Guide
What a FIRE number actually is
A FIRE number is annual spending divided by a withdrawal rate. At 4%, multiply spending by 25. It is a planning estimate from historical US portfolio research, not a promise you can retire, and not a number that knows your currency or your fees.
Last reviewed October 20, 2026 by Francis Eshun, Founder of Iron Wealth. He built the on-device tracker these pages describe. Iron Labs.
Why is 4% not a law?
People quote a 25× rule because 1 ÷ 0.04 = 25. William Bengen’s 1994 Journal of Financial Planning paper, and the 1998 Trinity study by Cooley, Hubbard, and Walz, looked at historical US stock-and-bond portfolios over about 30 years. They did not study your currency, your fees, or your lifespan. Treat 4% as a starting model you can change.
What does a worked example look like?
Annual spending of 6,000,000 NGN at 4% is 6,000,000 ÷ 0.04 = 150,000,000 NGN. Spending of 40,000 USD at the same rate is 1,000,000 USD. Drop the withdrawal rate to 3% and the naira target becomes 6,000,000 ÷ 0.03 = 200,000,000 NGN. The formula does not care which currency the spending is in. It also does not protect you if that currency loses purchasing power.
How is the FIRE number different from net worth?
Net worth is the balance you have today: assets minus liabilities. The FIRE number is a target you might need later. Planning fails when the target lives in a sheet and the balances live in five apps. Recalculate the target when spending or the rate changes, and keep the live net worth next to it.
What if you will spend in a different currency than you hold?
If you will spend in GHS or EUR and hold USD assets, the number hides an exchange-rate assumption. Keep each account in its own currency and pick the display currency you will actually spend. Educational only: this is not advice.
Where does a mobile money wallet fit?
Spending is the input that moves the target most. If daily life runs through MTN MoMo or M-Pesa, the wallet is where that spending shows up. Logging it, by hand or from Android payment alerts, gives a real annual figure instead of a guess. The wallet balance counts toward net worth. Count it toward the FIRE target only if you would invest it rather than spend it.
Questions
- Is the website FIRE calculator free?
- Yes. No account required. The in-app FIRE planner, forecasts, and what-ifs are Pro.
- Does a FIRE number guarantee I can retire?
- No. It is a planning estimate. Markets, inflation, spending, and health can all miss the model.
- Can I model FIRE in GHS, NGN, or EUR?
- Yes. Use the currency you plan to spend. Dual display helps if assets sit elsewhere.
Run the free calculator, then keep the live net worth in Iron Wealth so the target has a daily number next to it.
