What Is Coast FIRE?
Coast FIRE is a specific milestone on the path to full financial independence — the point at which your existing invested portfolio is large enough that, with no further contributions, compound growth alone will carry it to your full FIRE number by the time you reach your target retirement age.
Once you hit your Coast FIRE number, you can “coast” — like a cyclist who has reached the top of a hill and can freewheel to their destination without pedalling further. You stop the intense, high-savings-rate accumulation phase. From this point, you only need to earn enough to cover your current living expenses. There is no more pressure to save aggressively. The compound interest machinery is already set in motion, and all you need to do is wait.
This concept has genuine appeal for people who find the aggressive saving phase of FIRE unsustainable or exhausting, but who do not want to abandon the destination of eventual financial independence. Coast FIRE offers a middle path: a reduced financial burden today, with full FIRE delivered by compound growth in due course.
The Coast FIRE Formula
Calculating your Coast FIRE number requires three inputs: your full FIRE number, your target retirement age, and an assumed real rate of return on your investments.
Coast FIRE Number = FIRE Number ÷ (1 + annual real return)^years until retirement
Let us work through a concrete example. Suppose you:
- Want to fully retire at age 60
- Need £750,000 at age 60 to sustain your desired lifestyle (your FIRE number)
- Are currently 35 years old (25 years until retirement)
- Assume a 7% real annual return on a globally diversified equity portfolio
Coast FIRE Number = £750,000 ÷ (1.07)^25 = £750,000 ÷ 5.4274 = £138,200
Once you have £138,200 invested (not counting pension contributions you cannot access yet, or future contributions — just what is invested today), you have reached Coast FIRE. If you simply leave that money invested in a global equity index fund and make no further contributions, it should grow to approximately £750,000 by age 60, assuming 7% real returns.
The implications are significant. Instead of needing to save aggressively for another 25 years, you only need to cover your living expenses from this point. You could switch to lower-paid but more fulfilling work, reduce to part-time hours, take a career break to travel or raise children, or pursue creative or social projects — all without endangering your retirement.
Why Coast FIRE Appeals: The Psychology of Enough
One of the most common challenges in the FIRE movement is maintaining motivation over what is typically a 15–25 year journey from zero to full financial independence. The intense savings discipline required — often 50–65% savings rates — can lead to burnout, resentment, or lifestyle deprivation that eventually causes people to abandon their plans.
Coast FIRE provides an earlier intermediate milestone that genuinely changes the stakes. Once you hit your Coast number, you have effectively locked in future financial independence. The decision to keep saving or stop aggressive saving becomes an active choice rather than a necessity. Many people find that this psychological shift — from “I must save this much or fail” to “I could stop saving now and still retire eventually” — is profoundly liberating.
It also makes FIRE accessible to people who cannot or do not want to sustain extreme savings rates for two decades. A teacher who saves aggressively for 8 years and reaches their Coast number at 33 can then focus entirely on living costs for the remaining 27 years to retirement, without any further wealth accumulation.
The Catch: You Still Need Income to Cover Living Expenses
Coast FIRE is not free retirement. It is freedom from aggressive saving, not from earning. Once you stop adding to your portfolio, you still need income to pay your rent or mortgage, food, transport, and bills. You are no longer building wealth — you are just covering costs.
This means Coast FIRE is really only viable if:
- You can earn enough from part-time or lower-paid work to cover your living costs without further savings
- Or you have a genuine low-cost lifestyle that can be sustained on a modest income
- Or you have other income sources (rental income, a partner’s income, side business) that cover costs
The failure mode for Coast FIRE is being forced to draw down your invested portfolio to cover living expenses before you reach your target retirement age. If your “coast income” is insufficient and you dip into investments, you are no longer coasting — you are decumulating early, which undermines the entire plan.
Coast FIRE in the UK Context
UK tax wrappers and account structures interact well with the Coast FIRE concept in several useful ways:
ISA Compounding Without Top-Ups
A Stocks & Shares ISA that has reached your Coast number simply continues to compound tax-free without any further action required. You do not need to touch it, report it to HMRC, or manage it actively. A low-cost global index fund inside an ISA, left alone for 20–25 years, illustrates compound growth at its most straightforward.
Pension Contributions Can Be Reduced to Match Level
During the coast phase, you might reduce pension contributions to just the level required to capture any employer match — the free money you should always take — without additional salary sacrifice contributions. This maximises take-home pay and ensures your living costs are covered without touching investments.
National Insurance Continues Accruing
If you continue in some form of paid employment during the coast phase — even part-time — you continue to accrue National Insurance qualifying years towards the State Pension. Someone who coasts from 33 to 60 in part-time work may accumulate enough qualifying years for the full State Pension, which further enhances the post-67 financial picture.
Flamingo FIRE: The Halfway Variant
A related concept is Flamingo FIRE— named for the bird’s characteristic one-legged stance. Flamingo FIRE is reached when you have accumulated half of your full FIRE number. At this point, you can “semi-retire” — reducing working hours significantly — because the invested half will grow to roughly your full FIRE number by conventional retirement age, while part-time income covers current costs.
Flamingo FIRE sits between full accumulation and Coast FIRE in terms of aggressiveness. Half the FIRE number is typically reached earlier than the full Coast FIRE number, making it a useful earlier milestone. For some people, the possibility of halving their working hours at age 40 is a more motivating near-term goal than “full retirement at 55.”
Calculating Your UK Coast FIRE Number
To calculate your own Coast FIRE number, work through these steps:
- Determine your full FIRE number. This is your annual spending divided by your safe withdrawal rate (e.g., £30,000 ÷ 0.04 = £750,000), adjusted for any guaranteed income like the State Pension. See our article on calculating your UK FIRE number for a detailed walkthrough.
- Decide your target retirement age. This can be full FIRE (stop working completely) or Barista FIRE (part-time work for supplemental income). The later the target date, the smaller the Coast number.
- Choose a real rate of return assumption. Historically, globally diversified equity portfolios have returned approximately 5–7% per year in real terms. For a conservative estimate, use 5%; for a central estimate, 7%.
- Apply the formula: Coast Number = FIRE Number ÷ (1 + rate)^years
For example, targeting £600,000 at 60, currently aged 30, 5% real return: Coast Number = £600,000 ÷ (1.05)^30 = £600,000 ÷ 4.3219 = £138,800.
For the same target at 7% real return: Coast Number = £600,000 ÷ (1.07)^30 = £600,000 ÷ 7.6123 = £78,800.
The difference between 5% and 7% real return assumptions produces a Coast number that is £60,000 higher or lower — illustrating why the assumed return rate matters. Use the UK FIRE Calculator to model your specific Coast FIRE number with different return assumptions, and find your personalised milestone on the path to financial independence.