Compound interest calculator
Work out how much your savings will grow with compound interest and regular contributions, and how much of that total came from interest rather than from you.
Your figures
Result
- What you put in
- $58,000
- What the interest put in
- $82,204
- Share of the final total that is interest
- 58.6%
- Growth on what you contributed
- 141.7%
- After 20 years
- $140,204
- After 30 years
- $310,013
- After 40 years
- $644,053
- After 50 years
- $1,301,160
Year by year, out to 50
| Year | Contributed | Interest | Total |
|---|---|---|---|
| 1 | $12,400 | $776 | $13,176 |
| 2 | $14,800 | $1,774 | $16,574 |
| 3 | $17,200 | $3,011 | $20,211 |
| 4 | $19,600 | $4,502 | $24,102 |
| 5 | $22,000 | $6,265 | $28,265 |
| 6 | $24,400 | $8,319 | $32,719 |
| 7 | $26,800 | $10,686 | $37,486 |
| 8 | $29,200 | $13,386 | $42,586 |
| 9 | $31,600 | $16,443 | $48,043 |
| 10 | $34,000 | $19,882 | $53,882 |
| 11 | $36,400 | $23,730 | $60,130 |
| 12 | $38,800 | $28,015 | $66,815 |
What we assume
- The return is read as an effective annual rate: 7% a year gives exactly +7% after one year with no contributions.
- Compounding is monthly, using the equivalent monthly rate.
- The return is assumed constant. In reality no year looks like the one before.
- Tax, fees and inflation are not deducted. The result is in today money, unadjusted.
- It is a mathematical projection, not a forecast and not a guarantee.
- The 20, 30, 40 and 50-year milestones project the same scenario beyond the term you pick: they assume you keep paying in the same amount, at the same return, for the whole period. The table follows them out to year 50; the chart stays on the term you set.
- The longer the horizon, the more inflation matters. Fifty years of nominal money is not fifty years of spending power: to see it in today money, subtract expected inflation from the return.
How it is calculated
Compound interest is the effect of interest earning further interest. Each month the balance grows a little, and the following month that growth earns a return too. That is why the curve is not a straight line: it steepens over time.
The formula combines two parts:
- The starting amount, growing as
C x (1 + i)^n. - The monthly contributions, each compounding from the month it goes in.
Where i is the monthly rate equivalent to the annual one you entered, and
n is the number of months.
Why 20, 30, 40 and 50 years
Over five or ten years compound interest barely looks different from putting money in a drawer: most of what is there at the end is money you put there. What makes it interesting happens later, once the interest already earned has spent decades earning in turn. That is why the tool always shows those four horizons, whatever term you calculate: they are the scale at which the effect is visible.
They are there to compare, not to promise. Nobody knows what returns the next fifty years hold, and half a century of inflation eats a good part of the figure.
The number actually worth watching
More than the final balance, look at what share of the total is interest. Over short horizons you are putting in almost all of it. Past a certain point the proportion flips, and that is when compounding starts to matter.
An example
With 10,000 to start, 200 a month and 7% a year over 20 years, the final balance is 140,204. Of that, 58,000 came from you and 82,204 is interest: 58.6% of the total. More than half the final money is not money you saved.
Frequently asked questions
Is the return before or after tax?
Before. No tax or fees are deducted. Investment gains are usually taxed when you sell, so the amount actually available to you will be lower.
Does it account for inflation?
No. The result is in nominal terms. For a sense of purchasing power, subtract expected inflation from the return: with 7% growth and 2% inflation, use 5%.
Why does paying in at the start of the month give more?
Because each contribution has one extra month to earn a return. Over long horizons the difference is small but real.
Keep calculating
All tools →- How much to save each month Say what you want and when. We work out the monthly amount, and how much of it the return contributes instead of you.
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Updated on 2026-08-28. Calculations run in your browser; nothing you type is sent to a server.