ZBee vs Flexible Retirement Planner
Last reviewed: October 2026
Flexible Retirement Planner, often called FRP, is a free Monte Carlo planner first built in 2006. It runs your plan 10,000 times with random market returns and reports how often the money lasts. It lets you set almost every input year by year, it can let spending flex with the markets (the "flexible" in its name), and it runs on your own computer. This page won't pretend it isn't good at that. ZBee is a native Mac app that measures one thing from the accounts you actually hold: the most you can safely spend. The differences are in what the answer is, what it is built on, and how much of the tax it works out for you.
The one-line version: FRP tells you how likely the plan you type in is to last, with tax at rates you choose; ZBee measures the most your real accounts can safely pay you, with each year's tax worked out, on your Mac.
| ZBee | Flexible Retirement Planner | |
|---|---|---|
| Price | $132/year (30 days free, no account) | Free for personal use. An optional contribution (suggested $10–20) brings a license key that removes the "evaluation" notices |
| Platform | Native Mac app, plus a free iPhone & iPad app — a private, on-device estimate | A Java program you download, for Windows or for Mac and Linux. On a Mac, Java must be installed first |
| Where your data lives | On your Mac — by design, not policy | On your computer — the download runs without the internet and saves your plans there |
| Your real holdings | Linked read-only — every position, priced as of your last sync (or manual entry) | You type three balances: taxable, tax-deferred and tax-free |
| Core question | Most you can safely spend | The probability of success (the share of runs that don't run out of money) for the spending you enter. A goal-seek setting can solve for an input that reaches a probability you choose |
| Spending that flexes with the markets |
Yes — this year's trigger points in dollars: the level below which the rules trim (and to what), and where a raise is earned. Published Guyton-Klinger rules you can verify by hand | Yes — its namesake: its own rules skip the inflation raise in a year the portfolio falls while it is below its starting value, and the Flexible setting raises spending when it is above. A floor (75% of your target by default) and a ceiling are yours to set |
| Market risk | Every market sequence since 1928, plus a thousand Monte Carlo futures | 10,000 Monte Carlo runs from one average return and spread you set for the whole portfolio, drawn from a bell curve. You can add a one-year crash in a year you pick |
| Taxes | Computed year by year — federal, state, IRMAA, NIIT, Social Security taxation | Two flat rates you set: one on each year's gains in taxable accounts, one on withdrawals and income. You can change them by year. RMDs are an option, and early withdrawals carry a penalty |
| Social Security | Worked out from your benefit: the adjustment for the age you claim, spousal and survivor benefits, and how much of it is taxed | An income amount and start age you enter, taxed at your income rate (or a cash flow with its own inflation rule and taxable share) |
| Roth conversions | A plan you enter, taxed through the real brackets, Medicare IRMAA and Social Security taxation, with each year's bracket room shown. No optimizer, by design | A plan you enter, as a cash flow: the amount moves from tax-deferred to tax-free and is taxed at your one income-tax rate |
| Sensitivity analysis | The Trade-offs screen: how each choice you control (when to retire, when to claim Social Security, how much to leave behind) moves your safe yearly spend | A heatmap over any two of more than a dozen inputs |
| Shows its methodology & what it won't model |
Yes — cited sources, published | Yes — detailed documentation, plus the heart of its code |
Flexible Retirement Planner's price, downloads and features checked against its own site and documentation in October 2026; they change — check flexibleretirementplanner.com for the current details. ZBee isn't affiliated with Flexible Retirement Planner.
Where Flexible Retirement Planner does more
Being straight about this: FRP is free for personal use, and it gives you more dials than ZBee does. It plans the saving years as well as retirement, with yearly savings into each kind of account. You can change the return, inflation and tax rates for any span of years, give a cost such as medical care its own inflation rate, and model going back to work for a while if the markets fall hard. Its sensitivity heatmap shows how your odds change as any two inputs move at once, and the download saves several what-if versions of your plan. It runs on Windows and Linux as well as a Mac, it works offline, and its author publishes the core of the simulation's code so you can see exactly what it does. If you like building and testing a plan yourself, at no cost, those are fine reasons to pick it.
Where ZBee is different
The first difference is the answer itself. FRP starts from the spending you type in and tells you how likely it is to last; its goal-seek setting can work backwards to an input that reaches the odds you want. ZBee starts at the other end: the number it produces is the spending, the most your accounts support at the confidence you pick, with this year's guardrail trigger points in dollars beside it. FRP's spending rules compare your portfolio with its value when retirement began; ZBee's follow the published Guyton-Klinger rules, measured against what the whole plan can pay out, Social Security still to come included.
The second is tax. FRP does charge tax, which not every free tool does, but at two flat rates you choose. ZBee works each year's tax out from the rules: the federal brackets, how much of your Social Security is taxed, required withdrawals (RMDs) that push income up later on, Medicare's income-based surcharge (IRMAA), and your state's own rules. States differ a great deal in retirement: Illinois does not tax retirement income at all, and Georgia excludes up to $65,000 of it per person from age 65. Flat rates picked in advance, even ones you change by year, cannot follow your income as it moves, so the answer is only as right as the rates.
The third is what the number is built on. In FRP you type three balances and choose one average return and spread for the whole portfolio; its author explains that this is deliberate, to keep the model simple and in your hands. ZBee links read-only to your brokerage, brings in every position, and draws each year's returns for the mix you actually hold. It then replays your number through every complete stretch of real market returns since 1928, in their real order and with the inflation of those same years, and names any stretch it would not have survived.
And ZBee is a native Mac app rather than a Java program. Like FRP's download, it keeps your plan on your own computer: ZBee stores your financial data on your Mac and never on its servers.
Roth conversions in Flexible Retirement Planner and ZBee
Both let you enter a conversion plan. In FRP it is a cash flow: you give an amount and the years, it moves that amount from tax-deferred to tax-free savings, and it charges tax at the one income-tax rate you set. That makes the cost of a conversion something you supply. In ZBee the conversion is ordinary income in the year you make it, so it runs through the same brackets, Medicare IRMAA tiers and Social Security taxation as any other income, and the year-by-year table shows how much more income fits before the next tax bracket. ZBee measures the plan you enter and shows what it does to your safe yearly spend, your lifetime tax and what you leave behind; it does not recommend an amount. The full comparison explains where that line is drawn and why.
Which should you choose?
Choose Flexible Retirement Planner if you want a free simulator you set up by
hand, want to model the saving years too, like testing what-ifs and sensitivity heatmaps, or use
Windows or Linux.
Choose ZBee if the question you actually have is "how much can I safely spend?",
you want it measured from the accounts you hold with each year's tax worked out and real market
history built in, and you want it as a native app on your own Mac. Running both is cheap: FRP is
free, and ZBee is free for 30 days on your own numbers — every feature, no account.
Where the answers differ, the methodology shows every step of ZBee's.
See pricing · The full comparison · ZBee vs FIRECalc · ZBee vs cFIREsim · Guyton-Klinger spending guardrails · How much can you safely spend? · Read the methodology