How Compound Interest Works (Plain English)

A clear explanation of compound interest, why time matters most, and how compounding frequency affects results.

The idea in one sentence

Compound interest is when your money earns returns, and then those returns earn returns too — so growth accelerates over time.

Why time matters more than almost anything

If you keep the rate and contributions the same, the biggest driver of growth is time. Early years often look slow, then compounding starts to show a bigger effect.

Compounding frequency (monthly vs yearly)

Compounding more often usually increases the final value, but the difference is often smaller than people expect. The longer the time horizon, the more it can matter.

A simple way to build intuition

Try two runs in the calculator: 10 years vs 20 years with the same inputs. You’ll typically see the 2nd decade adds more than the 1st.

What this doesn’t include

Taxes, fees, inflation, and changing rates. Those factors matter in real life — the calculator is designed to be a clean baseline.