How Retirement Lab works

What you enter, what you get back, and what happens in between - in plain words, with the technical detail underneath for anyone who wants it.

How you use it

No account needed. Most people have a first answer in about two minutes.

  1. 1

    Start simple

    One short form: your age, when you want to retire, what you have saved, what you save each month, what you plan to spend, and any pension or Social Security. Or start from a Quick start example and change it.

  2. 2

    Read your result

    Your success rate in plain words, what a bad case would leave you with, the chance the money runs out by each age, and your savings, income and spending over time - all in today's money.

  3. 3

    Improve it

    Three levers - save more, retire later, spend less. Pick a target (75, 90 or 95%) to see how far each one would have to move on its own, then apply the changes and re-run.

  4. 4

    Go deeper when you want

    Advanced mode walks through five steps: profile, income, portfolio, spending and settings. A free account lets you save plans, duplicate them and compare two side by side.

Example plan

Success rate

85%

We ran 1,000 possible futures for this plan. The savings last to age 90 in about 850 of them.

Chance the money runs out

By age 75
0.4%
By age 85
9%
By age 90
15%

Improve this plan

~85% → ~96%

Save more+200 / month
Retire laterage 67
Spend less-250 / month

Same example plan, with all three changes

Market outlook

Same example plan, three sets of assumptions

Cautious

56%

Stocks 6% · bonds 3%

Balanced

85%

Stocks 8% · bonds 4.5%

Historical

92%

Stocks 11.9% · bonds 4.8%

Success rate for each outlook - Balanced is the default

Your savings over time

Same example plan, in today's money

0600k1.2M1.8MRetire5060708090
Typical: 794k at 90Middle half: 217k to 1.7M

What goes into your plan

All of this is in the free simulator. Simple mode covers the basics; Advanced mode adds the rest.

Taxes

One effective tax rate on your retirement income and withdrawals. The spending you enter is after tax; the simulator takes out enough to cover the tax too.

Inflation and today's money

Spending rises with inflation, and results are shown in today's money, so an amount means what it would buy now.

Retirement income

Social Security, pensions or rent, starting at the age you choose. Income can rise with inflation.

Your portfolio

Stocks, bonds and alternatives (for example gold or crypto), with a mix that can change as you get older.

One-time events

An inheritance or a house sale coming in, a new car or a wedding going out - at the age they happen.

Market outlook

Cautious, Balanced or Historical presets for returns and inflation, or your own numbers.

Under the hood

Each part in one paragraph, with the technical detail below it.

A thousand possible futures

Instead of one average return every year, the simulator plays out your retirement 1,000 times (50,000 on Pro), each with a different mix of good and bad years. Your success rate is the share of those futures where the money lasts to the end of your plan.

Technical detail

Monte Carlo simulation, year by year: returns are drawn for each asset, the portfolio grows or shrinks, contributions and income come in, spending and tax go out. Free runs 1,000 futures, Pro 50,000; the result shows its margin of error, which shrinks with more futures. Results are percentiles (a pessimistic, typical and optimistic path), not a single forecast. When you use Improve, each variant faces the same simulated markets, so a change moves the answer for a real reason rather than by chance. Everything runs in your browser.

Crashes as often as they really happen

PRO

A bell curve treats a year like 1931 or 2008 as a once-in-centuries event. Real markets have extreme years more often. Pro can simulate returns with "fat tails" and an optional downside skew, keeping your average and volatility the same - so you can see how deep the bad years go.

Technical detail

Student's t-distribution instead of a normal distribution, with an optional Fernandez-Steel skew. Two settings: moderate (5 degrees of freedom) and extreme (3). Draws are standardized, so your expected return and volatility stay exactly as set; only the shape changes. For a whole retirement the effect on the success rate is usually small (a 4% plan: 83.3% with a bell curve, 83.1% with fat tails, 82.5% with a U.S.-like skew) - your return and volatility assumptions matter far more.

Investments that move together

Stocks, bonds and alternatives don't move independently - in a bad year they can fall together. The simulator keeps a realistic link between them on every plan, free or Pro.

Technical detail

Returns are drawn together through a Cholesky decomposition of a fixed correlation matrix: stocks-bonds 0.3, stocks-alternatives 0.2, bonds-alternatives 0.1. These values are set by the model, not edited in the app.

Spending that can adapt

Free plans spend a fixed monthly amount that rises with inflation. Pro lets spending change with age, or react to markets - a little less after bad years, a little more after good ones - and shows what you would actually get to spend.

Fixed amount is free; the other strategies are Pro.

Technical detail

Fixed amount (free), plus with Pro: spending by age (a different monthly amount per age range); Guyton-Klinger guardrails (spending is cut by your chosen % when the withdrawal rate rises 20% above where it started, boosted when it falls 20% below, and never drops below half of the target); percentage of portfolio (an annual % of current savings); and floor and ceiling (a % of savings, kept between a monthly minimum and maximum). All amounts are in today's money.

Stress tests

PRO

What if the market drops sharply the year you retire? What if inflation spikes for a few years, or a big bill arrives out of nowhere? Pro lets you add these to your plan and see the effect.

Technical detail

Market crashes: a drop of a chosen size at a chosen age, applied to every simulated future. Inflation bursts: a higher inflation rate for a number of years, which raises spending. Emergency expenses: a chance each year, from a chosen age, of a one-off cost. Your results show how much these change your success rate.

Checked against real history

Would your plan have survived retiring in 1929, 1966 or 2000? The historical stress test runs your plan through every stretch of real US market history of the same length.

Free shows the chart; stepping through it and the historical success rate are Pro.

Technical detail

Actual annual S&P 500 and 10-year US Treasury returns, 1928-2025 (Damodaran). Each window starts with your typical savings at retirement from the simulation and uses your spending, income and tax. Alternatives are spread over stocks and bonds, since there is no long history for them; stress-test events are left out, because history has its own crashes. Where the record ends, your expected returns fill the remaining years.

Compare two plans

Retire at 65 or 67? Keep working part time? Save both versions and put them side by side to see which holds up better, and why.

Technical detail

With a free account (up to 2 saved plans, 10 on Pro): success rates and key numbers side by side, both savings paths on one chart, and a list of every input that differs.

How we check ourselves

A well-known study (the Trinity study) found that spending 4% of a 50/50 stock and bond portfolio, raised with inflation, lasted 30 years in about 95% of US historical periods. Run through our historical engine, the same rule lasts in 95.7% of the 30-year periods since 1928.

The engine is covered by automated tests, including checks that the same random seed gives the same result and that Pro features stay off on the free plan. A simulation is only as good as its assumptions, though - it shows a range of possible outcomes, not a forecast, and it isn't financial advice.

Try it with your own numbers

Free, no account needed.

Try the simulatorSee Free vs Pro