Coast FI, explained without the jargon
August 23, 2026 · Updated September 7, 2026 · 3 min read
- coast fi
- fire
- retirement math
Coast FI is the point where the money you've already saved, left alone and simply allowed to grow, will reach your full retirement number by the time you actually want to retire — with no more contributions required. You could stop saving today and still get there. Most people who hit it don't stop working; they just stop feeling like every dollar has to go to retirement, and redirect the difference toward a house, a career change, or just breathing room.
The number that actually matters
Coast FI isn't a net worth figure — it's a comparison between two projections. Take what you've already invested, project it forward at a reasonable growth rate with zero further contributions, and see what it becomes by your target retirement age. If that number already covers your full FI number (your annual spending times 25, the standard 4% rule), you've coasted there. If it falls short, the gap tells you exactly how much more you still need to contribute — not just that you're "behind."
Why this is easy to get wrong by hand
The two most common mistakes: using your current age instead of your target retirement age for the growth projection (which understates how much time your existing savings have to compound), and forgetting that any debt still being paid down reduces what's actually available to invest and grow. A number that ignores either one will look more optimistic than reality.
Run your own numbers
This is the same calculation Reign runs inside the app, with the same assumptions. It answers two questions: what balance would coast to your chosen retirement age from here, and what age your current balance coasts to on its own.
Are you at coast FI?
Four numbers. Nothing is saved. Assumes a 6% annual return, compounded monthly, and the 4% rule adjusted for how long the money must last.
Your coast number for age 60
$273,397
Left alone, your money reaches FI at
age 72, 6 mo
Not yet — you are $153,397 short of the balance that would coast to age 60 on its own. Contributions still have work to do.
The calculator leaves out what a spreadsheet leaves out too: debts still being paid down, Social Security, and the difference between what you spend now and what you will spend then. Reign's free Snapshot takes those into account, and the calculator lives on its own page at /tools/coast-fi.