Coast FI Checkpoint

A browser calculator that tells you whether your retirement savings can already grow to your target on their own, at what age they will if not, and how much that answer depends on the return you assume.

Date: 2026-10-06 Theme: Net-Worth + Investing Status: Prototype

What it is

A single-page calculator for "Coast FI" (coast financial independence): the point where the money you've already invested, left alone, compounds into the full amount you need at retirement. You type in your age, balance, contribution, and planned retirement spending. It shows your coast number, the age you cross it at your current pace, and a grid of how that number moves when you change the return or withdrawal rate.

Who it serves

Someone in their late 20s to 40s with a 401(k) or IRA balance that has started to look real, who is asking a specific question: can I ease off? Maybe that means dropping to just the employer match, taking a lower-paying job they'd like more, or going part-time. It also suits FIRE (financial independence, retire early) readers who know the coast formula but want to see their own numbers run month by month.

The money problem it addresses

Most retirement calculators answer "how much should I save each month?" The coast question runs the other way: how much do I need to have saved already, so that I can stop adding? The formula is short (FI number divided by growth over the years remaining), and that's the trap. It depends heavily on one guess, the real return, and a single-number calculator hides that.

Take one synthetic profile from the sample data: age 33, $200,000 invested, retiring at 60, $70,000 a year of spending, 4% withdrawal rate. At a 7% real return, their coast number is $281,628 and they cross it at 40 yr 3 mo. At 4% real, the coast number is $606,929 and they don't cross it before 60 at all. Same person, same savings, and the answer flips between "you can ease off in seven years" and "keep going." Financial Samurai calls Coast FIRE "the weakest type of FIRE" for exactly this reason. The app puts the return assumption next to the answer instead of burying it, so you can decide how much of a bet you're making before you cut contributions.

The withdrawal rate moves the target as well. The classic 4% rule turns $1 of annual spending into a $25 target. Morningstar's 2026 research puts a safe fixed starting rate for new retirees at 3.9%, which makes the same spending need about $25.64 per dollar. The app has presets for both, plus 3.5% for anyone who wants extra margin.

How it works

One screen, inputs on the left and results on the right. Everything recalculates as you type.

1. Pick a sample profile or enter your own: current and retirement age, invested balance, monthly contribution, annual retirement spending, and any guaranteed income such as a Social Security estimate or pension. Guaranteed income comes off the spending before the target is calculated.
2. Set a withdrawal rate (chips for 3.5%, 3.9%, 4.0%) and a real return, meaning the return after inflation. All dollars are in today's money.
3. The banner tells you whether you're past your coast number or short of it, by how much, and what stopping contributions today would leave you with at retirement, as a share of the target.
4. Four tiles show the FI number (with its formula), the coast number today, your coast age at the current contribution, and the age your balance reaches the full FI number.
5. The chart has three lines by age: your balance if you keep contributing, your balance if you stop today, and the dashed coast line, which climbs toward the FI number as retirement gets closer. Where the solid green line crosses the dashed one is your coast age. Hover for exact values, or open the table underneath.
6. The sensitivity grid shows your coast number across five returns (3–7% real) and four withdrawal rates. Cells your current balance already covers get a check mark. If you only clear the 7% column, you are betting on returns.

What's in this folder

• index.html: the app; open it directly
• style.css: layout and chart styling
• script.js: the math (integer cents, month-by-month projection), the SVG chart, the sensitivity grid, and an embedded copy of the sample profiles
• sample-data.json: 10 synthetic profiles, including someone already coasting, a new grad starting from $3,000, a pension-backed 50-year-old, and the 7%-vs-4% pair described above
• requirements.md: functional and technical requirements
• cover-image.png: cover art for the library listing
• metadata.md: generation details and sources

How to run / read it

Open index.html in any modern browser. No install, server, account, or internet connection needed. Choose a profile from the dropdown, then change any field to use your own numbers.

Estimated impact

The payoff is a clearer decision. A specific dollar figure is harder to promise. Two examples from the sample data:

• The "already coasting" 38-year-old ($420,000 invested, $60,000 spending, $18,000 guaranteed income, 4% withdrawal, 5% real) has a coast number of $281,241, so they are $138,759 past it. Their $1,500/month contribution, $18,000 a year, is optional under those assumptions. Whether they actually stop is up to them, but now they're deciding with numbers.
• The 7%-vs-4% pair above shows that the return guess alone can move the coast number by $325,301 for the same person. Anyone about to cut contributions on the strength of a 7% assumption should see that gap first.

For scale: the 2026 401(k) employee deferral limit is $24,500 (IRS). Someone who has truly reached coast can redirect up to that much a year to other goals, and someone who only reaches it in the optimistic column probably shouldn't yet.

Good to know

This tool estimates, and it can't tell you what to do with your money. It assumes the same return every year, so it ignores sequence-of-returns risk: a bad decade right after you stop contributing hurts far more than the average suggests. It doesn't model taxes, so a dollar in a traditional 401(k) and a dollar in a Roth count the same here even though they aren't the same at withdrawal. Withdrawal-rate research is updated every year, and Morningstar's figure for 2026 differs from earlier years. Check current numbers, and treat coasting as a choice you can reverse. You can always start contributing again.

Sources

• https://www.irs.gov/newsroom/401k-limit-increases-to-24500-for-2026-ira-limit-increases-to-7500: 2026 401(k) deferral limit ($24,500) and IRA limit ($7,500)
• https://www.boldin.com/retirement/safe-withdrawal-rate-morningstar/: Morningstar's 2026 safe starting withdrawal rate (3.9% fixed, up to 5.7% with flexible spending)
• https://www.morningstar.com/business/insights/research/the-state-of-retirement-income: Morningstar's State of Retirement Income report, the primary source for the 3.9% figure
• https://getfinny.app/blog/what-is-coast-fire: Coast FI formula (FI number ÷ (1 + return)^years) and the 25× (4% rule) FI number
• https://www.financialsamurai.com/what-is-coast-fire-financial-independence-retire-early/: the case against relying on Coast FIRE (return dependence, "the weakest type of FIRE")