26 May 2026 · Retirement Planning
Coast FIRE means saving enough now so your investments grow to fund retirement on their own — no more contributions needed. Here's how to calculate your coast number.
Coast FIRE is the most underrated retirement concept no one talks about at dinner parties.
The idea is simple: save aggressively until your portfolio reaches a "coast number," then stop saving for retirement entirely. Your investments grow on their own — compounding over decades — and arrive at your full retirement target right when you need them.
Between hitting your coast number and actually retiring, you only need to cover your living expenses. You can work less, switch careers, go part-time, or take risks you couldn't before. You've "coasted" to retirement.
Traditional FIRE (Financial Independence, Retire Early) means accumulating a portfolio large enough to live off indefinitely — typically 25–33x your annual spending.
Coast FIRE asks a different question: how much do I need invested today, so that compound growth alone gets me to my full FIRE number by retirement age?
The coast number is always smaller than the full FIRE number — because time and compound interest do the heavy lifting.
Want to know your exact retirement number?
Run your numbers in RetireGauge →The formula is straightforward:
Coast Number = Full FIRE Target ÷ (1 + annual return)^years to retirement
Or rearranged: how much, invested today at X% annual return, grows to your target in Y years?
Example:
Coast number = £1,000,000 ÷ (1.07)^30 = £1,000,000 ÷ 7.61 = £131,400
If you have £131,400 invested at age 35, you never need to contribute another penny. It'll grow to £1,000,000 by age 65 (in real terms, assuming 7% real returns).
Time is the biggest lever in the coast number calculation. The earlier you hit it, the smaller it is.
| Coast number at... | For a £1M retirement target (7% real return) |
|---|---|
| Age 25 | £66,600 |
| Age 30 | £94,000 |
| Age 35 | £131,400 |
| Age 40 | £183,000 |
| Age 45 | £258,000 |
| Age 50 | £362,000 |
| Age 55 | £508,000 |
The difference between coasting at 30 vs 40 is £89,000. Every year you wait, your coast number grows — because there's less time for compounding to work.
This is why it pays to front-load savings in your 20s and 30s. Money invested early has decades to compound. Money invested at 50 has less than half the time.
Your full FIRE target is the portfolio needed to sustain your retirement spending indefinitely.
The common approach: annual retirement spending × 25 (the 4% rule).
But as we've discussed elsewhere, you should adjust for:
If your desired retirement spending is £45,000/year and you'll receive £11,500/year from State Pension at age 66, your portfolio only needs to generate £33,500/year. That's a FIRE target of £837,500 — not £1,125,000.
Your coast number is calculated from that reduced target.
Want to know your exact retirement number?
Run your numbers in RetireGauge →Profile:
Years to compound: 27 years
Assumed real return: 6.5% (slightly conservative)
Coast number at 33: £938,000 ÷ (1.065)^27 = £938,000 ÷ 5.48 = £171,200
They already have £85,000 — about half their coast number. If they keep contributing for another 7–8 years, they'll reach £171,200 and can officially "coast." From that point, they only need to earn enough to cover living expenses, not retirement savings.
You may have heard of "Barista FIRE" — a related concept where you take a lower-stress job (famously, a barista) that covers living expenses while your portfolio grows.
The difference:
In practice, many people mix them. What matters is that you've reduced the pressure on your portfolio significantly.
1. You're betting on investment returns
If your portfolio grows at 4% instead of 7%, your coast number is much larger — or you'll fall short of the target. Run your coast calculation with conservative return assumptions (5–6% real) rather than optimistic ones.
2. Lifestyle costs can creep up
If you "coast" and lifestyle inflation pushes your spending up, your retirement target grows and your previously sufficient coast number might not be enough anymore.
3. Tax changes and state pension changes
Government rules change. The UK State Pension age has risen several times. Build a buffer into your plan.
4. You still need to work
Coast FIRE doesn't mean you stop working — you still need to cover living expenses. What changes is the financial pressure: you can work less, in lower-paid but more fulfilling work, without worrying about retirement savings.
The variables involved — current savings, return rate, retirement age, state pension timing, country tax rules — quickly get complicated to calculate by hand. A calculator that models these together gives you a much clearer picture.
RetireGauge calculates your coast number alongside your retirement readiness score, with real country-accurate tax and state pension baked in. Enter your current savings and income, set your target retirement age, and it tells you how close you are to coasting.
Your Coast FIRE number (or coast number) is the amount you need invested today so that, with compound growth and no additional contributions, your portfolio reaches your full retirement target by your chosen retirement age. It's always smaller than your full retirement target because time and compound interest bridge the gap.
Yes — for people who can save aggressively in their 20s and 30s. The key is front-loading savings while time is on your side. Many people hit their coast number without realising it because they've been contributing steadily to a pension and ISA for a decade. Run the calculation with your actual savings to find out.
Regular FIRE means accumulating enough to retire fully — typically 25x annual spending. Coast FIRE means accumulating a smaller amount earlier, then relying on compound growth to reach the full FIRE number by a later retirement date. You still need to work to cover expenses during the coasting phase, but you have much more freedom in how you work.
A conservative 5–6% real return (after inflation) is a reasonable assumption for a globally diversified equity portfolio over long periods. Some people use 7%, which is closer to the historical average, but markets vary. Running your coast number at 5%, 6%, and 7% gives you a range — and planning for the 5% scenario means you have a margin of safety.
Yes — and it can significantly reduce your required coast number. If your state pension covers £11,500/year of your retirement income, your private portfolio doesn't need to generate that amount. That lowers your full FIRE target, which in turn lowers your coast number. Always factor in state pension when calculating your coast number for a realistic result.
Answer a few questions about the life you want — RetireGauge gives you a single readiness score and the exact income or age that funds it.
Try the calculator →