Coast FIRE Planner

Overview

Estimate the invested amount you need today for your portfolio to coast toward a future FIRE target. The calculator is built for scenario thinking rather than one-off arithmetic. It gathers the variables that usually sit in a spreadsheet, updates the results immediately in the browser, and keeps the main outputs visible while you adjust assumptions. That makes it useful for comparing a base case, a conservative case, and a more ambitious case without losing track of the logic. The aim is not to replace expert judgment, professional advice, laboratory validation, or field measurement. It is to give you a transparent model that is quick to inspect and easy to challenge.

How to use it

Start by entering the assumptions that best describe your situation: Current portfolio, Annual spending, Withdrawal rate, Real return, Current age, Retirement age. Use measured values when you have them and sensible estimates when you do not. A good workflow is to change only one field at a time, because that makes sensitivity easier to see. If the answer changes dramatically after a small adjustment, treat that input as a planning risk and spend more time verifying it. The result cards update locally, so you can run several scenarios without sending your data anywhere or saving personal information.

Calculation method

The FIRE target is annual spending divided by the withdrawal rate; the coast number discounts that target back with the real return until retirement age. The formulas are deterministic, so the same inputs produce the same outputs every time. Intermediate values are kept as numbers until the display layer formats them for your language. The primary result cards are FIRE number, Coast number today, Gap to Coast FIRE. Supporting metrics such as Monthly saving needed, Portfolio at retirement, Coast progress add context so that the headline figure is not read in isolation.

Interpreting the results

Use the first result as the headline, but do not stop there. The surrounding metrics often explain whether the answer is strong, fragile, expensive, efficient, or simply a rough midpoint. For planning, it is usually better to compare three cases than to trust a single precise number. Try one realistic case, one cautious case with less favorable assumptions, and one upside case. The distance between those results is often more useful than any single estimate, because it shows how much room you have before the plan stops making sense.

Practical example

Suppose the default scenario is close to your situation. Record the headline result, then reduce the most optimistic input by ten to twenty percent. If the conclusion still holds, the plan has some resilience. Next, increase the cost, delay, loss, or uncertainty input and watch which supporting card moves first. This is the practical value of an interactive calculator: it turns a static formula into a conversation with the assumptions. You can also use the result labels as a checklist when discussing the model with a colleague, coach, client, teacher, or adviser.

Limitations

Every calculator simplifies reality. It assumes the inputs are internally consistent, uses the stated formula, and does not know about contracts, regulation, personal medical history, market shocks, instrument calibration, or local constraints unless those factors are represented by an input. Treat rounded results as planning estimates. For high-stakes financial, health, engineering, legal, or laboratory decisions, use this page as a starting point and confirm the final decision with qualified expertise and primary source data.