How to use the Coast FIRE calculator
The calculator starts with an example: someone who is 30, with $100,000 invested, who wants to retire at 60 and spend $50,000 a year. Change the inputs to your own situation.
- Current age and target retirement age. The years between them are how long your money can compound on its own.
- Annual spending in retirement. In today’s dollars. This and the withdrawal rate set the FIRE number you are aiming at.
- Safe withdrawal rate. 4% is common for a 30-year retirement; use a lower rate if you plan to retire early and need the money to last longer.
- Currently invested. Money set aside for retirement that you will not touch before then: 401(k), IRA and long-term brokerage accounts.
- Expected real return. The average yearly return after fees and inflation. This is the input Coast FIRE is most sensitive to.
- Monthly contribution. What you invest now. The calculator uses it to find the age at which you reach Coast FI.
The chart shows the coast number you need at each age (amber), your balance if you keep contributing (blue) and your balance if you stop today (grey dashes). Where blue crosses amber, you can coast.
How the Coast FIRE number is calculated
Coast FIRE number = FIRE number ÷ (1 + r)n
Balance at retirement if you stop now = Current balance × (1 + r)nr is the real annual return and n the number of years until your target retirement age. All amounts are in today's dollars.
To find the age you reach Coast FI, the calculator adds your monthly contribution at the end of each month and grows the balance at the monthly equivalent of the annual return, (1 + r)1/12 − 1. The first month in which the balance, left alone from then on, would reach the FIRE number by retirement is your Coast FI date. The coast number rises every year as retirement gets closer, so you are chasing a moving target; contributions and growth have to outpace it.
A worked example
You want to spend $50,000 a year from age 60 with a 4% withdrawal rate, so your FIRE number is $1,250,000.
Retirement is 30 years away. At a 5% real return, every dollar invested today grows to about 4.32 dollars by then, so the Coast FIRE number today is $289,222: the FIRE number divided by that growth.
You have $100,000, which leaves a gap of $189,222. If you stopped saving today, your balance would grow to $432,194 by 60, enough for about $17,288 a year at 4%.
Investing $1,500 a month, you reach Coast FI at age 44.8, when the coast number has risen to $596,402. From then on you could stop contributing and still arrive at $1,250,000 on time. If you kept contributing all the way, you would retire with $1,655,258 instead.
What changes your Coast FIRE number
Two inputs dominate: how many years the money has to grow, and how fast it grows.
| Change from the example | FIRE number | Coast number today | Coast FI age |
|---|---|---|---|
| The example (retire at 60, 5% return) | $1,250,000 | $289,222 | 44.8 |
| Retire at 55 | $1,250,000 | $369,128 | Not before retirement |
| Retire at 65 | $1,250,000 | $226,613 | 38.7 |
| Real return 4% | $1,250,000 | $385,398 | 54.9 |
| Real return 6% | $1,250,000 | $217,638 | 38.3 |
| Withdrawal rate 3.5% | $1,428,571 | $330,539 | 50.3 |
Retiring five years earlier needs the same FIRE number but gives it five fewer years to compound, so the coast number jumps to $369,128, and $1,500 a month no longer gets you there before you retire. A one-point change in the assumed return moves the coast number further than most people expect, from $217,638 at 6% to $385,398 at 4%. Check your date at a cautious return as well as your central one.
Your monthly contribution changes only the date, not the number. At $1,200 a month you would reach Coast FI at age 51.1; at $2,500, at age 37.6. Below a certain contribution you never reach it before retirement, and the plan becomes ordinary saving all the way to the finish.
Coast FIRE, Barista FIRE and full FIRE
All three come from the same community vocabulary, and none has an official definition:
- Full FIRE: your portfolio already covers your spending, so work becomes optional now. The FIRE calculator sizes that target.
- Coast FIRE: your portfolio will cover your spending at a later retirement age without further saving. You still need income to cover today’s bills, but you can take a lower-paid or part-time job, a sabbatical, or simply spend more of what you earn.
- Barista FIRE: you leave full-time work earlier and part-time income covers part of your spending, so the portfolio needs to support only the rest. The name comes from the idea of a part-time job with benefits.
The two ideas combine naturally. If part-time work in retirement covered $20,000 of the example’s $50,000, the portfolio would only need to fund $30,000 a year. The FIRE number falls to $750,000, the coast number today to $173,533, and you would reach it at age 34.6. The catch is that the plan now depends on finding and keeping that work in your 60s.
Reading the result
Reaching the number is a milestone, not a guarantee. Recheck it each year with your real balance: after a weak year in the markets the coast line may be above you again, and a few months of contributions put you back on track. Many people keep saving a little after they cross it, as a margin against a lower return than they assumed.
What this calculator does not include
- Market volatility. The projection uses one steady return. A real portfolio can lag the coast line for years; the retirement Monte Carlo calculator shows a range of outcomes.
- Taxes and account rules. Withdrawals from pre-tax accounts are taxed in the US, and early withdrawals can carry penalties. Include expected taxes in your spending figure.
- Social Security and pensions. Income you will receive in retirement reduces the portfolio you need. Subtract it from your spending before entering it.
- Fees, unless they are already deducted from the return you entered.
- Spending between now and retirement. Coasting assumes your income always covers your current costs, so the invested money is never touched.