Retirement Calculator — Projections & Monte Carlo
Project retirement savings, withdrawals and Social Security year by year, then stress-test the plan with Monte Carlo survival odds and percentile fan charts.
A clear path from inputs to results.
Project your retirement savings year by year from today to your life expectancy, then test whether withdrawals, inflation and Social Security can carry you through. A deterministic engine gives the single expected path, and an optional Monte Carlo mode runs thousands of randomized market paths to show the probability the money lasts.
How to use
- Set your current age, target retirement age and life expectancy — these frame the accumulation and withdrawal windows.
- Enter current savings, your annual contribution and an annual contribution increase (contributions grow each year by the percentage you enter).
- Set the assumed annual returns before and after retirement, your desired retirement income in today's dollars, expected Social Security, and inflation.
- Choose a region: US mode adds employer match, age-50 catch-up contributions and Social Security claim-age adjustments; UK mode adds SIPP tax relief, salary sacrifice and state-pension gating; universal keeps it plain.
- Read the deterministic results: projected wealth at retirement (nominal and in today's dollars), the monthly income needed at retirement, contributions versus growth, and the projected depletion age.
- Optionally open the Monte Carlo section, choose a volatility (default 18%) and a number of paths (1,000 to 50,000, default 10,000), and read the survival probability and the 5th–95th percentile fan chart.
The formula
Reference: Deterministic year-by-year accumulation with user-set returns (Broverman, Mathematics of Investment and Credit); Monte Carlo mode draws log-normal returns r = exp(mu − sigma²/2 + sigma·z) − 1 with z by Box-Muller.
Worked example
Age 35 to 67, $6,000 a year growing 2%, $60,000 income target
- Current age 35; retire at 67; savings $30,000; contribution $6,000/yr rising 2%/yr; returns 7% before and 4% after retirement; desired income $60,000/yr in today's dollars; Social Security $20,000/yr; inflation 3%; life expectancy 90; US region.
- 1
Contributions and 7% growth compound for 32 years, taking the account to a projected $1,142,464.84 at 67 — about $443,661.41 in today's dollars. Total contributions over the career are $265,362.18 against $1,793,157.80 of investment growth. Inflation lifts the $60,000 target to roughly $154,505 by 67 (about $12,875 a month), of which Social Security covers an inflation-adjusted $20,000 from age 67; the rest is withdrawn from the fund. Withdrawals plus 4% growth on what remains leave about $711,461 still invested at 90, so the projection never depletes.
Peak wealth $1,142,464.84 at 67 ($443,661.41 in today's dollars); contributions $265,362.18 versus growth $1,793,157.80; the fund survives past age 90 with about $711,461 remaining.
Interpreting the result
The depletion age is the headline: if it is below your life expectancy the plan runs short, and the shortfall year tells you when. Compare the nominal peak with the real (today's-dollars) peak — the distance between them is what three decades of inflation does to a portfolio. In Monte Carlo mode, the survival probability is the share of 10,000 randomized paths that never hit zero; a good average is not enough if the 5th-percentile path depletes early, which is exactly what the fan chart exposes.
Assumptions & limitations
- Deterministic returns are constant rates you choose — arithmetic illustrations, not market forecasts; real returns arrive in an unpredictable sequence.
- Monte Carlo draws are log-normal annual returns at your chosen volatility; real markets have fatter tails and correlations the model does not capture.
- Social Security is treated as a flat benefit inflated at your inflation input: zero before your claim age, 30% reduced if claimed at 62, 24% boosted from age 70 (US mode).
- Taxes on withdrawals, contribution limits and required-minimum-distribution rules are not modeled.
- The projection ends at the life expectancy you enter; a fund that survives to that age is reported as never depleting, not as guaranteed permanent income.
Frequently asked questions
How much do I need to retire?
It depends on the gap between your target income and reliable outside income. In the worked example the plan supports a target of roughly $72,900 a year in today's dollars through age 90 — the $60,000 target leaves about $711,000 unspent at 90. Push the target above that boundary and the projected fund runs out before 90.
Is the simulation seeded (same result every run)?
No. The worker samples returns from an unseeded Math.random stream via Box-Muller, so each run is a genuinely fresh simulation and the percentiles move a little between runs. Running more paths (up to 50,000) tightens the estimates if you want stabler numbers.
What do the employer match and catch-up inputs do?
In US mode, the match percentage adds that share of your contribution on top every year (a 50% match on $6,000 adds $3,000 annually), and the age-50 catch-up option adds the higher catch-up limits to contributions from age 50 onward. Both raise the accumulation line directly; the growth engine itself is unchanged.
What does the Monte Carlo survival probability mean?
It is the percentage of simulated lifetime paths — each with a different random sequence of annual returns — in which the fund never hits zero before your life expectancy. A 95% survival rate means 95 of 100 random markets succeeded, not that your account is insured; the 5th-percentile fan line shows how badly the failing paths unfold.
How much does retiring two years later help?
In the worked example, retiring at 69 instead of 67 raises the projected peak from $1,142,464.84 to $1,340,333.68 — about $197,869 more — because two extra years add contributions and growth while removing two years of withdrawals from the end of the plan.
Why is the income needed at retirement so much higher than my target?
The tool inflates your desired income from today to your retirement date: $60,000 at 3% inflation becomes about $154,505 a year by 67, roughly $12,875 a month. That is the purchasing-power-equivalent withdrawal the plan must fund, and Social Security offsets it only from your claim age onward.
Methodology last reviewed 2026-10-06. Results are informational and not financial or engineering advice — see our disclaimer.