deliberately not modelled — benefit amounts depend on your earnings record and claiming age, and the honest source is your own statement at ssa.gov. Two clean ways to use it here: treat the benefit as a safety buffer on top of the plan, or subtract a conservative fraction of the estimated benefit from your monthly expense input.
Enter their combined balances as current savings and your total monthly contributions (including any employer match) as the saving amount. The wrappers differ in tax treatment — contribution limits also change year to year, so check current IRS figures — but the corpus math is the same pool of invested money. Use post-tax return assumptions if most of your savings are pre-tax.
More than intuition suggests. The default plan started at 45 instead of 30 is only 48.6% funded and needs $1,305 more per month to close the gap — versus $800 total contributions had it started at 30. Each year of delay removes the cheapest, most-compounded dollars from the plan.