I Want To Retire Today vs cFIREsim: Which FIRE Calculator Should You Use?
cFIREsim has been a go-to FIRE calculator for years — deep historical backtesting, multiple withdrawal strategies, granular control over every variable. It's a powerful tool built for people who want to model every edge case.
I Want To Retire Today takes the opposite approach: 5 inputs, 5 seconds, a clear answer. Monte Carlo simulation instead of historical-only backtesting. Automatic sensitivity analysis instead of manual tweaking. Different tools for different needs — here's how they compare.
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Side-by-side breakdown of what each tool offers.
| Feature | I Want To Retire Today | cFIREsim |
|---|---|---|
| Inputs | 5 | 30+ variables |
| Time to result | 5 seconds | 5–10 minutes setup |
| Simulation | Monte Carlo 1,000 runs | Historical backtest only |
| Withdrawal strategies | 4% rule (adjustable) | Guyton-Klinger, VPW, CAPE, etc. |
| Account types | Single pool | Single pool |
| Mobile experience | Optimized | Functional but cramped |
| Sensitivity analysis | Auto "Retire faster" | None |
| Data freshness | Current | Last updated Nov 2024 |
| Maintenance | Active | Creator moved to FIREproof |
| Sharing | OG image + URL + card | Simulation link |
| Price | Free | Free |
When to use I Want To Retire Today
You want a quick sanity check
Five inputs, five seconds, a clear retirement date. No need to understand 30 configuration options before you can see a result. If you're wondering "am I on track?" — this gives you the answer immediately.
You're on your phone
I Want To Retire Today is built mobile-first. cFIREsim works on mobile but the dense input forms and wide result tables are hard to navigate on a small screen. If you're checking your numbers on the go, the experience difference is significant.
You want automatic sensitivity analysis
After your result, I Want To Retire Today shows you exactly how cutting expenses or increasing contributions would change your retirement date. cFIREsim requires you to manually re-run simulations for each scenario. The "Retire faster" analysis surfaces actionable levers without extra work.
When to use cFIREsim
You want advanced withdrawal strategies
cFIREsim supports Guyton-Klinger, Variable Percentage Withdrawal, CAPE-based, and other dynamic withdrawal methods. If you're comparing how different withdrawal rules perform across historical periods, cFIREsim is purpose-built for this. I Want To Retire Today uses an adjustable fixed-rate approach.
You're a historical data purist
cFIREsim runs your plan against every historical cycle since 1871, showing you exactly how your portfolio would have performed in 1929, 1966, 2000, and every other period. If you trust historical backtesting more than Monte Carlo probability modeling, cFIREsim speaks your language.
You need granular income/expense modeling
cFIREsim lets you add Social Security start dates, pension income, one-time expenses, and other detailed cash flow events. If your retirement plan includes multiple income sources kicking in at different ages, cFIREsim's input depth handles that complexity.
Frequently asked questions
Is cFIREsim still being maintained?
cFIREsim's creator has shifted focus to a newer tool called FIREproof. The site is still online and functional, but the last data update was November 2024. It remains a solid tool for historical backtesting, but don't expect new features or frequent data refreshes going forward.
Which calculator is more accurate?
Neither is inherently more accurate — they answer different questions. cFIREsim tells you how your plan would have performed in every historical period since 1871. I Want To Retire Today runs 1,000 Monte Carlo simulations to model a range of possible futures. Historical backtesting is precise about the past; Monte Carlo is better at capturing uncertainty about the future. Both are valid approaches.
Can I use both tools together?
Yes, and many FIRE planners do exactly that. Start with I Want To Retire Today to get a quick read on your retirement readiness and explore sensitivity scenarios. Then use cFIREsim to stress-test specific withdrawal strategies against historical data. The two approaches complement each other well.
Why does I Want To Retire Today only have 5 inputs?
Because research shows that your savings, contributions, expenses, expected return, and time horizon explain 90%+ of retirement outcomes. Adding 25 more variables increases complexity without proportionally improving accuracy. If you need to model Guyton-Klinger rules or CAPE-based withdrawals, cFIREsim is the right tool. For most people, 5 inputs is enough to make a confident decision.
Which tool should I use if I'm just starting to think about FIRE?
I Want To Retire Today. It takes 5 seconds to get a result, shows you exactly when you can retire, and includes automatic sensitivity analysis so you can see how small changes affect your timeline. You don't need 30 inputs when you're still figuring out your target number. Once you have a plan and want to stress-test specific withdrawal strategies, cFIREsim is a great next step.
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