How our calculators work
All calculators run in your browser and use the same open formulas. Money is in today’s dollars and returns are real (after inflation).
FIRE number
(annual spending − other income, floored at zero) ÷ withdrawal rate.
Coast FIRE number
FIRE number ÷ (1 + r)n, where r is the real return and n the years to your target retirement age. For the “when will I coast” estimate we add the monthly contribution at the end of each month, grow the balance at the monthly equivalent rate (1 + r)1/12 − 1, and report the first month where the balance meets FIRE number ÷ (1 + r)remaining years.
Years to financial independence
Spending = income × (1 − savings rate); target = spending ÷ withdrawal rate. The balance grows at r and receives one year of savings at the end of each year until it meets the target, capped at 100 years. From zero this matches n = ln(1 + T·r/s) ÷ ln(1 + r), rounded up.
Barista FIRE
(spending − part-time income) ÷ withdrawal rate, with months to reach it using the same monthly convention as above.
Withdrawal longevity
The withdrawal is taken at the start of each year and the remainder grows at r. A final partial year is counted. The flat-balance withdrawal is P × r ÷ (1 + r).
Testing
Unit tests check every formula against hand-calculated values and closed-form solutions, and a browser smoke test checks that each calculator renders a result. We do not model taxes, fees, variable returns or benefit timing; see each tool’s notes.