Coast FIRE Number: The Math for When You Can Stop Saving for Retirement

A 35-year-old with $231,377 invested can stop contributing to retirement accounts entirely and still land at roughly $1 million by age 65, assuming a 5% real return. Not $1 million in inflated dollars — $1 million in today’s purchasing power. That threshold has a name, and the arithmetic behind it is the most useful three minutes of math in personal finance.

Your coast FIRE number is the portfolio balance that, left completely alone, compounds into a full retirement nest egg by your target date. Hit it and every dollar you earn from that point forward is yours to spend, give away, or use to buy a shorter work week. This guide walks through how to calculate your coast FIRE number, why the return assumption you pick matters more than anything else, what coasting actually changes about your cash flow, and the five ways the strategy quietly fails in practice.

What a Coast FIRE Number Actually Is (and What It Isn’t)

Traditional financial independence asks a single question: how much do I need to never work again? Coast FIRE asks a narrower and far more achievable one: how much do I need right now so that compounding alone finishes the job?

The distinction matters because the two numbers are wildly different. If your target nest egg is $1 million and you’re 30 years out, the full-FIRE number is $1 million and the coast number is roughly $231,000 at a 5% real return. Same destination. One requires you to have saved 4.3x less.

What coast FIRE is not is early retirement. You still work. You still need income to cover today’s expenses. What changes is that the retirement-savings line item in your budget drops to zero, which for a lot of households is the single largest discretionary outflow. The Bureau of Labor Statistics Consumer Expenditure Survey puts average household contributions to retirement plans at $1,991 in 2024, and total personal insurance and pensions at $9,797 — about 12.5% of the average household’s $78,535 in annual spending. Freeing that up is a meaningful raise you give yourself.

It’s also not a one-time calculation. The figure is a moving target that responds to your spending, your timeline, and the returns markets actually deliver. Treat it as a checkpoint you re-run annually, not a finish line you cross once.

How to Calculate Your Coast FIRE Number in Three Steps

The formula is short:

Coast FIRE number = Target nest egg ÷ (1 + real return)^{years until retirement}

Step 1: Set your target nest egg. The common shortcut is 25 times annual spending, which flows from the 4% withdrawal guideline. If you expect to spend $40,000 a year in retirement beyond Social Security, your target is $1 million. That multiplier is a starting point rather than a law — our breakdown of what a safe withdrawal rate looks like in 2026 covers why 4% is a range, not a constant, and how much you actually need to retire at 55 shows how the target shifts when the timeline stretches.

Step 2: Pick a real (inflation-adjusted) return. Working in real terms means your target stays in today’s dollars and you skip a separate inflation adjustment. More on the assumption below, because it does most of the work.

Step 3: Discount backward. Divide the target by (1 + r) raised to the number of years left. Here is what that produces for a $1 million target at age 65, across three return assumptions:

Read across any row and the point lands immediately: the gap between the optimistic and pessimistic column is larger than most people’s entire portfolio. A 30-year-old’s coast number is either $93,663 or $355,383 depending on nothing but the assumption typed into a spreadsheet.

The Return Assumption Is the Whole Ballgame

Every coast FIRE calculator on the internet defaults to 7% real, and that number has a real pedigree. Using NYU Stern’s long-run dataset maintained by Aswath Damodaran, the S&P 500 compounded at about 10.0% annually from 1928 through 2025 with dividends reinvested, which works out to roughly 6.5% to 7% after inflation. Ninety-eight years is a serious sample.

The problem is that your coasting window isn’t 98 years. It’s 25 or 30, and 30-year windows have varied enormously. Forward-looking institutional forecasts are also notably more cautious right now: Vanguard’s 2026 economic and market outlook projects annualized U.S. equity returns of roughly 3.9% to 5.9% over the next decade — a nominal figure, which makes the real number lower still.

You don’t have to resolve that disagreement. You have to decide which error you’d rather make. Assume 7% and you’ll declare victory early; if returns come in at 4%, you discover the shortfall at 60, when there’s no time to fix it. Assume 4% and you’ll oversave, which costs you optional years of lower-intensity work but leaves you with a surplus.

Same starting balance, same thirty years, and a retirement that is either genuinely comfortable or roughly a third of what you planned for. Note also that fees come straight out of the real return — a point worth reading alongside what expense ratios actually cost over 30 years, because a 0.60% fund quietly converts a 5% plan into a 4.4% one.

What Coasting Actually Changes About Your Cash Flow

The appeal of hitting the threshold is rarely the spreadsheet. It’s the optionality: taking the lower-paying job with better hours, going part-time, starting something risky, or simply not panicking during a layoff.

That optionality is real, but the cash-flow math has a wrinkle most write-ups skip. Stopping contributions often means forfeiting an employer match, which is an immediate guaranteed return you can’t replicate anywhere else. Vanguard’s How America Saves research found that plan participation among eligible employees reached a record 86%, with automatic enrollment now used by 61% of Vanguard plans as of year-end 2025 and average deferral rates of 7.7% for auto-enrolled participants. If your employer matches 4%, coasting past that match means declining a 100% instant return to avoid saving money you were going to save anyway.

The sensible version: coast on your contributions, keep contributing exactly enough to capture the full match, and redirect the rest. That’s not purist coast FIRE, but it’s better arithmetic.

There’s also the question of where the freed-up money goes. Against a backdrop where the U.S. personal saving rate sat at 3.0% in July 2026 per the Bureau of Economic Analysis, and where the Federal Reserve’s 2025 Survey of Household Economics and Decisionmaking found only 63% of adults could cover a $400 emergency expense with cash, redirecting former retirement contributions into a cash buffer is frequently the higher-value move. Parking that buffer matters too: the FDIC pegged the national average savings rate at 0.38% in August 2026 while high-yield accounts averaged around 1.59% and top-tier accounts paid close to 3.9%.

Five Ways a Coast FIRE Number Quietly Fails

1. Sequence risk at the wrong end. The average return over 30 years can hit your assumption exactly while a bad stretch in the final five years wrecks the outcome. This is the same mechanism described in our piece on sequence of returns risk — it applies to accumulation, not just withdrawal.

2. Spending drifts upward after you stop saving. Freed-up cash flow has a way of becoming permanent lifestyle. If your retirement target was built on $40,000 of annual spending and you settle into $55,000, your coast number was calculated against a target that no longer exists.

3. Healthcare before Medicare. Downshifting to part-time work often means losing employer coverage a decade or more before Medicare eligibility. That’s a line item that can consume the entire raise coasting was supposed to deliver.

4. The money is locked up. A balance sitting entirely in a 401(k) isn’t accessible at 50 without planning. If part of the plan is retiring before 59½, you need a bridge — taxable brokerage assets, Roth contributions, or Rule 72(t) substantially equal periodic payments.

5. Never re-running the number. A coast calculation done at 33 and never revisited is a guess with a decade of drift baked in. Recalculate annually using your actual balance and actual spending.

How to Stress-Test Your Number Before You Downshift

Three checks worth doing before you cut contributions:

Run it at two return assumptions and use the lower one as your trigger. Calculate at 4% real and 6% real. Coast when you clear the 4% number, not the 6% one. The cost is a few extra years of contributions; the benefit is that a decade of mediocre returns doesn’t invalidate the plan.

Keep contributing to the match, always. Whatever your employer matches, that portion is not part of the coast decision. Contribution room is generous in 2026 — the IRS set the 401(k) elective deferral limit at $24,500 and the IRA limit at $7,500, with an $8,000 catch-up at 50 and $11,250 for ages 60 through 63 — so there’s headroom to restart quickly if you need to.

Define the un-coast trigger in advance. Write down the portfolio balance below which you resume full contributions, and check it once a year. Deciding this while calm is dramatically easier than deciding it during a 30% drawdown.

I ran the calculation on my own portfolio a few years ago mostly to see whether the concept held up outside of forum posts, and the result was more sobering than liberating. At a 7% real assumption I was comfortably past it. At 4.5% I wasn’t close. As a software engineer I have a professional bias toward not shipping something whose behavior changes completely based on one unvalidated input, and this is exactly that: one cell in a spreadsheet swings the answer by a factor of three. What I actually changed was smaller than “stop saving” — I stopped treating the 401(k) contribution as sacred, kept the match, and let the surplus go toward a taxable account that I can actually reach before 59½. The behavioral-economics angle is the interesting part, though. Knowing the number changed how I felt about work well before it changed anything in my budget, which is probably most of the real benefit.

This article is for general information and is not personalized investment advice. Return figures are historical or forecast and are not guarantees.

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Written by Chris Steve

Chris Steve is a software engineer with a deep interest in personal finance, behavioral economics, and AI. He started Money & Planet to share clear, research-backed money guides — the kind that explain the math instead of pushing products. His writing focuses on long-term wealth building, the psychology behind spending and investing decisions, and the practical tools regular people can use to make smarter financial choices.