Teapot vs FI Calc: Tax Planning, Couples Support, and Monte Carlo

Picture someone who found FI Calc a few years before they retired. They plugged in their savings target and their planned annual spending, and the tool ran their scenario through every market cycle since the 1920s. It told them their plan would have succeeded 94% of the time. That felt reassuring. They kept saving.

Then they actually retired. And the questions changed.

Now they have $1.2 million split between a traditional account, a Roth, and a taxable account. They have a spouse who is five years younger, with her own accounts and her own Social Security timeline. They are in a state with a 5.5% income tax that exempts Roth withdrawals but not traditional ones. They want to know whether to start Roth conversions, how much to convert each year, and whether their Medicare premiums might jump in three years because of what they do today.

FI Calc cannot answer any of those questions. That is not a criticism — it was never designed to. It answers a different question: would this portfolio have historically survived a given spending rate? That question is worth asking. But for most retirees, it is not the only question, and it is not the most expensive one to get wrong.

FI Calc is excellent at what it was built for. It runs your portfolio through every retirement cohort in recorded US market history and shows you how many of those cohorts would have survived your spending plan. The interface is clean, fast, and requires no account. You can model several different withdrawal strategies — constant dollar, percent of portfolio, guardrails — and see historical results instantly.

For anyone in the accumulation phase asking whether a target savings number is large enough to support a target spending level, FI Calc gives a clear and well-grounded answer.

FI Calc does not model taxes. Its own documentation states that taxes and RMDs are not included, and that users must fold their estimated tax bill into their withdrawal amount as a manual adjustment.

That means the tool cannot tell you how much of your traditional account withdrawal is actually taxable versus what comes out of Roth tax-free. It cannot show you how income stacks across brackets in a year when you take a large distribution. It cannot flag whether a planned conversion would push your modified adjusted gross income past an IRMAA threshold two years before your Medicare enrollment begins. And it cannot tell you whether your state taxes retirement income at all.

For anyone holding a mix of pre-tax, Roth, and taxable accounts — which describes most people who have been saving seriously — the tax layer is not a detail. It is often where the largest planning decisions live.

FI Calc treats your entire portfolio as a single figure. It does not distinguish between a traditional account, a Roth, and a taxable account.

In practice, those three account types behave completely differently. A dollar in a traditional account will be taxed as ordinary income when you withdraw it. A dollar in a Roth comes out tax-free. A dollar in a taxable account may carry years of embedded capital gains at a preferential rate, or it may be nearly free of gain and efficient to sell. The sequence in which you draw from these accounts over a 25-year retirement can mean tens of thousands of dollars in lifetime tax savings or losses. A calculator that treats them as one number cannot help with that decision.

FI Calc does not support two spouses with different ages, different retirement dates, and different Social Security timings. A couple with a five-year age gap has meaningfully different planning needs than a single individual. The younger spouse may have a 30-year retirement horizon where the older spouse has 20. Their Social Security benefit timings, RMD schedules, and survivor income needs all interact — and they interact differently depending on which accounts each person holds and who draws down first.

FI Calc uses historical return sequences rather than Monte Carlo simulation. Historical backtesting is genuinely useful, but it is bounded by what markets have already done. Monte Carlo simulation generates thousands of forward-looking scenarios — including conditions outside the historical record — and produces a probability-of-success estimate calibrated to your own inputs rather than to past cohort matching.

FI Calc is best suited for the accumulation phase. If you are still building toward a retirement number and want to stress-test whether your savings target and spending plan would have survived historical markets, it gives you a clear and credible answer quickly.

It is a harder fit for the distribution phase — for someone who has already retired or is close enough to retirement that the questions have shifted from "am I saving enough" to "what do I do with what I have." Those questions involve taxes, account types, couples coordination, and income cliff management. FI Calc does not address them.

Teapot covers both phases. During accumulation it models year-by-year savings growth, contributions, and projected balances across Traditional, Roth, and Taxable accounts so you can see exactly where you are headed. As you approach and enter retirement it shifts to the distribution questions: federal and state taxes under real brackets for all 50 states, 10,000 Monte Carlo scenarios as a long-term risk view, and automatic comparison of predefined Roth conversion strategies ranked by after-tax ending wealth with IRMAA and ACA effects included in every projection year.

FI Calc is a good tool for a specific question, and it answers that question faster and more simply than most alternatives. If your question is whether your portfolio would have historically survived a given spending rate, it is an efficient place to start.

If your questions have shifted to the actual mechanics of retirement — what to convert, when, from which account, at what cost — you need a tool that can see your tax situation, your account types, and your household. Both tools are free to try, and they serve genuinely different stages of the planning process.