Coast FIRE is the point where your invested pot is big enough that compound growth alone — with no further contributions — will reach your retirement target. You stop saving, not working. The pot needed today is your target pot ÷ (1 + real return)years to retirement. For a UK target of £750,000 at 67 at 5% real, a 28-year-old's Coast number is about £112,000.
What "Coast FIRE" actually means
Coast FIRE is the point in your life when your existing investment pot is big enough that, with no further contributions, compound growth alone will carry it to your retirement target. You don't stop working. You stop saving. From that day forward, anything you earn above your living costs can be spent — no guilt, no required pension contributions, no monthly transfer to the ISA.
It sits next to the more familiar FIRE labels but it's a different beast. Regular FIRE (sometimes called Full FIRE) is the destination — enough pot to retire fully and live off it. Lean FIRE is the same idea at a lower spending target. Fat FIRE is the high-spend version. Coast FIRE is a milestone on the way to any of those — the year you stop having to add to the pot.
What I find most useful about Coast FIRE as a frame is that it splits a person's financial life into three distinct phases instead of two:
- Working & contributing. Salary in, ISA / pension out. The pot grows from contributions and compound returns together.
- Coasting. Salary still in, but no contributions to investments. The pot grows on compound returns alone. You're still working — you just don't need to save any more.
- Retired. No salary, drawing income from the pot, State Pension, DB pension, etc.
Almost every UK retirement planning tool I tested while building WealthR conflates phases 1 and 2 — they assume you contribute right up to the day you retire. For the FIRE-curious user that's wrong by years.
The maths in one formula
Coast FIRE has one equation. Memorise this one and you can run the calculation in your head:
A note on the × 25 multiplier: it comes from the 4% "safe withdrawal rate", a US rule of thumb (the Trinity Study) for a 30-year retirement. Plenty of UK FIRE writers trim it to 3.5% — multiplying by ~29 — for a longer horizon or a more cautious plan. A lower withdrawal rate means a bigger target pot and a bigger Coast number, so it's worth knowing which rate a calculator is using before you trust its output.
The intuition: a pot today, left untouched, doubles roughly every 14 years at 5% real returns. Two decades of compounding turns £100,000 into about £265,000. Four decades turns £100,000 into about £700,000. The maths is brutal and quiet — you don't notice it day to day, but it's the reason starting early dwarfs everything else.
The headline output of the formula is the Coast number — the amount you need in your investment pot today, with no further contributions, for it to grow into your target retirement pot by the time you retire. If your invested pot today is larger than your Coast number, you've already passed the milestone. If it's smaller, the gap tells you roughly how many more years of saving you need before you can stop.
A worked UK example
Let's run the numbers for someone who matches a fairly common UK situation: 28 years old, £50,000 already invested across an ISA and a workplace SIPP, aiming to retire at 67 with the PLSA "Moderate" retirement living standard of around £30,000 a year.
| Input | Value |
|---|---|
| Age now | 28 |
| Currently invested | £50,000 |
| Planned retirement age | 67 |
| Years to retirement | 39 |
| Target retirement income | £30,000/yr |
| Target retirement pot (× 25) | £750,000 |
| Expected real return | 5% per year |
| Coast number today (age 28) | £111,888 |
So at 28 with £50,000 invested, you're about 45% of the way to Coast FIRE. You need another £62,000 (roughly) before you can stop contributing and still hit £750,000 at 67 on compound growth alone.
If you're saving £600 a month from here on — close to the average UK workplace pension contribution including employer match — you'd hit the Coast number at around age 40. After that you could stop saving entirely. The £204,000 you'd have at 40 would compound to roughly £750,000 by 67 with no further contributions, assuming real returns hold up.
The Coast FIRE insight: at 28, the choice isn't "save for 39 years or don't." It's "save hard for 12 years, then stop." The maths is the same. The lifestyle after 40 is wildly different.
Once past Coast FIRE, anything you earn above living costs is genuinely yours. You could downshift to part-time work, take a sabbatical, change careers to something lower-paid but more meaningful, or just stop pretending you need to obsess about an ISA top-up every spring. The pot keeps going.
What's your Coast number — and are you past it?
The example above is a stranger's. Put in your own ISA and pension figures and WealthR runs this exact formula for you — your Coast number today, how many saving years you've got left, and your three-phase trajectory (working → coasting → retired) drawn out to your retirement age. No bank connection, just your numbers, and it remembers them so you can watch the gap close.
Start free → or run the quick calculator first — no signupCoast number by age and real return
The single biggest lever is how early you start — and the return assumption you trust. Here's the Coast number today for the same £750,000 target pot (the worked example above, before any State Pension deduction), at different current ages and real returns. Read it as: "at this age, with this pot already invested, I never have to contribute again."
| Age today | Years to 67 | 4% real | 5% real | 6% real |
|---|---|---|---|---|
| 25 | 42 | £144,000 | £97,000 | £65,000 |
| 30 | 37 | £176,000 | £123,000 | £87,000 |
| 35 | 32 | £214,000 | £157,000 | £116,000 |
| 40 | 27 | £260,000 | £201,000 | £156,000 |
All figures assume a £750,000 target pot and use Coast number = £750,000 ÷ (1 + real return)years to 67, rounded to the nearest £1,000. Subtract guaranteed income (State Pension, DB) from your spending target first and every number here drops — see below.
UK Coast FIRE calculator
Enter your age, current pot, target retirement income and assumed real return. The calculator tells you your Coast number today, how close you are, and what your projected retirement pot looks like if you stop contributing now. UK-specific defaults (4% SWR, 5% real return, State Pension Age 67). No signup.
Open the calculator →
Why UK savers should care (and why most planners miss it)
Coast FIRE is a US-originated concept and most of the writing about it online assumes 401(k)s, traditional IRAs, and US tax bands. Translating it to the UK takes more than a currency swap — there are real structural differences worth knowing about.
Pension access age 57 — and the bridge years
You can't draw from a UK SIPP or DC workplace pension before 55 (rising to 57 from April 2028). If you plan to retire before that age, your pot of pension money is functionally locked, and you need accessible-now savings — typically a Stocks & Shares ISA — to bridge the years between actual retirement and pension-access age.
This shapes the UK Coast FIRE strategy. The standard approach for a high earner is: max the ISA first (£20,000/yr 2026/27) to build the bridge, take any workplace pension match (it's free money — usually 3-6% of salary), then route the rest into a SIPP for the 40% higher-rate tax relief. When you hit Coast FIRE on the combined total, you've also typically built the bridge to age 57.
State Pension and DB pensions change the maths a lot
The £750,000 target in the worked example above assumes you fund the full £30,000/yr from your invested pot. In practice, most UK retirees have at least the State Pension (about £12,500/yr from 2026/27 at the full new rate) and some have a workplace DB pension on top.
Treat guaranteed retirement income as a deduction from your spending target before calculating the Coast number. If your target is £30,000/yr and the State Pension covers £12,500/yr, you only need to fund the other £17,500/yr from invested money — which means a pot of about £437,500, not £750,000. The Coast number drops by 42%.
If you have a DB pension paying £15,000/yr at retirement, the maths drops further. Don't make the mistake of running Coast FIRE without subtracting guaranteed income — it's the most common reason UK savers think they need a much bigger pot than they actually do.
The £100k tax-trap angle
For high earners between £100,000 and £125,140 of taxable income, the marginal tax rate is effectively 60% (personal allowance taper). Pension contributions — usually via salary sacrifice — are one of the few ways to get income back under £100,000. That's a strong reason for a high earner to delay coasting on the pension side — every £1,000 contributed while in the trap saves about £600 of tax, which doesn't apply when you stop.
What real-return assumption to use
UK consumer financial planning tools tend to default to a 5% real return based on a global-equity portfolio. That's a defensible long-term average but the sensitivity is high: a 3% real return moves your Coast year out by about 5 years; 6% brings it in by 3. Running two or three scenarios rather than picking one number is what shows you the range. WealthR's three-scenario forecast (Pessimistic / Realistic / Optimistic) does this by default.
How to model your own Coast year in WealthR
The three-phase trajectory is built into WealthR's Forecast tab. Once you've added your DOB, current investments and a retirement age, the chart automatically shows a Working phase (green) and a Retired phase (amber). To add the Coasting phase, set your planned stop-saving age in the Adjust Assumptions panel:
Once you set an age, the trajectory line picks up a third colour — blue — for the years between when you stop contributing and when you start drawing. The legend updates to show the Coast and Retirement transition dates. You can drag the scenario growth slider to see how a different real-return assumption changes the picture. The standalone calculator at /tools/coast-fire-uk/ answers a single one-shot question; the in-app chart answers the same question but reads your actual tracked finances, factors in your real income streams (State Pension, DB, drawdown), and lets you save and compare scenarios.
Five common UK Coast FIRE mistakes
Things I see again and again when people start running their own Coast FIRE numbers:
- Treating it as American. The US writing on Coast FIRE assumes 401(k) access at 59½, Social Security at 62-70, and no workplace DB pensions. UK tools that bolt on a £ symbol without changing the underlying maths give you the wrong answer. SIPP at 57, State Pension at 67, and any DB pension all change the calculation materially.
- Ignoring State Pension and DB pensions. If you treat your invested pot as needing to fund the full retirement income, you'll overstate the Coast number by 30-50% for most UK households. Subtract guaranteed income before doing the maths.
- Optimistic real return assumptions. 7% real returns sound reasonable on a global equity portfolio, until you look at any 30-year window that included the 1970s or 2000-2010. 5% is the honest default; 4% is the conservative one. Run the numbers at multiple rates.
- Confusing "stop saving" with "stop working". Coast FIRE is not retirement. You still need to cover living costs from somewhere — usually continued employment — for years or decades after hitting it. The pot just doesn't need to grow from contributions any more.
- Building a coast plan that doesn't survive a market crash. A 30% drawdown in the first three years of your coast phase can extend your retirement age by 5-10 years because you don't have new contributions to buy at the lower prices. The mitigation is sequence-of-returns awareness: keep some flexibility in the plan, especially in years 1-5 of coasting. WealthR's stress test models this explicitly.
What I'd actually do
If I were 28 today with £50,000 invested — the worked example above — the practical sequence would be:
- Run the Coast FIRE numbers with realistic UK inputs. Include State Pension, any workplace DB, and a 5% real return. See what your real Coast number is — it's usually smaller than the headline £700k figures floating around online.
- Front-load contributions while you're still under the £100k tax trap. Maximum tax-efficient savings in your twenties and thirties — workplace pension match first, ISA bridge, SIPP for the higher-rate relief.
- Map your projected Coast year onto the Forecast trajectory chart. Seeing the blue Coasting phase between Working and Retired changes how the plan feels — it's a real milestone, not just a number.
- Don't actually stop saving the day you hit Coast. Cross the line by a meaningful buffer — 20-30% above the Coast number — before deciding to stop. The buffer protects you against sequence-of-returns risk in the first few coast years.
- Keep checking in. Coast FIRE isn't a one-shot calculation. Your spending target changes. Real returns vary. The wrapper rules change. Re-run the maths annually and let the trajectory chart show you the updated picture.
The point
Coast FIRE is the part of UK retirement planning almost no consumer tool surfaces well. The maths is a single formula. The implications are huge: for a 28-year-old, the difference between "save until 67" and "save until 40" is twenty-seven years of optionality. The pot ends up in the same place either way.
The free calculator at /tools/coast-fire-uk/ answers the one-shot question. The in-app three-phase Trajectory inside WealthR shows the same answer integrated with your real finances — your actual pension pots, your retirement income streams, your scenario growth assumptions, all in one chart. Whichever you use, the goal is the same: stop treating "retire at the State Pension Age" as the default and start treating "stop saving as early as the maths allows" as a real planning choice.
Inside WealthR · Free
WealthR's Forecast tab shows your real net worth trajectory across Working, Coasting and Retired phases, factoring in your tracked pots, State Pension, DB pensions and ISA bridge. Add a planned coast age and watch the line bend at the right year. Free at the core tier — no signup needed to try, no expiry, no Open Banking, no ads.
Try WealthR free →Related reading
- Investing explained (UK): index funds, ISAs and dividends — the growth engine behind Coast FIRE, part of the UK Money Playbook.
- Your full FIRE number (free UK calculator) — Coast FIRE is the milestone on the way; this works out the destination pot.
- Will your money last? 10,000 simulated UK retirements — the sequence-of-returns risk behind the coast-phase crash warning above.
- UK salary sacrifice pension: the complete 2026/27 guide — how to front-load pension contributions tax-efficiently before you coast.