How these numbers are calculated.
Every FIRE Projection tool runs on the same small set of assumptions and the same plain-math approach. This page lays them out so you can judge the numbers for yourself — and see who maintains them.
How we calculate
Everything is in today's dollars. Rather than project inflated future balances and then deflate them, we collapse your nominal return and inflation into a single real return — so your spending target, your FIRE number, and the projection all live in the same units. The formula is real return = (1 + nominal) ÷ (1 + inflation) − 1.
The FIRE number is the 25× rule. A 4% safe withdrawal rate is the inverse of multiplying annual spending by 25, drawn from the Trinity Study — the research that established the 4% rule for a 30-year retirement. Annual spending of $50,000 implies a FIRE number of $1,250,000.
From there, a Coast FIRE number is just that target discounted back to today at your real return, over the years until you retire. The same building blocks power every calculator on the site.
The default assumptions, all of which you can change:
- 10% nominal investment return — the long-run U.S. equity average with dividends, before inflation (≈6.8% real after 3% inflation).
- 3% inflation — keeping every figure in today's dollars.
- 4% safe withdrawal rate — the Trinity Study baseline (≈25× spending).
All figures are pre-tax estimates. Taxes, fees, and the messy reality of markets that never return a steady 10% will move your real result. These tools are built to give you intuition and a defensible starting number — not a guaranteed plan.
Who's behind this
FIRE Projection is built and maintained by bbmaxwell, an indie developer building a small portfolio of focused, honest tools. The FIRE calculators come from actually wanting to answer these questions for myself — not from a content farm chasing search traffic.
Editorial & update policy. Assumptions and copy are revisited as the underlying research moves — if a better long-run figure or a stronger source emerges, the defaults change and the last-updated date moves with them. Corrections are welcome; the fastest way to reach me is on X.
Educational, not financial advice. Nothing here is a recommendation to buy, sell, or hold any investment, and it isn't personalized to your situation. Markets are uncertain and these projections are estimates — use them to build intuition, and consult a qualified professional before acting on any number.