What Is Your 'Coast FI' Number? The Formula That Shows When You Can 'Stop' Saving for Retirement

A retirement shortcut is spreading through personal finance forums and social media feeds, and it comes with a bold promise: work out one number, and you will know the exact moment you can stop saving for retirement. Its followers call it 'Coast FI'. Financial advisers are less convinced.

The idea sits within the wider FIRE movement, short for Financial Independence, Retire Early. Where traditional FIRE asks people to save hard enough to quit work for good, Coast FI sets a gentler target. Reach a certain figure, its proponents say, and compound growth alone will carry your pot to a full retirement fund by 65 or 67, even if you never add another penny.

That is the claim. Whether it holds depends almost entirely on market assumptions that no one can guarantee.

How the 'Coast FI' Number Actually Works

The maths behind it is straightforward. First comes the FIRE number, the total you would need to live off your investments indefinitely. Most followers use the 4% rule, which means multiplying your expected annual spending by 25. Someone planning to spend £40,000 ($54,000) a year in retirement lands on a target of £1M ($1.35M).

The Coast FI number then discounts that target back to today. The formula divides the full figure by (1 plus your expected real return) raised to the number of years until retirement. A 30-year-old aiming to stop contributing by 65, assuming a 5% return after inflation, would need roughly £181,000 ($245,000) invested now. From there, the pot should grow to £1M on its own by 65.

Once someone reaches the figure, the pitch is that they need only earn enough to cover day-to-day bills, with no further pension contributions required.

Why Younger Savers Are Embracing the Formula

Part of the appeal is timing. Coast FI arrives years before full independence, which makes it feel reachable to people in their twenties and thirties. It also reframes work as optional, opening the door to shorter weeks, career changes, or lower-paid jobs with less stress.

Ryan Sterling, who founded the New York wealth-management firm Future You Wealth, set out the trade-off in comments to Scripps News. 'The one issue with FIRE is it creates a lot of anxiety to retire fast, retire as soon as possible,' he said.

The milestone is not a quick win, though. Michela Allocca, founder of the personal-finance platform Break Your Budget, says reaching a Coast FI number usually takes 10 to 15 years of front-loaded saving and rarely happens in a person's twenties, especially on a standard salary.

The Risk Experts Keep Flagging

The warnings centre on a single weak point: the whole model rests on returns that may never turn up. Most calculators assume a 7% return, which drops to around 5% after inflation. A poor run of years early on can undermine the projection, and because the saver has stopped paying in, there is nothing new going in to buy through the dip.

Market analyst Jahanzeb Nawaz put figures on it in comments to Moneywise. 'Yet if the decade right after you stop contributing delivers 2% real instead of 5%, the compounding base never recovers, because there are no fresh contributions to dollar-cost-average through the dip,' he said. 'A Coast FIRE number that pencils out to $1 million at 65 with a 5% real return drops to closer to $700,000, and that savings gap is not recoverable by working a few more years coasting.'

The number is a projection, not a promise. Rising costs, a longer life, or an unexpected bill can all move the goalposts after someone has eased off. Allocca recalculates her own figure every year, treating it as a moving milestone that shifts with income, spending, and the markets rather than a fixed finish line.

What 'Coast FI' Means for UK Savers

British savers face extra wrinkles the US-born strategy tends to skip. Most Coast FI money sits inside a pension, which cannot be touched until 55, rising to 57 on 6 April 2028. Coasting therefore frees you from saving, not from working, until then.

The State Pension softens the target too. At £12,548 ($16,990) a year for 2026/27, it acts as an income floor worth roughly £313,700 ($424,750) in portfolio terms at a 4% withdrawal rate, shrinking the pot your own savings must build.

Even so, many UK planners lean on a more cautious 3.5% withdrawal rate, which lifts both the FIRE number and the Coast FI figure, precisely because the market cannot be counted on to behave.