Compound Interest Calculator
See how a starting amount and regular contributions grow over time. Choose how often interest is added and read the full year-by-year breakdown below the results.
Year-by-year breakdown
| Year | You put in (total) | Interest (total) | Balance |
|---|
Why time does most of the work
Each period's interest is added to the balance, so the next period earns interest on a slightly bigger number. Early on this is barely noticeable. Over decades it dominates: with a long enough horizon, the interest can exceed everything you contributed. Starting earlier, even with smaller amounts, usually beats starting later with larger ones.
Rate, time, and contributions
Three levers drive the result. A higher rate helps but is largely outside your control and comes with more risk. More time is the most powerful and the one to protect. Bigger contributions are the lever you control most directly, and raising them whenever your income rises has an outsized effect.
What this leaves out
The calculator assumes one fixed rate forever. Real returns vary and can be negative in a given year. It also ignores fees, tax on gains and inflation, all of which reduce what you keep. For a rough real-growth view, enter a rate equal to your expected return minus those three. This tool is for comparing scenarios, not for predicting a specific outcome, and is not investment advice.
Frequently asked questions
What is compound interest?
Interest earned on both your original money and the interest it has already earned. Because each period's interest is added to the balance, the next period's interest is slightly larger, and the effect grows over time.
Does compounding frequency matter much?
Less than people expect. At the same annual rate, moving from yearly to monthly compounding adds a small amount; monthly to daily adds almost nothing. The rate, the time invested and how much you keep adding matter far more.
Is this a forecast of what I will actually earn?
No. It assumes a single fixed rate every period. Real savings rates change, and investment returns go up and down and can be negative. Use it to compare scenarios, not to predict a specific outcome. It is not investment advice.
How do fees, tax and inflation change the picture?
They all reduce what you actually keep. A rough way to see real, after-cost growth is to enter a rate equal to your expected return minus fees, minus tax on the gains, minus inflation.
What is the rule of 72?
Divide 72 by the annual rate to estimate the years for money to double. At 6% that is about 12 years; at 8%, about 9 years. It is a mental shortcut, not exact.