Finance Calculators & Guides

Compound Interest, Retirement Math, and the FIRE Number

Most personal finance decisions come down to one idea: how money grows over time, and how fast you can safely spend it back down. This category covers both halves of that equation, starting with how compound interest actually works and where it quietly gets misunderstood.

On the growth side, you'll find a full breakdown of how compound interest grows your money, the difference between daily, monthly, and yearly compounding, and how simple interest compares to compound interest on the same loan or savings balance. We also cover the specific compound interest mistakes, like starting late or interrupting the compounding period, that cost people the most money.

On the spending side, the guides explain how to calculate your FIRE number for retirement, what the FIRE movement actually involves day to day, and whether the classic 4% withdrawal rule still holds up for retirees today. Each article works through the math with real examples rather than just stating a rule, so you can apply the same logic to your own savings rate, timeline, and target retirement age.