What Is Financial Planning Software Unable to Predict?

About four in 10 retirees say they stopped working earlier than they planned. That one number answers most of the question of what financial planning software is unable to predict. It can't predict your life. It doesn't know the order in which markets will hand out returns, or when a layoff or a diagnosis will end your career. It doesn't know how long you'll live, or what you'll do when your balance falls hard in a single year.

Source: Greenwald Research/EBRI, 2025 Retirement Confidence Survey report · Apr 2025

Good software does something else, and it's more useful. It shows you the odds across many possible futures, so you can see how sturdy or fragile your plan is before real life tests it.

A quick note on scope: this article covers personal retirement planning tools. If you came here about business cash-flow or FP&A forecasting software, some of the blind spots are the same, but the fixes are different.

What is financial planning software unable to predict? The short list

No tool can see the future. The blind spots fall into four groups: market, personal, policy and behavior.

Here's what each one means for your plan.

Market blind spots

Sequence of returns

Two retirees can earn the same average return over 30 years and end up in very different places. If the bad years come first, while you're withdrawing, you sell more shares at low prices, and those shares never get the chance to recover.

A calculator that assumes "7% a year, every year" can't show you this at all. The real order of returns is unknowable, so the honest approach is to test many possible orders and see how many of them your plan survives.

Fat tails and the assumptions underneath

This is where the experts disagree, and you should know both sides.

The paper Stress Testing Monte Carlo Assumptions, from the Wharton/Pension Research Council, points out that most Monte Carlo tools assume returns are normally distributed and independent from year to year. Real markets have "fat tails," meaning extreme years happen more often than a bell curve predicts.

Michael Kitces pushes back. In his analysis of fat tails versus safe withdrawal rates, he finds that when you look at annual returns, most of the downside fat tail goes away.

The practical point: every model is a set of assumptions. That's why different tools give different answers for the same person. So don't treat any single tool's output as gospel. Pick a sensible withdrawal rate and leave yourself room for error.

Personal blind spots

When you actually stop working

This is the biggest unpredictable input, and most tools treat it as fixed. You type in "retire at 67," and the software believes you.

Life often doesn't. Besides the four in 10 who retired earlier than planned, an earlier EBRI survey found that 38% of people who left work early named health problems or disability as the main reason.

Source: EBRI Retirement Confidence Survey, as reported by Benzinga · Nov 2024

Retiring early hits twice. You lose the years when you'd still be saving, and you gain extra years when you're withdrawing. If your plan only works when you retire on your own schedule, it's weaker than it looks.

How long you live

About one in three 65-year-olds may live to at least 90. If your plan stops at 85, it is quietly betting against a third of retirees.

Source: Social Security Administration, Retirement Information for Medicare Beneficiaries · 2026

Software can't tell you your lifespan. But it can make you plan for a long one. Running out of money at 88 is a much worse outcome than dying with some left over.

Health care

Fidelity estimates that a single 65-year-old retiring in 2025 will spend $172,500 on health care over retirement. That covers Medicare premiums, deductibles and prescriptions. It's up from $165,000 the year before and $80,000 in 2002. It also leaves out long-term care, most dental care and over-the-counter costs.

Source: Fidelity Investments, 2025 Retiree Health Care Cost Estimate · Jul 2025

No software knows whether you'll need years of care or none at all. What it can do is stop treating health care as a rounding error.

Policy blind spots

Tax brackets, Social Security rules, Medicare premiums and the income-based surcharges added to them all change over time. No tool can predict what lawmakers will do.

You can't control policy. You can control how often you check. A plan built on this year's rules should be re-run when those rules change, not left alone for a decade.

Behavior blind spots

This may be the most underrated gap. Software assumes you'll follow the plan. People don't always. They sell after a crash, or let spending creep up in good years, or freeze and change nothing when they should adjust.

Kitces names a related gap in Is Financial Planning Software Incapable of Formulating an Actual Financial Plan?. A basic question, how much markets have to decline before you need to cut spending, can't easily be answered with the planning software available today. The software gives you a probability. It doesn't give you a decision rule.

So you have to write the rule yourself, before you need it.

How Monte Carlo helps (and what it doesn't fix)

"Monte Carlo" sounds technical. The idea is simple. A typical calculator assumes one average return every year. A Monte Carlo test draws many different sequences of market returns instead, some good, some bad, some awful early on. Then it counts how many of those futures your money survives.

That's the approach behind Morningstar's research. Its 2025 report puts the safe starting withdrawal rate at 3.9% for a 30-year retirement. "Safe" here doesn't mean the average outcome works. It means money was still there in 90% of the simulated futures.

Source: Morningstar, The State of Retirement Income 2025 · Dec 2025

What a success rate actually means

Say your plan shows an 85% success rate. That means 850 out of 1,000 simulated futures stayed funded to the end of the plan.

The other 150 aren't predictions that you'll go broke. They're the futures where something would have had to change: spending less, working a bit longer, or adjusting after a bad stretch. Real retirees make those changes. A lower number tells you to look closer. It isn't a verdict.

Where Monte Climb fits

The calculator at Monte Climb takes three numbers: your age, your retirement savings and your monthly spending. It runs them through 1,000 simulated futures and gives you a success rate. Plans run to age 95 by default, and you can push them further. The engine charges Medicare premiums and income-based surcharges year by year instead of using a flat guess. It runs in your browser, and nothing is saved until you say so.

It still can't predict your future. No software can. What it can do is show how your plan holds up across a wide range of futures, using numbers you understand.

A practical routine for the things no software can predict

Prediction isn't possible, so aim for preparation. Here's a simple routine.

1. Stress-test your retirement date. Run your plan again as if you stopped working a few years early. If the success rate falls off a cliff, you've found your biggest risk.
2. Compare your withdrawal rate to the research. Divide your yearly spending from savings by your total savings. If it's well above 3.9%, you're relying on better-than-cautious markets.
3. Plan past 90. Given the longevity data, a plan that ends in your mid-80s is optimistic.
4. Give health care its own line. Treat it as a real expense that grows, and remember that the big estimates leave out long-term care.
5. Write your guardrails now. Decide ahead of time what you'll trim if your portfolio drops meaningfully. You'll make better decisions calm than mid-crash.
6. Re-run on a schedule. Do it once a year, and again after any big change: a job loss, a health event, a market slump, a change in the rules.

None of this is personalized advice. It's a way to turn unknowns into tests you can run.

The bottom line

What is financial planning software unable to predict? Your future. Markets, your health, your lifespan, the law and your own reactions are all beyond any model. But "can't predict" doesn't mean "can't help." A plan tested across 1,000 futures will tell you far more than one built on a single average return. It gives you a probability, not a promise.

Want to see yours? Show my answer with three numbers, then ask William, our AI planner, whether you're on track, whether you can retire at 62, or what you can sustainably spend. He shows his work. Planning software, not advice.

Frequently asked questions

Can financial planning software predict the stock market?

No. No software can predict future returns or the order they'll come in. Monte Carlo tools don't try to. They run many possible return sequences and show how often your plan survives across them.

What does a Monte Carlo success rate mean?

It's the share of simulated futures in which your money lasted to the end of the plan. An 85% success rate means 850 of 1,000 futures stayed funded. The other futures usually mean you'd have had to adjust, for example by spending less or working longer. They don't mean you'd certainly run out.

Is Monte Carlo better than an average-return retirement calculator?

For retirement planning it's usually more informative. An average-return projection hides sequence-of-returns risk, the damage bad early years can do. Morningstar's 2025 research, for example, judges a safe withdrawal rate by whether money lasts in 90% of simulated outcomes, not by the average result.

Why do different retirement calculators give different answers?

Each tool makes its own assumptions about returns, inflation, lifespan and costs, and different assumptions produce different results. The experts don't agree either. A Wharton/Pension Research Council paper argues that most tools underweight fat-tailed returns, while Kitces finds that effect mostly fades in annual data. Treat any single result as an estimate and leave a margin for error.

How often should I re-run my retirement plan?

At least once a year. Also re-run it after a major change, such as a job loss, a health event, a big market move or a change to tax, Social Security or Medicare rules. Software can't predict those changes, but it can show how they affect your odds.