About coastfire.info

coastfire.info exists to answer a single, surprisingly slippery question: when does the money you have already saved become enough to grow into your retirement, without you adding another dollar? That moment — Coast FIRE — is where compound growth quietly takes over the job you have been doing by hand. The calculator on the home page finds it, and these pages explain the reasoning behind it.

Who builds this

coastfire.info is built and maintained by Sandhill.Software, an independent software studio that makes small, considered tools. It is not a brokerage, an advisory firm, or a lead-generation funnel dressed up as a calculator. There is no sign-up, no email capture, no premium tier, and nothing is sold. The site is supported by unobtrusive advertising, which is the only reason it can stay free and account-free.

If you want to know exactly how your visit is handled — short version: it isn't, beyond the ads — the privacy policy lays it out in full.

Why it's free

Most retirement calculators sit behind a financial planner, a brokerage account, or a subscription. The math behind Coast FIRE does not justify any of that. It is compound interest and a withdrawal rate — arithmetic that should be inspectable by the person whose future it describes. We decided the most useful version of this tool was one that ran entirely in your browser, kept your inputs in the URL so you could bookmark or share a scenario, and showed its formulas openly rather than treating them as a trade secret.

How the math is derived

The projection rests on two well-established ideas, both spelled out under The Method on the home page:

• A real (inflation-adjusted) rate of return. Every figure the calculator reports is in today's dollars, so a number you see is a number you can actually reason about. We derive the real return from your nominal growth assumption and your inflation assumption rather than asking you to do that subtraction in your head.
• A safe withdrawal rate. The retirement target is built from the long-studied 4% rule — the finding that a diversified portfolio can typically sustain inflation-adjusted withdrawals of roughly 4% per year over a long retirement. You can change that assumption; the calculator does not pretend it is a law of nature.

We are deliberately transparent about the limits of these inputs. The guides — for instance, Is 7% a realistic return assumption? — exist precisely because the default numbers deserve scrutiny, not blind trust. Where reasonable people disagree, we show the disagreement instead of hiding it inside a single confident output.

What this tool is — and isn't

It is a fast, honest, back-of-the-envelope projection that helps you see when contributions can stop and growth can carry the rest. It is a good way to test how sensitive your plan is to a single assumption: nudge the return, the spending, or the retirement age and watch the Coast FIRE date move.

It is not financial advice, a guarantee, or a substitute for a professional who knows your full situation. It does not model taxes, sequence-of-returns risk, Social Security, healthcare, or the dozens of real-world complications that a one-page calculator cannot. Treat the result as a starting point for your own judgement, not a verdict.

Going further

The field guides work through the questions the calculator raises but cannot answer on its own — how Coast FIRE compares to Barista and Lean FIRE, how couples should run the math together, and what the target looks like at different ages.

Found a mistake, disagree with an assumption, or have an idea? That kind of correspondence genuinely improves the tool — the contact page tells you how to reach us.