An FI Calc alternative for planning from today to retirement

What is FI Calc?

FI Calc, at ficalc.app, is a free retirement calculator built around backtesting^[2]. Its documentation describes the approach William Bengen used for the 4% rule: start a simulated retirement in one historical year, run it forward year by year, then repeat from the next year, and count how often the money lasted^[3]. FI Calc lets you recreate that study or change the inputs to see how success rates move.

Its configuration covers the retirement itself: starting age, duration, starting portfolio and asset mix, how often to rebalance, a withdrawal strategy, extra one-off withdrawals, income such as Social Security, and which years of historical data to use^[1]. The withdrawal strategies are its standout feature. Alongside a constant inflation-adjusted amount, it offers percent of portfolio, 1/N, variable percentage withdrawal (VPW) and a custom version, dynamic SWR, an endowment strategy, Guyton-Klinger, the 95% rule, a CAPE-based rule, Sensible Withdrawals, Hebeler Autopilot II and Vanguard Dynamic Spending^[1]. Results can be exported as CSV^[3].

FI Calc processes your inputs in the browser. If you create an account, it stores your saved calculations; it uses Google Analytics, and its policy notes that shared or saved calculation URLs can carry your inputs^[7].

How do FI Calc and FIRE Calculator Pro compare?

Do the two tools answer the same question?

Mostly they answer neighboring ones. FI Calc asks: given this portfolio on the day I retire, and this way of withdrawing from it, how often would it have lasted? That is the right question once you know your number, and its range of withdrawal strategies makes it a strong tool for the years just before and after you stop working.

Our calculators start further back. You enter what you have invested, what you save and what you spend, and the FIRE calculator finds your FIRE number and the age you reach it. The Coast FIRE calculator finds the balance where you could stop saving; the Barista FIRE calculator finds the balance where part-time pay could cover the gap. Each plan then runs from today to your plan-until age through both risk tests, so a poor stretch of markets while you are still saving counts too.

That difference shapes how Social Security works in each. In FI Calc it is an income source added to the retirement simulation^[6]. In ours it also feeds into the target: if a benefit will cover part of your spending from 67, the portfolio only has to carry the full amount until then, so the FIRE number and FI age come down.

A practical way to see the split: the further you are from retirement, the more your answer depends on how much you save and the return you earn before you stop, and the less it depends on the fine print of a withdrawal rule. Years out, the savings rate drives the date. Within a year or two of retiring, the balance is largely known and the withdrawal rule becomes the main lever, which is where FI Calc does its most detailed work.

Why does looping historical data matter?

Every backtest runs into the same problem: long retirements need long stretches of data, and recent start years have not finished yet. FI Calc's default is to continue from the first year once a simulation reaches the last one, so every year appears in the same number of simulations and long retirements get as many runs as short ones. Its documentation is clear that a looped sequence, with the most recent year followed by the oldest, is a modeled sequence rather than a historical one, and that you can turn looping off^[4].

We made the other choice: a start year only counts if every year of the retirement exists in our 1928-2025 data. That keeps every cohort a real sequence, but long retirements have fewer cohorts, and they all start in earlier decades. To cover the rest, we add a Monte Carlo test that draws whole historical years at random. The table shows the classic case through our historical test: $1,000,000, $40,000 a year rising with inflation, 80% stocks, 0.1% fees.

Our calculation with the FIRE Calculator Pro engine, US data 1928-2025^[8]. Monte Carlo: 2,000 runs, seed 2026. Not FI Calc output.

Notice the 60-year row. Its historical success rate, 82%, is higher than the 50-year rate of 80%, because the 60-year test can only use start years up to 1966. That is the cost of refusing to loop, and it is why we show the Monte Carlo result next to it: 80% at 50 years and 76% at 60. Looping makes the opposite trade. Neither is wrong, but you should know which one a success rate comes from before comparing tools. Both point the same way as the research: 4% was built for 30 years^[9], and long early retirements call for something nearer 3.25%-3.5%^[10].

Why do withdrawal strategies change the answer?

A success rate depends on the spending rule behind it as much as on the markets. Our FIRE calculators, like Bengen's original study^[9], assume you withdraw the same inflation-adjusted amount every year whatever happens. That is the simplest rule to plan around and the strictest to test, because spending never adapts to a crash.

Most of FI Calc's other strategies let spending move. A percent-of-portfolio rule withdraws a fixed share of whatever the balance is, so the money cannot run out but your income rises and falls with markets. Guardrail rules such as Guyton-Klinger cut spending by a set amount when the withdrawal rate drifts too high and raise it when markets have done well^[11]. Each one trades a steadier income for a higher chance that the money lasts, or the reverse.

That makes the tools complementary rather than interchangeable. A constant-spending success rate tells you how a plan holds up if you change nothing. FI Calc shows what flexibility would buy you. If you are willing to cut back in a bad decade, its strategies can justify a higher starting withdrawal than a fixed rule would.

How do you check an FI Calc plan here?

FI Calc's inputs map most closely to our savings longevity calculator, which also starts from a balance on the day you retire:

• Starting age and duration. Enter the starting age as your current age, and the starting age plus the duration as the age the money must last to.
• Portfolio and spending. Enter the starting portfolio as your savings balance and the annual spending goal as your yearly withdrawal.
• Asset mix. We test stocks and bonds only, so add any cash share to bonds. Rebalancing is yearly.
• Income. Social Security and a pension have their own fields with a start age. Other income is better modeled in FI Calc.
• Withdrawal strategy. Compare against FI Calc's constant dollar strategy, which matches a fixed inflation-adjusted withdrawal.

Expect small differences even then, from the data window, looping and fees. If you are still saving, use the FIRE calculator instead, which projects the years before you retire as well.

What does Social Security do to the same plan?

Take the 50-year case, retiring at 45, and add a $20,000 benefit from 67. Kept at $1,000,000, the plan lasted in 92% of historical start years in our test, up from 80%. Our engine also lowers the target: the portfolio needed at 45 falls to $818,736, because the full $40,000 only has to come from savings for 22 years. At that lower amount, the plan lasted in 71% of start years, against 80% for the 4% plan with no benefit. The smaller portfolio carries its heaviest withdrawals in the first 22 years, when a bad sequence of returns does the most damage, so keep some margin above the target. In FI Calc you would add the benefit as income and adjust the starting portfolio yourself to find the same point.

When should you use FI Calc instead?

• You want to compare withdrawal strategies. Its 13 strategies, from Guyton-Klinger to Vanguard Dynamic Spending, are the reason to use it^[1].
• You want the longest history. Shiller's data from 1871 gives more start years, and you can restrict it if you prefer^[4].
• You plan one-off expenses. Extra withdrawals handle known future costs^[1].
• You want to save and revisit scenarios. An optional account keeps them across devices^[7].

When should you use FIRE Calculator Pro?

• You are still saving. Find your FIRE number and the age you reach it from your savings, contributions and spending.
• You are weighing Coast or Barista FIRE. Dedicated calculators find the point where you could stop saving or go part-time.
• You want two independent risk tests. A Monte Carlo test alongside the historical replay, for every plan.
• You plan in pounds or Canadian or Australian dollars. Our FIRE and Coast FIRE calculators have UK, Canada and Australia editions with local accounts and government pensions.
• You want no account. No sign-in, and the numbers you type never leave your browser. See our privacy policy.

Used together they cover the whole path: our calculators to get to your number, then FI Calc to choose how you will spend it down. Our formulas are documented on the methodology page.