ZBee vs cFIREsim
Last reviewed: August 2026
cFIREsim is what the FIRE community built when FIRECalc wasn't flexible enough: a free historical simulator with real spending strategies — Guyton-Klinger, VPW, percentage-of- portfolio, floors and ceilings — plus adjustable allocations and custom income streams. For a pre-tax portfolio experiment, it's the most capable free tool there is, and this page won't pretend otherwise. ZBee shares its core conviction (test the plan against history, not averages) and differs in one deliberate place: the tax layer.
The one-line version: cFIREsim runs sophisticated spending strategies on a pre-tax portfolio; ZBee runs the same kind of gauntlet on your actual accounts, with every year's taxes, RMDs, and Social Security in the math.
| ZBee | cFIREsim | |
|---|---|---|
| Price | $99/year (free demo) | Free |
| Historical stress-test | Every market sequence since 1928, plus a thousand Monte Carlo futures | Historical start-years (longer record), plus constant-growth modes |
| Spending guardrails | Guyton-Klinger on the after-tax plan — this year's trim and raise trigger points in dollars | Guyton-Klinger and others available — applied pre-tax |
| Taxes | Computed year by year — federal, state, IRMAA, NIIT, Social Security taxation | Not modeled — spending is treated as pre-tax |
| Account types & RMDs | Taxable / tax-deferred / Roth, withdrawal order, required distributions | One portfolio total |
| Your real holdings | Linked read-only — every position, live prices (or manual entry) | You type a total |
cFIREsim's features as published on cfiresim.com, August 2026. ZBee isn't affiliated with cFIREsim.
When cFIREsim is enough
If you think in pre-tax terms — a portfolio number, an annual draw, a strategy to flex it — cFIREsim will let you test almost any idea you have, free, with a longer historical record than ZBee's 1928 replay. For accumulation-phase what-ifs and strategy comparisons it's excellent, and its Guyton-Klinger implementation is a fine way to learn how guardrails behave.
Where the pre-tax frame breaks down
Guardrails move real money: a trim that cuts spending 10% lands differently when the spending was funding taxes too. And the tax bill isn't a constant you can subtract once — it depends on which account each year's withdrawal comes from, when RMDs start forcing tax-deferred money into income, how much of Social Security becomes taxable at your draw level, and whether a withdrawal tips a Medicare IRMAA cliff. Once meaningful money sits in tax-deferred accounts, the pre-tax answer and the after-tax answer can diverge by five figures a year. That divergence is invisible from inside a pre-tax model — which is no knock on cFIREsim; it simply drew its scope line there.
ZBee draws the line differently: your actual accounts (linked read-only, or by hand), each year's taxes computed from where the money comes from, RMDs and Social Security timing applied — and then the guardrails and the historical gauntlet run on that after-tax reality, reporting the most you can safely spend in today's dollars.
A fair way to decide
Run both. cFIREsim is free; ZBee's demo mode is the full app on realistic sample data with no account. If your cFIREsim number minus your best tax guess lands near ZBee's after-tax answer, the simple model is serving you well. If it doesn't, the methodology will show you exactly where the difference came from.
See pricing · The full comparison · ZBee vs FIRECalc · How much can you safely spend? · Read the methodology