Coastline vs cFIREsim
cFIREsim is a beloved historical-cycle simulator for testing retirement withdrawal strategies. Coastline covers the whole journey — building wealth and spending it — with real federal and state tax math and a full breakdown of every number. They overlap, but they’re built for different questions.
Coastline vs cFIREsim, feature by feature
| Feature | Coastline | cFIREsim |
|---|---|---|
| Free, no signup | Yes | Yes |
| Historical-cycle / Monte Carlo simulation | Yes | Yes |
| Accumulation (working years) modeling | Yes | No |
| Federal + state income tax math | Yes | Limited |
| Roth conversion / withdrawal-order strategy | Yes | No |
| Social Security claim-age modeling | Yes | Basic |
| All 50 states + DC, itemized deductions, IRMAA, ACA, 72(t) | Yes | No |
| Shows the exact math behind every number | Yes | No |
| Save / share scenarios by link | Yes | Limited |
The same plan, run through Coastline
cFIREsim takes a portfolio, a yearly spend and an allocation and replays them against every historical cycle. The spend is pre-tax: it does not know which account the money comes from or what the IRS takes on the way out. One retiree, $1,000,000 at 65 split 60% taxable / 30% traditional / 10% Roth, single filer, no-income-tax state, no Social Security, spending $40,000 a year in today’s dollars to 95:
| Figure | Coastline |
|---|---|
| Spending the accounts delivered, 65 to 95 (today’s $): the $40,000 a year plus Medicare premiums from 65 | $1,287,081 |
| Gross withdrawn to deliver it | $1,295,071 |
| Tax on the way out, federal + state (today’s $) | $7,990 (0.6% of withdrawals) |
| Real net worth left at 95, straight-line 6% return | $382,148 funded to 95 |
| Most the plan can spend after tax and still last to 95, retiring at 65 | $47,000 (4.7%) |
| … retiring at 45 instead | $28,000 (2.8%) |
| $40,000/yr across 67 historical cycles (1928–2024), 70/30: runs that lasted to 95 · bad luck / median / good luck ending | 88% · $0 / $1,675,806 / $3,895,172 |
The tax row is what a pre-tax simulator leaves for the reader to guess, and for this particular retiree it is small: a single filer drawing about $12,000 a year from a traditional account and the rest from a taxable account whose gains fall in the 0% capital-gains bracket has little to tax. A larger traditional balance, a pension or Social Security stacking on top, or a state with an income tax moves it — which is the case for running your own plan rather than reading this one. The figures are identical on every comparison page on this site because they are one plan run once; only the tool being compared changes.
When to use cFIREsim
Reach for cFIREsim when your question is purely "would my withdrawal strategy have survived history?" — it shines at running a spending plan against every historical market cycle and showing the range of outcomes. If you’re already retired or near it and want a fast historical stress-test, it’s excellent and time-tested.
When to use Coastline
Use Coastline when you want the full picture: how your wealth builds during your working years, the taxes you’ll actually pay in both accumulation and retirement — federal brackets, all 50 states and DC, itemized deductions, IRMAA, ACA credits — Roth-conversion and withdrawal-order strategy, Social Security timing, and a click-through breakdown of the math behind every figure.
Both tools are free. This comparison is written by Coastline, so weigh it accordingly — try both and use whichever answers your question. cFIREsim is a well-regarded tool; the differences here are about focus, not quality.
Common questions
Is cFIREsim or Coastline more accurate?
They answer different questions. cFIREsim is a rigorous historical-cycle backtester for withdrawal strategies. Coastline models the full tax and accumulation picture and shows its work. For a withdrawal-only stress test, cFIREsim is purpose-built; for whole-plan tax-aware modeling, Coastline goes further.
Are both really free?
Yes. Both are free to use. Coastline needs no account.
Does Coastline do historical simulation like cFIREsim?
Yes — Coastline’s Simulation Tools replay real historical market sequences (and Monte Carlo draws) to stress-test your plan, in addition to the year-by-year deterministic projection.