Compound interest calculator
Compounding is the quiet engine behind almost every long-term money outcome, and it’s easier to feel with your own numbers than to read about. Enter a starting balance, a monthly contribution, a rate, and a timeline - the calculator shows the ending balance and how much of it is growth rather than deposits.
What the table is really showing
Look at the gap between “contributed” and “balance” as the years pass: early on, deposits do all the work; later, growth-on-growth takes over. That handoff is the whole argument for starting early - time in the market is the input that can’t be substituted later.
Related reading
What compound interest is and why it matters explains the concept from zero, and starting to invest with little money covers the practical first steps. For the parking-money side, see high-yield savings accounts, and for whether growth math should ever wait, pay off debt or save first.
The formula this calculator uses
r = annual rate / 100 / 12
each month: balance = balance * (1 + r) + monthly contribution
contributed = starting amount + monthly contribution * months
growth = balance - contributed
What it assumes without asking
- The contribution never changes and never stops
- A single steady return applied every month, which no real investment has
- No fees, no taxes and no withdrawals
One example, computed in full: $200 a month for 30 years at 7%
| Year | You contributed | Growth | Balance | Growth as % of balance |
|---|---|---|---|---|
| 5 | $12,000 | $2,319 | $14,319 | 16.2% |
| 10 | $24,000 | $10,617 | $34,617 | 30.7% |
| 15 | $36,000 | $27,392 | $63,392 | 43.2% |
| 20 | $48,000 | $56,185 | $104,185 | 53.9% |
| 25 | $60,000 | $102,014 | $162,014 | 63.0% |
| 30 | $72,000 | $171,994 | $243,994 | 70.5% |
What this calculator does not model
- Inflation. Every figure above is in future dollars and buys less than it looks
- The order returns arrive in, which matters as much as the average once you are drawing money out
- Fees, which compound against you in exactly the same shape
The guide behind this tool
A number on its own does not tell you what to do with it. How compound interest works has the full 30-year table and the chart of where growth overtakes contributions.
Sources & further reading
This tool is general education, not personalized financial advice. Steady-rate compounding is an illustration - real investments fluctuate and can lose value. See our disclaimer.