Compound Interest Calculator

Why time matters more than the amount

Compound growth is exponential, and human intuition is famously bad at exponentials. The consequence is a result that seems unfair and is simply arithmetic: someone who saves diligently for ten years in their twenties and then stops entirely will frequently end up with more at retirement than someone who starts at forty and saves for twenty-five years. The early saver contributed far less money, but their contributions had decades in which to compound, and it is the compounding that does the work, not the contributing. Every year of delay removes a year from the far end of the curve — the steepest part, where the growth is largest. This single fact is the most valuable thing in personal finance, and it is almost entirely about when you start rather than how much you put in.

The same force works against you

Compounding is indifferent to whose side it is on. Credit card debt at 24% annual interest, compounding monthly, grows on exactly the same accelerating curve as an investment — only now you are the one on the wrong end of it. A balance left to compound at that rate roughly doubles in three years without a single new purchase. This is why paying down high-interest debt is so often the single best financial decision available: clearing a 24% debt is mathematically equivalent to earning a guaranteed, tax-free 24% return, which no legitimate investment will offer you. The compounding curve is a tremendous ally and a merciless creditor, and it is the same curve.