Retirement Calculator

About the Retirement Calculator

The retirement calculator projects the corpus you will accumulate by a chosen retirement age, given your current age, current savings, monthly contribution, and expected annual return. It then applies the 4% rule to estimate the safe annual withdrawal your corpus can support, and estimates how long the corpus would last under a fixed annual withdrawal at the expected return. The result is a back-of-the-envelope projection you can use to stress-test your savings rate against your retirement spending target.

The 4% rule originates from the Trinity Study (Cooley, Hubbard, and Walz, 1998, "Sustainable Withdrawals from the Stock-Market Boom"), which examined historical U.S. stock and bond returns from 1926 through 1995 and concluded that a 4% initial withdrawal rate, adjusted annually for inflation, was sustainable over a 30-year retirement in roughly 95% of historical scenarios. The rule has become the standard planning heuristic, though recent research — particularly Pfau (2012) — argues that lower withdrawal rates (3-3.5%) may be more appropriate given current equity valuations and lower bond yields.

The corpus projection uses the same compound growth and annuity formulas as the compound interest and savings goal tools, but extended over a multi-decade horizon where the time value of money is most dramatic. A 30-year-old starting with $25,000 and saving $500/month at 7% until age 65 will accumulate about $1,188,000 — of which only $235,000 is contributions ($25,000 starting plus $210,000 monthly over 35 years) and $953,000 is investment growth. Starting 10 years later at age 40 with the same starting balance and contribution cuts the corpus to roughly $548,000, demonstrating why starting early matters more than the contribution level itself.

This calculator does not model Social Security benefits, pension income, tax-advantaged account contribution limits (such as the IRC Sec. 402(g) limit of $23,000 for 401(k) employee contributions in 2024), required minimum distributions under IRC Sec. 401(a)(9), or inflation. For real retirement planning, you need a more sophisticated model — but this tool gives you the foundational math to sanity-check your savings rate.

How It Works

The corpus at retirement is the sum of the future value of current savings and the future value of monthly contributions:

FV_existing = PV x (1 + r/12)^(12 x years)
FV_contrib = PMT x (((1 + r/12)^(12 x years) - 1) / (r/12))

Corpus = FV_existing + FV_contrib

where:
  PV = current retirement savings
  PMT = monthly contribution
  r = expected annual return (decimal)
  years = retirement age - current age

The 4% rule check estimates the safe first-year withdrawal:

Safe annual withdrawal = Corpus x 0.04

(per the Trinity Study, 1998)

To estimate how many years the corpus lasts under a fixed annual withdrawal at the expected return, the calculator uses the present value of an annuity formula inverted to solve for time:

Months = -log(1 - (Corpus x r/12) / Withdrawal_monthly) / log(1 + r/12)

where Withdrawal_monthly = annual_withdrawal / 12

If Corpus x (r/12) >= Withdrawal_monthly, the corpus grows
faster than withdrawals deplete it (indefinite duration).

This is a simplified model that assumes a constant return (no volatility) and a constant withdrawal (no inflation adjustment). Real-world retirement involves sequence-of-returns risk — the order in which returns occur matters as much as their average. A market crash in the first few years of retirement can deplete a corpus much faster than the same crash occurring a decade later.

The tool handles two edge cases: when the rate is zero, the corpus is simply the sum of current savings plus contributions (no growth), and the withdrawal sustainability reduces to a simple division. When the annual withdrawal is zero, the calculator skips the sustainability computation. The "Years of withdrawals (no growth)" output is the corpus divided by annual withdrawal — a worst-case scenario assuming zero return.

Worked Examples

Using the default inputs: age 30, retire at 65 (35 years), $25,000 current savings, $500/month contribution, 7% expected return, $60,000/year withdrawal target. FV of current savings: 25000 x (1.005833)^420 = $287,654. FV of contributions: 500 x ((1.005833^420 - 1) / 0.005833) = $900,527. Total corpus = $1,188,181. 4% rule safe withdrawal = $47,527/year. Years of withdrawals (no growth) = 1,188,181 / 60,000 = 19.8 years. Years of withdrawals (with 7% growth) is indefinite — the corpus earns about $83,172/year in interest at 7%, which exceeds the $60,000 withdrawal, so the corpus continues to grow rather than deplete.

The same saver starting at age 40 (25 years to retirement) accumulates: FV of current savings = 25000 x (1.005833)^300 = $143,135. FV of contributions = 500 x ((1.005833^300 - 1) / 0.005833) = $405,036. Total corpus = $548,171. 4% rule safe withdrawal = $21,927/year — less than half the earlier starter's. Starting 10 years later cuts the corpus by roughly $640,000, even though total contributions differ by only $60,000 ($210,000 vs. $150,000 in monthly contributions over the working years, plus the same $25,000 starting balance). The remaining ~$580,000 gap is forgone investment growth.

For an aggressive saver at age 35, retiring at 60 (25 years), $50,000 current savings, $1,500/month contribution, 7% return: FV of current savings = 50000 x (1.005833)^300 = $286,317. FV of contributions = 1500 x ((1.005833^300 - 1) / 0.005833) = $1,215,342. Total corpus = $1,501,659. 4% rule safe withdrawal = $60,066/year. This corpus would support a $60,000/year retirement income, before Social Security, for 30+ years under most historical scenarios per the Trinity Study.

For a smaller corpus of $500,000 at 7% return with a $60,000/year withdrawal target, the corpus earns about $35,000/year in interest, falling short of the $60,000 withdrawal. Using the withdrawal-sustainability formula: months = -log(1 - 500000 x 0.005833 / 5000) / log(1.005833) = -log(1 - 0.5833) / log(1.005833) = 150.5 months ≈ 12.5 years. This is the kind of insight a retirement calculator should provoke: the line between a corpus that lasts forever and one that runs out in 12 years is simply whether returns exceed withdrawals.

When to Use This Tool

Use the retirement calculator when you need to:

• Project your retirement corpus from current savings and ongoing contributions.
• Stress-test whether your savings rate is on track for a target retirement income.
• Compare the impact of starting now versus delaying 5 or 10 years.
• Estimate the safe annual withdrawal using the 4% rule (Trinity Study).
• Model how a higher expected return changes your corpus (with appropriate risk caveat).
• Decide whether to increase your 401(k) or IRA contribution to hit a target.
• Demonstrate the power of long-time-horizon compounding to younger savers.

Limitations & Disclaimer

This calculator uses constant-return projections and does not model investment volatility, sequence-of-returns risk, inflation, taxes, Social Security, pension income, RMDs under IRC Sec. 401(a)(9), tax-advantaged contribution limits under IRC Sec. 402(g), or fee drag. Real investment returns vary widely year to year and may be negative for extended periods. The 4% rule is a historical heuristic, not a guarantee, and may be too optimistic at current valuations. For real retirement planning, consult a licensed financial advisor. See our disclaimer for full details.

Frequently Asked Questions

Last updated: September 9, 2026  ·  Author: HT99 Tools Editorial Team