What is Coast FI, and how do you work out your number?
Coast FI is the amount you'd need invested today for compound growth alone — with no further contributions at all — to carry you to your full financial independence number by the age you plan to retire. Cross it and the retirement half of your finances is settled: it's funded, and all it needs now is time. Your income only has to cover this year's expenses.
The arithmetic is a discount backwards. If your FI number is $1.2 million, you're 32, you're aiming at 60, and you assume a 6% real return, then $1.2M ÷ 1.06²⁸ is roughly $236,000 — the balance that gets there by itself. This calculator goes one step further than that formula and solves the FI number itself the same way the FI Calculator does, with a year-by-year check that the portfolio survives your actual expense schedule, so a pension or a mortgage payoff you already know about is reflected in the target instead of being ignored.
The more useful question, though, is when you'll get there. Two lines move over time: the coast requirement rises every year, because each year that passes is one less year of compounding runway, while your portfolio rises faster, earning that same return plus whatever you're contributing. Where they cross is your Coast FI date. Because both sides compound at the same rate, the gap between them closes purely on the strength of what you contribute — so with any positive savings rate the two lines always converge eventually. The only real question is whether they meet before the retirement age you picked. The chart carries on past that point into the drawdown, where the same dashed line becomes the balance you'd need in order to last to the end of your plan: it falls away to nothing if you've set a finite planning age, or holds level on "forever", which is what capital that never runs down looks like.
Everything here is in today's dollars against a real, inflation-adjusted return, so there's no separate inflation input to set. Because a single return assumption gets compounded across decades, the tool shows your coast number at three different rates side by side rather than presenting one figure as though it were certain. The Intermediate tab lets you run the question backwards — pick the age you want to be coasting by and it solves for the contribution that gets you there — and adds income streams: a government or workplace pension, rental income, or a side business that pays indefinitely, which usually move the number more than anything else on the page. The Advanced tab layers in lasting expense changes, one-time windfalls, and a partial "coast on reduced savings" plan that tells you the income your work would actually need to bring in. Any of these — expenses, withdrawal rate, return, portfolio, age, the planning horizon, income streams, expense changes, or windfalls — can be linked individually to your FI Calculator figures, so the two stay matched without re-entering the same pension or windfall twice.