Compound Interest Calculator
See how your money grows over time with compound interest and regular contributions — with a growth chart and year-by-year table. 看看复利加定期追加，长期下来钱能长成什么样。
Contributions / 投入
| Year年份 | Balance结余 | Contributions累计投入 | Interest累计利息 |
|---|
Compound interest is the closest thing finance has to a law of nature: money earns returns, and those returns earn their own returns. This calculator projects what a starting principal grows into over years or decades at a given rate, with optional monthly contributions, and shows the result as a growth chart, three headline numbers and a year-by-year table.
The example that convinces most people: $10,000 invested at 7% for ten years, with $200 added each month, becomes about $54,700. You put in $34,000 — the remaining $20,700 is pure compound growth, earned while you slept. Change the horizon to twenty years and the same inputs reach roughly $130,000, because the curve steepens: early years are dominated by your contributions, later years by compounding itself. That geometry is the entire argument for starting early rather than saving harder later.
Two details shape the numbers. Compounding frequency — yearly, quarterly, monthly, daily — nudges the effective rate slightly higher as frequency rises, but at 7% the gap between yearly and monthly over a decade is small compared with one extra year of saving. Contributions are modelled at the end of each compounding period, which is the conservative convention most projections use.
The year table keeps the growth honest: every row shows balance, total contributions and cumulative interest side by side, so you can watch interest overtake contributions around the middle of a long horizon. And the usual caveat applies — this assumes a constant rate. Real markets vary, inflation takes its share, and fees quietly compound too. For an honest picture, run the numbers again at 5% and see how wide the range is.
One practical trick: use the table, not just the final number. Find the year when interest first exceeds your yearly contributions — that is the moment the plan starts carrying itself, and it arrives sooner than intuition suggests. From there, protecting the habit of contributing matters more than chasing a slightly better rate.
Frequently asked questions
Does monthly vs. yearly compounding make a big difference?
Compounding frequency matters, but less than people think. Monthly versus yearly compounding at 7% adds roughly 0.2 percentage points of effective return over ten years — worth having, but your savings rate moves the needle far more.
How much of the final balance is interest vs. my own money?
Interest earned = future value − everything you put in. At 7% with $10,000 starting and $200/month for 10 years, you put in $34,000 and the interest contributes about $20,700 — compound growth does roughly 40% of the work.
Why does starting early matter more than saving more?
Yes — the growth chart shows that contributions (the teal line) dominate early on, while the compounding curve steepens with time. That is exactly why starting early beats saving more later: time is the input you cannot buy back.
What annual rate should I assume?
Long-run averages: broad stock markets have historically returned around 7% per year after inflation in the US, government bonds 2–4%, bank deposits often less than inflation. Any single decade can differ wildly, so run the projection at several rates rather than trusting one number.