Quick Answer: How long will my money last calculator?
A “how long will my money last” calculator projects a starting balance forward month by month, subtracting withdrawals and adding investment returns, until the balance hits zero. That final date is your answer, and you can run it yourself with free tools from consumer-protection bodies, university extension services, or a simple spreadsheet.
The catch is that the output is only as good as the assumptions behind it. Some tools give a single deterministic answer; others run thousands of simulations and report a probability instead. The same inputs can produce “28 years” from one calculator and “85% chance of lasting 30 years” from another.
Before trusting any result, check the inflation rate, the expected return, the fee assumption, your withdrawal amount, the time horizon, and whether taxes and required minimum distributions are included.
Why These Calculators Feel So Reassuring, and Why That’s a Trap
There’s something deeply satisfying about typing a number into a box and getting a clean answer back. Thirty-one years. Done. Plan made.
But a longevity calculator isn’t a fortune-telling machine. It’s a spreadsheet wearing a nice interface. It takes what you tell it, applies arithmetic, and hands the result back. If you feed it optimistic assumptions, it will happily tell you that you’ll be fine until age 103.
The real skill isn’t finding a calculator. It’s knowing which inputs actually drive the answer, which ones calculators quietly ignore, and where the whole exercise breaks down. That’s what this guide covers.
What a Depletion Calculator Actually Does
The math is simple. A depletion calculation takes your starting balance, subtracts your monthly withdrawal, adds whatever return the portfolio earns, and repeats. When the balance crosses zero, the calculator reports how long you lasted.
This is the same logic as a retirement sustainable-withdrawal model, just run in reverse. Instead of asking “how much can I safely take out,” you’re asking “how long does this amount of money survive at this withdrawal rate.”
Most calculators ask for a handful of inputs:
| Input | What it controls | Common default |
|---|---|---|
| Starting balance | The pot you’re drawing from | Whatever you enter |
| Monthly or annual withdrawal | How fast the pot drains | Often set by the user |
| Expected return | How fast the pot refills | 5–7% nominal |
| Inflation rate | How fast withdrawals must grow | 2–3% |
| Time horizon | When the projection stops | 25–30 years |
| Fees | Drag on returns | Frequently 0% |
That last row is where trouble starts. Plenty of free calculators don’t ask about fees at all, which means they assume you invest for free. A 1% annual fee can cut several years from a portfolio’s duration in a long retirement. A tool that ignores it will overstate how long your money lasts, sometimes by a wide margin. (Long do macbooks last realistic lifespans kills them)
The 4% Rule and Where It Came From
Many calculators default to a 4% initial withdrawal rate, and that number has a specific origin. It comes from research by Philip L. Cooley, Carl M. Hubbard, and Daniel T. Walz, published in the Journal of Financial Planning in 1998, with an earlier version appearing in 1994. Their work found that a 4% initial withdrawal, adjusted annually for inflation, historically survived 30-year periods in a high-equity portfolio with high probability.
The 4% rule became famous because it’s easy to remember. It’s also widely misunderstood.
It was derived from U.S. historical data, assumes a specific asset mix, and was built around a 30-year horizon. It doesn’t personalize for fees, taxes, or the order in which returns arrive. Treating it as a guarantee rather than a rule of thumb is one of the most common mistakes people make when they plug numbers into a calculator and walk away satisfied.
Sequence of Returns: The Risk a Single Average Hides
Here’s a subtlety that trips up even careful planners. Two portfolios can have identical average returns and wildly different lifespans.
Imagine two retirees with the same balance, same withdrawal, same 30-year average return. One retires into a bull market; the other retires into a crash. The second one keeps selling assets at depressed prices to fund withdrawals, and those shares never get the chance to recover. Same average, very different outcome.
This is sequence-of-returns risk, and it’s the reason a calculator that uses one fixed average return can overstate how long money lasts in bad early-return scenarios. If a tool lets you enter only a single return figure, treat its answer as an optimistic case, not a plan.
The fix is to look for tools that run many simulated market paths. Those are called Monte Carlo calculators, and they report a probability rather than a single year count. The distinction matters enough that any article or advisor quoting a result should say which type produced it.
Deterministic vs. Monte Carlo: Two Different Answers to the Same Question
This is worth spelling out because the gap between the two methods surprises people.
A deterministic calculator gives one answer: “Your money lasts 28 years.” It assumes smooth, constant returns every single year. Real markets don’t do smooth.
A Monte Carlo calculator runs thousands of randomized scenarios and reports something like: “There’s an 85% chance your money lasts 30 years.” That’s a fundamentally different kind of statement. It acknowledges uncertainty instead of hiding it.
Neither is wrong. They answer slightly different questions. But if you compare results from two calculators and one says 28 years while the other says 85% probability of 30 years, you’re not looking at a bug. You’re looking at two different methods.
Practical rule: use deterministic tools for quick sanity checks and Monte Carlo tools when the decision actually matters.
The Assumption That Moves the Needle Most: Inflation
Ask most people which input matters most, and they’ll guess investment returns. In practice, the inflation assumption often drives the result harder.
A calculator using 0% inflation will report a dramatically longer duration than one using 2–3% annual inflation, because it never grows your withdrawals to keep pace with rising prices. Over a 30-year retirement, that difference compounds into years of runway that don’t actually exist.
If a tool doesn’t let you set inflation, or buries it in a default you can’t see, that’s a red flag. Look for the setting. Change it. Watch how much the answer moves. That single experiment teaches more about your plan than any single output number.
Longevity: The Unknown You Can’t Calculate Away
Every longevity calculator has to guess how long you’ll live. That guess is the biggest source of uncertainty in the whole exercise.
Life expectancy at age 65 has run roughly 19 to 20 additional years in recent U.S. data, but individual outcomes vary enormously. Many retirement tools default to planning until age 90 or 95, which is conservative by design. Others stop at 85, which quietly assumes you won’t be around to need money in your late eighties.
Two practical habits help here. First, run the calculator at several horizons rather than one. Second, remember that a couple has a real chance that at least one partner lives well past average. Planning to the average is a coin flip, not a plan.
What Calculators Usually Leave Out
Even good calculators tend to omit a few things that matter in real life. Knowing the gaps is half the battle.
Taxes. Withdrawals from tax-deferred accounts are taxed as ordinary income. A calculator that treats your balance as spendable cash will overstate your runway.
Required minimum distributions. In the U.S., RMDs from tax-deferred accounts begin at age 73 for those born 1951–1959 and age 75 for those born 1960 or later, under SECURE 2.0, enacted in December 2022. Simple longevity tools often ignore RMDs entirely, even though they can force larger withdrawals than you planned. Current IRS guidance is worth checking directly.
Fees. As noted, a 1% annual fee is a silent subtraction that many tools skip.
Healthcare and long-term care. A big, lumpy expense that doesn’t fit neatly into a smooth monthly withdrawal model.
One-time costs. A roof, a car, a wedding, a grandchild’s tuition. These don’t appear in a straight-line projection.
If you want to see how dramatically real-world conditions can shorten a product’s useful life, the same logic applies to hardware: just as a calculator’s output depends on assumptions, real-world data on how long MacBooks actually last shows how much lifespan varies once heat, charging habits, and repairs enter the picture. The parallel is useful: projections are clean, reality is messy.
How to Use a Calculator Without Fooling Yourself
A calculator is a conversation starter, not a verdict. Here’s a routine that actually works.
- Run three scenarios, not one. Use a pessimistic return, a middle case, and an optimistic case. If your plan only survives the optimistic one, you’ve learned something important.
- Set inflation honestly. Use 2–3%, not zero. Then run it again at 4% and see how much the answer shrinks.
- Add fees manually if the tool won’t. Subtract 1% from your expected return and rerun. The difference is your fee drag.
- Test a bad early decade. Drop the return in the first several years and raise it later, keeping the average the same. That approximates sequence risk.
- Stress-test the horizon. Run to age 85, 90, and 95. The spread tells you how sensitive your plan is to longevity.
- Check the type of tool. Note whether it’s deterministic or Monte Carlo, and read the output accordingly.
Common mistakes to avoid: entering your full portfolio when part of it is earmarked for something else, forgetting that withdrawals rise with inflation, and treating a single successful run as proof of safety. (SEC Investor.gov investing glossary)
Where Free, Neutral Tools Beat Commercial Ones
Plenty of calculators exist, and they are not created equal. Many are lead-generation tools dressed up as planning software, designed to capture your email and route you to a sales conversation. That doesn’t make them wrong, but it does mean the defaults may be tuned to make you feel either safe or nervous, depending on which reaction sells better.
Public-sector and nonprofit tools are usually a better starting point for factual planning. Consumer-protection bodies publish retirement-planning resources, and many university extension services offer calculators built for education rather than conversion. These tools tend to be plainer, but their assumptions are usually visible and adjustable.
The same principle applies to any longevity question. If you want to understand how long a water leak sensor‘s battery really lasts, you go to independent testing, not the manufacturer’s marketing page. If you want to know how long EV batteries hold up in the real world, you look at warranty terms and field data rather than a spec sheet. Money is no different: neutral sources, visible assumptions, and a healthy skepticism about defaults.
When the Answer Changes, Ask Why
One of the most useful habits is to notice when a calculator’s answer shifts and figure out which input caused it. Change one variable at a time. If bumping inflation from 2% to 3% knocks five years off your runway, that’s a signal your plan is fragile. If changing the return assumption barely moves anything, your withdrawal rate is probably the dominant factor.
This kind of sensitivity testing is what separates people who use calculators well from people who use them as reassurance machines. The number isn’t the point. The relationships between the numbers are.
Conclusion
A “how long will my money last” calculator is a genuinely useful tool, as long as you understand what it’s doing. It runs a depletion calculation, applies your assumptions, and reports when the balance hits zero. That’s it. It doesn’t know how long you’ll live, what inflation will do, or what the market will hand you in your first five years of withdrawals.
The people who get the most out of these tools treat them as experiments rather than answers. They run multiple scenarios. They set inflation honestly. They add fees the calculator forgot. They test bad early returns and long horizons. And they check whether the tool is giving them a single year count or a probability, because those are different things wearing similar clothes.
Run the numbers. Then run them again with worse assumptions. If the plan still holds, you’ve earned some confidence. If it doesn’t, you’ve learned something far more valuable than a clean answer.
FAQ
What is a safe withdrawal rate for a long retirement?
There’s no universal number, but 4% is the most cited starting point, drawn from research published in the Journal of Financial Planning in 1998. That figure assumed a specific asset mix and a 30-year horizon, and it doesn’t account for fees or taxes. Many planners suggest testing lower rates, such as 3% to 3.5%, for retirements longer than 30 years.
Why do two calculators give me different answers?
Usually because they use different methods or different hidden defaults. One may be deterministic and the other Monte Carlo. One may assume 0% fees and 2% inflation; the other may assume 1% fees and 3% inflation. Before comparing outputs, compare the assumption screens.
Should I use a Monte Carlo or a simple calculator?
Use a simple deterministic calculator for a quick sanity check. Use a Monte Carlo tool when the decision is significant, because it shows a range of outcomes instead of one smooth line. A single deterministic answer can hide serious risk from a bad early decade.
Does the calculator account for taxes and required minimum distributions?
Most free calculators don’t, or handle them only crudely. Taxes on withdrawals and required minimum distributions can both force larger-than-planned withdrawals, especially later in retirement. Treat any calculator output as pre-tax and pre-RMD unless the tool explicitly says otherwise.
How do fees change how long my money lasts?
Fees compound against you. A 1% annual fee can reduce a portfolio’s duration by several years over a long retirement. If a calculator doesn’t ask about fees, subtract 1% from the expected return and rerun the projection to see the real-world version.
What age should I plan to?
Planning to age 90 or 95 is common and conservative. Average life expectancy at 65 is roughly 19 to 20 additional years in recent U.S. data, but averages hide wide variation, and couples face a real chance that one partner lives well beyond the average. Run your projection at several end ages and see how sensitive the result is.
