Coast FIRE Calculator

The point where compounding takes over

Coast FIRE is the moment your invested balance is large enough that growth alone will carry it to your retirement target. You keep working and paying your bills from income, but the retirement saving is done. Everything after that is optional and only moves the date earlier. It is usually reached years or decades before full financial independence, which is what makes it a useful thing to measure.

The formula

Two steps: size the retirement target, then discount it back to today.

① FIRE number
   annual retirement spending ÷ withdrawal rate
   $40,000 ÷ 4% → $1,000,000

② Coast FIRE number
   FIRE number ÷ (1 + r) ^ t
   r = real return, t = years until retirement
   $1,000,000 ÷ 1.05^30 → about $231,000 today

The exponent is why timing dominates this calculation. At a 5% real return, every extra year before retirement cuts the amount you need today by about 5%. Someone aged 30 targeting retirement at 60 needs roughly $231,000 invested. The same person targeting 65 needs about $181,000, and targeting 50 needs about $377,000. Nothing changed except the runway.

Why the number is in today's dollars

The calculator uses a real return, meaning growth after inflation. That keeps the target, the coast number, and the by-age table all in money you recognise, so the annual spending you type does not need inflating year by year. A 5% real return corresponds roughly to a 7% to 8% nominal return with 2% to 3% inflation. If you would rather work in nominal terms, raise the return and inflate the spending yourself, but the real-return version is harder to get wrong.

What the model deliberately leaves out

This is an educational planning tool, not personal financial advice. It assumes a smooth constant real return, while real markets are volatile and the order in which returns arrive genuinely matters near retirement. It does not model taxes, investment fees, pensions, social security, windfalls, or the way spending shifts across a life. No output here is a promise or a guaranteed retirement date. Use it to see the shape of the decision, then revisit as your real numbers move.