Coast FIRE Calculator

What is a coast FIRE number?

A coast FIRE number is the amount that grows into your full retirement target on its own, with no further contributions, by the age you pick. Say a $40,000 retirement at a 4% withdrawal rate needs $1,000,000: reaching that at 65 takes $231,377 invested today at 35.

That gap is the whole idea. The coast number is a fraction of the target because 30 years of compounding does the rest of the work, and it is why the milestone arrives decades before financial independence does. Passing it does not mean stopping work. It means the retirement portion of your saving is finished: earned income still covers what you spend, but nothing more has to go into the pot for that particular target to be met.

What it does not cover is anything you plan to spend before that age. A coast number is committed to a date, so a house deposit, a career break, or a year off has to be funded from somewhere else. The lifestyle change calculator is built for that kind of plan.

The discounting math, in the open

Two steps, both reversible by hand. The retirement target is annual spending divided by the withdrawal rate, and the coast number is that target divided by growth over the years remaining.

1. $40,000 of spending at a 4% withdrawal rate gives a target of $1,000,000.
2. 30 years at 5% real multiplies a balance by 4.3 times.
3. Dividing the target by that multiple gives $231,377, the coast number.

The return entered is a REAL return, above inflation, which is why no separate inflation rate appears anywhere on this page. Mixing the two is the most common way this calculation goes wrong: a nominal rate applied to a target expressed in today’s money overstates the result badly across three decades.

Against $150,000 invested today, that leaves this example 65% of the way to coast, $81,377 short. Adding $500 a month closes it at age 58. Plotted over time, a climbing balance converges on a climbing requirement, because every year of waiting is one less year of compounding.

Coast FIRE at 30, 35 and 40

The requirement rises at exactly the return every year you wait, so at a 5% real return the same retirement costs 2.1 times as much to coast to at 45 as it does at 30. Holding the $1,000,000 target and that return fixed, this is what the number looks like across starting ages.

Read down the last column rather than across. The share of the target needed is what changes, and it changes faster than the years do, which is the practical argument for treating an early lump sum differently from a later one of the same size. Those figures assume one target and one return; entering your own spending redraws every row.

Would coasting have actually worked?

That is the question none of the arithmetic above can answer. A projection grows the balance by the same rate every year; markets do not, and the order returns arrive in changes the outcome even when the average matches. So the same plan is run through the historical engine behind the FIRE simulator: real returns, real inflation, one run for every start year the data supports.

Falling short of the number is what the comparison is for: a plan starting at $150,000, 65% of the way there, survived 80 of 95 windows instead of 95. The coast number is not a cliff edge, but the distance to it is measurable in outcomes rather than only in dollars.

Three things bound that result. The windows overlap heavily, so 95 of them are nothing like 95 independent trials. The last one starts in 1965, because a 60-year window cannot start any later and still have finished, so nobody who began recently is represented. And the backtest holds a fixed 80% stocks and 20% bonds throughout, ignoring the expected return field entirely: that field drives the projection, this section replays history, and they answer different questions on purpose.

Overlapping windows are the binding limit on that count, and the way past it is to stop counting windows. The retirement probability calculator gives the exact odds for the retirement half, weighing every sequence the return distribution could produce rather than only the runs history happened to deal.

Open this coast scenario in the full simulator to vary the allocation, change the withdrawal strategy, or see the individual start years rather than the summary.

Coast, barista, lean and full FIRE

These four are not rungs on one ladder, and treating them as sizes of the same thing is what makes them confusing. What separates them is whether earned income still covers your spending, and whether the portfolio is being drawn on before retirement.

The barista toggle on this calculator is the one worth trying, because its result surprises people: turning it on raises the number, not lowers it. The next section runs the numbers on what the toggle trades.

For the full target rather than the milestone, the simple FIRE calculator answers it from spending alone, and the full FIRE plan adds income, housing equity and a year-by-year path.

Barista FIRE calculator: coasting with part-time income

A barista FIRE calculator answers a different question than a coast FIRE calculator: whether you can cut back to part time rather than stop saving. Switch on Barista FIRE above and part-time pay covers some of your spending instead of all of it, so the portfolio starts funding what earned income does not, years before retirement rather than after.

That earlier drawdown is what the toggle prices, and it moves the number the opposite way most people expect. At $20,000 of part-time income against $40,000 of spending, the portfolio has to fund a $20,000 annual gap starting the day you cut back, so the balance required today rises from $231,377 to $538,826.

Barista FIRE asks for more than plain coasting, never less, because the pot is drawn down years before it finishes compounding. It asks for less than stopping work outright, and that comparison is the one worth running before ruling it out.

What a coast plan is betting on

One assumption carries almost all the weight: the real return, applied for decades, with no further contributions to absorb a bad run. Coasting deliberately removes the lever that rescues most plans, which is saving more when things go badly.

The sensitivity runs in the direction people find least intuitive. A lower assumed return does not mean a slightly larger coast number, it means a substantially larger one, because the assumption is compounded over the whole horizon. Running the calculator twice, once at the return you expect and once a point or two below it, shows the size of that exposure more clearly than any single figure can.

A second bet is quieter and gets checked less often. A coast number is computed from a spending estimate for a life that is decades away, and it silently assumes that estimate holds. Children, care, moving country, or paying off a mortgage all move the target, and a plan declared finished at 35 rests on a forecast made at 35. Re-running the number every few years against actual spending is what keeps it honest, and it costs nothing.

This is also the reason the historical section exists rather than a reassuring paragraph. A backtest cannot promise the future, but it can show which assumption broke plans in the past and by how much, which is a better basis for a decades-long decision than a single smooth curve.

What this calculator does not model

These omissions keep a decades-long projection auditable by hand.

• Tax and account types: Nothing here is taxed, and every account is treated as one pot. Money in a retirement account you cannot reach until a set age is counted the same as money in a taxable account, which matters a great deal for a plan whose whole premise is a long wait. Enter figures you have already netted down.
• A single smooth return: The projection grows the balance by the same real rate every year. Real markets do not, and the order returns arrive in changes the outcome even when the average matches. That gap is the reason the historical section on this page exists.
• Changes to your spending: The retirement target is fixed from one spending figure. Children, care, a move, or a paid-off mortgage all move it, and a target set decades out is a forecast rather than a measurement.
• Anything you add later: Past the point where the projection reaches coast, the number assumes nothing further arrives. Most people do keep saving something, so the plan is usually a floor rather than a ceiling.
• State and workplace pensions: No state pension, workplace pension, or other later income is counted. Any of them reduces what the portfolio has to cover from the age they begin, so leaving them out is the cautious direction rather than the flattering one.

Once coasting turns into actually retiring, the historical FIRE simulator is the next step: it replays that retirement against more than 150 years of market history across a dozen withdrawal strategies, something a single smooth return cannot show. For a simpler check on the drawdown side, the simple drawdown calculator strips the question back to a pot of money falling against rising costs.