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Investment SimulatorMonte Carlo retirement calculator with withdrawals

See how contributions, time, taxes, allocation and withdrawals interact, one day at a time.
Projected balance
$0
This tool is for education only. It is not personalized financial, tax, or investment advice. Every projection here rests on simplified assumptions and constant or randomly modeled rates that real markets will not follow. Talk with a licensed financial, tax, or investment professional before making decisions with real money.

Your inputs

Open a section below to adjust it. The graph on the right updates instantly and stays put while you work.
Starting point & contributions
Money invested when you start. It has the entire timeline to grow, so it carries the most compounding of any dollar you put in.
These three are separate deposits that all add together. Enter only the schedules you actually use. For example, $500 monthly plus $100 weekly means both are invested. There are 12 monthly, 26 biweekly, or 52 weekly deposits a year, and each one grows from its own deposit date.
%
The average yearly growth the model assumes for your investments. Real investments won't grow by exactly this amount every year.
years
How long you keep contributing. Time is what lets returns earn their own returns, and it's usually the biggest lever in this model. If the withdrawal phase is on, retirement starts when this timeline ends.
Work backward from a goal
Instead of projecting forward from a contribution amount, tell the calculator a dollar target and a year, and it works backward to find the periodic contribution required to get there.
Taxable vs. tax-advantaged
Shows a taxable account next to a tax-advantaged account (like a Roth IRA or 401(k)) on the same graph, so you can see the cost of yearly taxes on dividends and realized gains.
What if the market crashes early or late?
This is called sequence-of-returns risk. The same size downturn hits very differently depending on when it happens. Early, your contributions buy shares cheaply and ride the recovery. Late, it hits your largest balance right before the finish line.
Compare stock and bond mixes
This is called your investment allocation. Each mix combines stocks and bonds in different proportions, so you can see how changing the mix affects projected growth. With randomized markets on, it also reflects each mix's volatility.
Inflation adjustment
Flips every dollar figure on the page (the graph, the goal, the final balance) from nominal future dollars into what those dollars would buy today.
Withdrawal phase
Retirement starts when your timeline ends. Contributions stop, and the graph keeps going, drawing money out of whatever balance you built, so you see the whole arc: growth, then drawdown. Turn on randomized markets (below) too, to see the odds the money lasts.
Investment fees
Deducted from your return every year. Even a small ongoing fee compounds into a real drag, and this section shows exactly how many dollars it costs, not just the percentage.
Recessions & booms
Each event swaps in a different return for a window of years, then goes back to your expected return. Add up to five, in any mix.
What if the market doesn't behave as expected?
Instead of assuming one smooth path, the calculator runs 150 randomized market paths (60 per mix in the allocation comparison) and shows the median outcome plus the middle 80% range. This is commonly called a Monte Carlo simulation.

Growth over time

How compounding produces this chart

Growth can itself earn growth. The model steps through the investment period one day at a time, applies contributions or withdrawals on their real schedule, then applies that day's return to the whole balance.

New balance = old balance + contribution (or − withdrawal) + investment growth
The math behind the settings

Expected annual return, converted to a daily rate and compounded every day:

Daily rate = (1 + annual)^(1/365) − 1

Taxable account, each year:

Yearly tax = (that year's growth) × (share taxed) × (tax rate)

Stock and bond mix:

Mix return = stock% × stock return + bond% × bond return

Inflation adjustment:

Real value at year N = nominal value ÷ (1 + inflation)^N

Built by Justin Wiggins. This is an educational tool, not financial advice.

The Investment Simulator is a Monte Carlo retirement calculator with withdrawals. It follows your money through the years you save and the years you spend, then runs 150 randomized markets to show how often it lasts.

How to use the Investment Simulator

  1. Enter your starting amount and what you'll add each month, week or two weeks.
  2. Set an expected return and a timeline. The graph and headline balance update as you type.
  3. Open the other sections to layer on inflation, fees, taxes, a market crash, a withdrawal phase or randomized markets.

How this calculator works

The simulator moves one day at a time. Each day it adds any deposit that's due, then grows the whole balance by your expected return, converted to a daily rate. Repeat that for decades and growth starts earning growth of its own.

Turn on the withdrawal phase and, when your timeline ends, deposits stop and withdrawals begin. A percentage withdrawal is set from your balance on the day you retire, then held steady in buying power if inflation adjustment is on.

One smooth average return is a comforting fiction, though. Monte Carlo simulation means running the same plan through many random futures. Here, each day's return gets a random swing sized by your volatility, a measure of how much returns bounce around (16% is close to the U.S. stock market's long-run figure), across 150 paths.

You'll see the median path, the range the middle 80% of paths fall in, and the share of paths where the money lasted every year of retirement. That last number is the one to watch.

A worked example

Say you're five years from retiring with $450,000 saved, and you'll add $1,000 a month until then. You assume a 6% return and 3% inflation, and you plan to withdraw 4% a year for 30 years. Every figure below is in today's dollars.

On the smooth, fixed-return line, you retire with $579,409, withdraw $23,176 in the first year, and still have about $284,094 left after 30 years. It looks comfortable.

With randomized markets on, across 40 runs of 150 paths each, the money lasted all 30 years in 59% to 74% of paths, about two in three on a typical run. Drop the withdrawal rate to 3.5% and that rose to 71% to 82%, about three in four. (Illustrative figures from this simulator's own runs; randomized results shift a little each time.)

Why isn't it 100%? Half the paths do worse than typical, and a slump right after you retire does lasting damage, because you're selling while prices are down.

Methodology and sources

Returns compound daily: daily rate = (1 + annual return)^(1/365) − 1. Randomized markets add a bell-curve swing of volatility ÷ √365 to each day's rate, centered so the typical path still grows at your expected return.

The Social Security estimate uses SSA's 2026 early and delayed claiming rules and assumes a full retirement age of 67. The model doesn't handle taxes on withdrawals, required minimum distributions, or how stocks and bonds move together. It shows how a plan behaves under its assumptions, not what markets will do.

Last updated: October 2026

Frequently asked questions

What is a Monte Carlo retirement calculator?

It runs your plan through many randomized market paths, 150 here, instead of one smooth average. It reports the middle result, the range most outcomes fall in and how often the money lasted. That shows how much luck and timing can change a retirement.

What's a good Monte Carlo success rate for retirement?

Many planners aim for roughly 75% to 90% of paths lasting. Higher feels safer but can mean working longer or spending less than you could. A lower number isn't doom; it means you'd likely need to trim spending if markets disappoint early.

Is the 4% rule still safe?

It held up over 30-year retirements in most of past U.S. market history, but it isn't a promise. Here you can test 4% with randomized markets on, then 3.5%, and compare how often the money lasts.

How does compound growth work?

Your investments earn returns, and those returns start earning returns of their own. Over long periods that snowball does most of the work, which is why the timeline is usually the biggest lever in this simulator.

What return should I assume?

There's no single right answer. Broad U.S. stock indexes have averaged roughly 10% a year before inflation over long periods, with big swings along the way, and mixes with bonds have grown more slowly. Many people plan with a more conservative figure and turn on inflation adjustment.

Related tools

FIRE CalculatorFind your FIRE number and check whether you've hit Coast FIRE. Monthly Expense CalculatorTotal your spending to choose a realistic withdrawal. Social Security Break-Even CalculatorCompare claiming at 62, 67 and 70 and find the age waiting pays off.

Related reading: Am I Ready to Retire? and What Investing Actually Is, and Why Anyone Bothers