Coast FIRE calculator.

What is Coast FIRE?

Coast FIRE is a way to describe a retirement savings milestone. You have enough invested today that, under a set of growth assumptions, those investments could reach your retirement target without additional deposits. The “coast” period is the time between that milestone and the date when you need the portfolio to pay your bills.

That is a different question from retiring immediately. During the coast period, living expenses still need funding. Work, a business or another income source would have to cover them. The investments are left alone to grow; withdrawing from them before the target date changes the calculation.

The milestone depends on your own spending plan. A smaller retirement budget produces a smaller target. More years until retirement give assumed growth more time to work. Higher inflation reduces that growth in purchasing-power terms. A lower withdrawal-rate assumption requires a larger portfolio.

Think of the result as a comparison between two scenarios: continuing to save and stopping new deposits today. The calculator does not decide which is appropriate for you. A balance above the threshold is a result of the assumptions entered, rather than evidence that future returns, expenses or retirement needs will follow the model.

How this calculator works

The calculation starts with your retirement spending in today’s dollars. Monthly spending multiplied by twelve gives an annual budget. Dividing that budget by the initial withdrawal rate produces the Full FIRE target. This step sizes a portfolio; it does not simulate withdrawals or establish that a particular rate is sustainable.

Next, we adjust the investment-return assumption for inflation. We use the ratio of the two annual growth factors, rather than simply subtracting the percentages. Both inputs are effective annual rates. The resulting real growth factor expresses how purchasing power could change in a year.

fullTarget = monthlySpending × 12 / (withdrawalRate / 100)
realGrowthFactor = (1 + nominalReturn / 100) / (1 + inflation / 100)
years = retirementAge − currentAge
coastNumberToday = fullTarget / realGrowthFactor ** years

The Coast number discounts the future target back to today. If your current investments equal that number, the no-new-deposits path reaches the target in this smooth-return model. If inflation is greater than the return assumption, real growth is negative and waiting does not make the target easier to fund.

To estimate a Coast age, we also test what happens while you keep saving. Monthly deposits arrive at month end. They stay constant in purchasing-power terms, meaning their nominal dollar amount would rise with inflation. Each month we compare that path with the Coast threshold for the years still remaining. The first qualifying month becomes the displayed Coast age.

The additional savings figure solves a separate question: the extra monthly amount that would reach the Full FIRE target by your retirement age. It is not an instruction to save that amount, and it does not check whether your income supports it. The chart keeps the ongoing-deposit path running through retirement so you can compare outcomes, even after the first Coast crossing.

Coast FIRE vs. Full, Lean and Barista FIRE

These labels describe different relationships between work, spending and investments. Coast FIRE focuses on funding a later retirement while covering current expenses elsewhere. Full FIRE asks whether the portfolio could cover the whole spending budget when financial independence begins. Lean FIRE uses a smaller personal budget; this calculator does not assign a universal spending cutoff. Barista FIRE assumes continuing part-time income covers some spending, leaving a smaller gap for investments.

The labels do not change market risk or tax rules. Moving between them changes which cash flows the model must cover. Read the comparison of the four FIRE approaches before treating the targets as interchangeable.

Worked example: age 32, with $80,000 invested

This is the calculator’s default example, not a representative household or a recommended plan. Assume retirement at 65, monthly savings of $1,500, monthly retirement spending of $4,000, a 7% effective nominal return, 3% annual inflation and a 4% initial withdrawal-rate assumption.

1. There are 33 years between ages 32 and 65. Annual spending is $48,000.
2. The Full FIRE target is $48,000 ÷ 0.04 = $1,200,000 in today’s dollars.
3. The annual real growth factor is 1.07 ÷ 1.03 = 1.0388349515. Real growth is approximately 3.8835% annually.
4. The Coast number today is $1,200,000 ÷ 1.0388349515³³ = $341,307. The gap from $80,000 is $261,307.
5. With no more deposits, $80,000 grows to $281,272 at 65. With $1,500 month-end real deposits through 65, the comparison balance is $1,468,004.
6. The first modeled Coast crossing while saving is age 53 years, 3 months. The monthly amount needed to reach the target by 65 is approximately $1,161, so the entered $1,500 requires $0 extra in this scenario.

Dollar figures above round to the nearest dollar; the engine uses unrounded values. The crossing is checked monthly, so the displayed date is the first qualifying month. Changing return, inflation, spending or retirement age changes this answer. See the full recurrence and monthly-deposit formula.