ZBee vs FIRECalc
Last reviewed: August 2026
FIRECalc is the grandfather of honest retirement calculators, and this page won't pretend otherwise. Its core idea — don't trust average returns; walk your plan through every actual market start-year in history and count the survivors — is the right idea. It's free, it has no account to create, and it has talked more people off the "7% average growth" ledge than any tool on the internet. ZBee is built on the same conviction. The difference is what each tool is willing to leave out.
The one-line version: FIRECalc stress-tests a portfolio number you give it, before tax. ZBee stress-tests your actual accounts, with the tax layer — brackets, RMDs, Social Security taxation, IRMAA — computed year by year.
| ZBee | FIRECalc | |
|---|---|---|
| Price | $99/year (free demo) | Free |
| Historical stress-test | Every market sequence since 1928, plus a thousand Monte Carlo futures | Every start-year since 1871 — a longer record |
| Taxes | Computed year by year — federal, state, IRMAA, NIIT, Social Security taxation | Not modeled — you must fold taxes into the spending number yourself |
| Account types & RMDs | Taxable / tax-deferred / Roth, withdrawal order, required distributions | One pre-tax portfolio total |
| Your real holdings | Linked read-only — every position, live prices (or manual entry) | You type a total |
FIRECalc's features as published on firecalc.com, August 2026. ZBee isn't affiliated with FIRECalc.
When FIRECalc is enough
Honestly: often. If your savings are mostly in one bucket, you're years from RMD age, and you just want to know whether a spending level is in the right ballpark, FIRECalc's history-based survival count is a fine first answer — and its 1871-to-present record is actually longer than ZBee's 1928 replay. Plenty of careful planners should start there, and some never need more.
When the tax layer starts to change the answer
FIRECalc asks for your spending including taxes — which means it asks you to solve the hardest part yourself. The tax bill of a retirement isn't a flat number: it depends on which accounts each year's withdrawal comes from, when Social Security starts (and how much of it becomes taxable), when RMDs force tax-deferred money into income whether you want it or not, and whether withdrawals push you over IRMAA's Medicare surcharge cliffs. For a retiree with meaningful tax-deferred savings, those effects can swing the sustainable spend by five figures a year — in either direction. A guess baked into the input becomes a guess baked into the verdict.
ZBee's whole reason to exist is doing that layer properly: it takes your actual accounts (linked read-only, or entered by hand), computes each year's taxes from where the money actually comes from, applies RMDs and Social Security timing, and then runs the same kind of historical gauntlet FIRECalc taught everyone to demand — reporting the most you can safely spend, after tax, in today's dollars.
A fair way to decide
Run both. FIRECalc is free; ZBee's demo mode is the full app on realistic sample data with no account. If the tax-aware number and your FIRECalc ballpark are close, FIRECalc was serving you well. If they aren't, you've learned exactly why — and the methodology shows every step of the difference.
See pricing · The full comparison · ZBee vs cFIREsim · How much can you safely spend? · Read the methodology