FIRE Number and Coast FIRE
How a FIRE number, savings rate, and Coast FIRE number are calculated, with the withdrawal rate and real return on screen.
By Prosperics Editorial Board · DIGITI LLC
Prosperics is published by DIGITI LLC, a California company. Calculators and guides are written and maintained by the Prosperics editorial board. Figures are checked against primary sources — IRS, CRA, SSA, and central bank publications — and each page shows the date it was last reviewed.
Your FIRE number
A FIRE number is annual spending divided by the withdrawal rate you choose. At 4% that is 25 times spending. At 3.5% it is higher. The rate stays on screen because a longer retirement often uses a lower rate. This is an educational model in today's dollars, not a prediction and not advice.
Key takeaways
- ✓FIRE number = annual spending ÷ withdrawal rate
- ✓4% is the familiar 25× rule, not the only answer
- ✓Results are in today's dollars
Coast FIRE
Coast FIRE asks whether money you have already invested can grow to your FIRE number by a traditional retirement age without more contributions. The Coast number is that FIRE number discounted by a real return. Coast is a tab on the FIRE calculator. A government benefit, counted at 100%, 83%, or 0%, can lower the spending the portfolio must cover. The US default is $2,000 a month. Canada uses $1,400 a month for CPP and OAS. The screen also shows the Coast number with none of the benefit counted. 83% leaves a margin. In the US it is also the 2026 Social Security trustees payable share. It is not a CPP or OAS rule.
Key takeaways
- ✓Coast number = FIRE number ÷ (1 + real return) ^ years
- ✓You can test 100%, 83%, or 0% of a government benefit
- ✓Stopping extra saving is optional only if the gap is closed
Assumptions on screen
The savings rate here is savings divided by savings plus spending, the Mr. Money Mustache definition, not savings divided by gross pay. The default real return is 5% and the default withdrawal rate is 4% so the classic table stays comparable. For longer horizons, published research in 2026 often cites about 3.25% to 3.5% for 50 to 60 years, about 3.3% for 40 years, and about 3.9% for 30 years. Bengen's 30-year worst case of 4.7% is a reference, not the default. Markets, taxes, healthcare, and longevity can change the outcome.
Key takeaways
- ✓Savings rate uses spending, not gross pay
- ✓Default is 4% withdrawal and 5% real return
- ✓A longer horizon usually suggests a lower withdrawal rate
Sources
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Educational content only. Not financial, legal, or tax advice.