# Coast FIRE calculator

Find the invested balance that can grow to a retirement target with no new contributions, including Social Security, side income, partial withdrawals, and real or nominal dollars.

## The model, in one pass

1. Capitalize retirement spending that is not covered by Social Security or by side income long enough to treat as ongoing. Divide that remainder by your withdrawal rate.
2. If Social Security starts later, walk backward over the bridge years, withdrawing uncovered spending and growing the portfolio at the constant rate.
3. Walk backward from retirement to today. Each year, grow at the same rate and add any partial withdrawal that side income does not cover.
4. The result is the balance required today. Your gap is that number minus what you have invested.

On track means today's invested balance is at least that Coast number. It does not mean the plan survives a bad market, a cut in benefits, or a spending increase. The projection chart is your current balance with no new contributions, not a Monte Carlo.

> Type benefits you have a source for. A made-up Social Security number makes a confident Coast number that you do not have.

## Questions

### What is Coast FIRE?

You have enough invested today that, if you never add another dollar, a constant growth rate reaches your retirement portfolio by the age you picked. You may keep working and spending your paycheck. The portfolio is not asked to pay for life today. That is the difference from a full FIRE number.

### How is Social Security handled?

You type an annual benefit in today's dollars and the age it starts. The calculator does not look up your benefit. If the benefit starts after you retire, the years in between are a bridge: the portfolio pays full uncovered spending, then only has to equal the smaller capitalized amount once the benefit starts. If you leave the benefit at zero, the portfolio covers everything you typed.

### What is the difference between real and nominal?

Real mode converts your nominal return and inflation into a real rate and keeps every cash flow in today's dollars. Nominal mode grows spending, benefits, and withdrawals with inflation and reports the retirement target in future dollars. The Coast number you need today matches between the two modes when you are not withdrawing along the way. The labels do not. Read the one the result prints.

### Why is this higher than target divided by (1 + return) to the years?

That shortcut assumes you never touch the portfolio and that the full FIRE number is the right target. Partial withdrawals, a gap before Social Security, and inflation all change it. Side income reduces the capitalized target only if it is still running then and lasts at least 20 more years. Shorter income only offsets withdrawals while it exists. Extra side income is not invested back, because Coast means no new contributions.

## Default illustration

Age 35 to 60, invested $200,000, spending $60,000, Social Security $24,000 at 67, 7% nominal, 3% inflation, real mode, 4% withdrawal.
Coast number $410,867. Gap $210,867.

POST https://wealthcompounds.com/api/coast-fire with the same fields as decimals (0.07, not 7). Contract: https://wealthcompounds.com/openapi.json.

HTML: https://wealthcompounds.com/coast-fire

---

Educational only. Not investment, tax, or financial advice. Canonical HTML: https://wealthcompounds.com