This compound interest calculator shows what your savings become when interest starts earning interest of its own. Enter your starting amount, your monthly contribution, the rate and the number of years: it returns the final balance, the cumulative interest, a chart and the year-by-year schedule. The options add fees, inflation, rising contributions, withdrawals and the monthly amount needed to reach a goal. A free online compound interest calculator, no sign-up.
The rate you enter is annual and effective by default: monthly contributions compound at the equivalent monthly rate. A negative contribution reads as a withdrawal. This is a constant-rate projection, not a forecast.
Compound interest is interest calculated on the principal AND on the interest already earned. Each year, what the investment made is added to the balance, and the next year the rate applies to that larger total. Simple interest is only calculated on the original amount, so it pays the same sum every year. Cumulative interest is the total of all that interest over the whole period: this calculator shows it as interest earned, and year by year in the schedule.
Here is the gap on $10,000 invested at 5% a year, with no contributions:
| Period | Simple interest | Compound interest | Difference |
|---|---|---|---|
| 10 years | $15,000 | $16,289 | $1,289 |
| 20 years | $20,000 | $26,533 | $6,533 |
| 30 years | $25,000 | $43,219 | $18,219 |
Over ten years, the difference is $1,289. Over thirty years, it is more than $18,000: time does the work, far more than the rate.
The starting amount is what you invest today, the monthly contribution what you add every month. Either can be zero. A negative contribution models a withdrawal, to see how long a balance lasts.
The rate is an assumed average annual return, the period a number of years. The longer the period, the more the rate weighs: at 5%, $10,000 becomes $16,289 in 10 years and $43,219 in 30.
In the options: your fund or account fees, inflation to read the result in today's money, a yearly rise in your contributions, and your goal. The calculator then returns the balance net of fees and the monthly amount needed to reach the target.
The compound interest formula on a lump sum: final balance = starting amount × (1 + rate)^years. With contributions, the calculation runs month by month: the balance is multiplied by (1 + rate)^(1/12), fees are taken out, then the contribution is added. That is what this tool does, over the whole period.
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See portfolio tracking →Lump sum only, no contributions, interest compounded once a year. The rates are return assumptions, not those of any particular product.
| Annual rate | 10 years | 20 years | 30 years | 40 years |
|---|---|---|---|---|
| 2% | $12,190 | $14,859 | $18,114 | $22,080 |
| 4% | $14,802 | $21,911 | $32,434 | $48,010 |
| 6% | $17,908 | $32,071 | $57,435 | $102,857 |
| 8% | $21,589 | $46,610 | $100,627 | $217,245 |
Read the 8% row: the balance roughly doubles every nine years, so it grows from $10,000 to more than $200,000 in forty years. Now read the 40-year column: between 2% and 8%, the result is multiplied by almost ten. Rate and time do not add up, they multiply.
No starting amount, $100 paid in at the end of every month. Each cell shows the final balance, and below it what you actually paid in.
| Annual rate | 10 years | 20 years | 30 years | 40 years |
|---|---|---|---|---|
| 3% | $13,945 $12,000 paid in | $32,685 $24,000 paid in | $57,871 $36,000 paid in | $91,719 $48,000 paid in |
| 5% | $15,436 $12,000 paid in | $40,580 $24,000 paid in | $81,538 $36,000 paid in | $148,252 $48,000 paid in |
| 7% | $17,105 $12,000 paid in | $50,754 $24,000 paid in | $116,945 $36,000 paid in | $247,154 $48,000 paid in |
At 5% over 40 years, you paid in $48,000 and end up with $148,252: interest is more than two thirds of the result. Over ten years, it is not even a quarter.
$200 a month until 65, at 6% a year, depending on the age you start:
| You start | Years to 65 | Total paid in | Balance at 65 |
|---|---|---|---|
| at 25 | 40 years | $96,000 | $381,536 |
| at 35 | 30 years | $72,000 | $194,903 |
| at 45 | 20 years | $48,000 | $90,688 |
Ten extra years almost double the result: $381,536 against $194,903, for $24,000 more paid in. The most telling case: $200 a month from 25 to 35 only, then nothing, left at 6% until 65, gives $186,633. That is almost as much as thirty years of contributions started at 35, for a third of the money paid in.
A 1% annual fee looks small. Yet it is charged on the whole balance, every year, and what it takes out never earns interest again. Example: $10,000 to start, $200 a month, 30 years at 6% before fees.
| Annual fees | Final balance | Shortfall | Of which fees paid |
|---|---|---|---|
| 0% | $252,338 | – | – |
| 0.2% | $241,694 | $10,644 | $5,696 |
| 0.5% | $226,651 | $25,687 | $13,648 |
| 1% | $203,847 | $48,491 | $25,459 |
| 2% | $165,597 | $86,741 | $44,475 |
At a 1% fee, you lose $48,491 over thirty years, while the fees actually charged come to only $25,459. The rest is the interest those fees never earned. That is why a fund's expense ratio is best compared in points of return lost, not in dollars paid each year.
To find how many years it takes money to double, divide 72 by the rate. At 6%, 72 ÷ 6 = 12 years. The rule is an approximation: the calculator returns the exact time, ln 2 ÷ ln(1 + rate).
| Rate | Rule of 72 | Exact time |
|---|---|---|
| 2% | 36.0 years | 35.0 years |
| 3% | 24.0 years | 23.4 years |
| 4% | 18.0 years | 17.7 years |
| 6% | 12.0 years | 11.9 years |
| 8% | 9.0 years | 9.0 years |
| 10% | 7.2 years | 7.3 years |
| 12% | 6.0 years | 6.1 years |
It is spot on around 8% and a year off at 2%. For a rough idea the rule is enough; for a low-rate savings account, use the exact figure.
A 5% rate does not earn the same amount depending on how often interest is added to the balance. Here is $10,000 invested for 20 years at 5%, with the calculator's three settings:
| Compounding | Balance after 20 years | Effective rate |
|---|---|---|
| Annual (effective rate) | $26,533 | 5.00% |
| Monthly (5% ÷ 12 each month) | $27,126 | 5.12% |
| Daily (5% ÷ 365 each day) | $27,181 | 5.13% |
The gap is modest, and it mainly tells you what to check. Many savings accounts compound daily and credit interest monthly, and a rate quoted as APR is nominal while APY is already the effective annual rate: compare APY with APY.
$67,113 in twenty years will not buy what $67,113 buys today. With 2% annual inflation, it is worth $45,165 in today's money. Enter inflation in the options: the calculator adds this real value to the result and a column to the schedule. An investment whose rate stays below inflation loses value, even while its balance rises.
The calculation also works backwards. To have $100,000 in twenty years starting from $10,000 at 5%, you need to put in $181.04 a month. With nothing to start, aiming for $300,000 in thirty years at 6% takes $307.85 a month. Enter your goal in the options: the calculator returns this amount, fees and rising contributions included.
The calculator tells you what a rate produces. To decide which sum goes where, depending on when you will need it, the guide to where to put your money in France walks through the method, investment by investment.
This page is educational. It is not investment advice, and past performance does not guarantee future results.
On a lump sum: final balance = starting amount × (1 + rate)^years. $10,000 at 5% for 20 years: 10,000 × 1.05^20 = $26,533. With regular contributions, the calculation runs period by period, which this calculator does month by month.
Simple interest is calculated on the original amount only, compound interest on the amount plus the interest already earned. At 5% for 30 years, $10,000 becomes $25,000 with simple interest and $43,219 with compound interest.
At 5% a year, $100 paid in every month grows to $15,436 after 10 years, $40,580 after 20 years and $81,538 after 30 years, for $12,000, $24,000 and $36,000 paid in. These are constant-rate projections.
Annual. By default it is effective: monthly contributions compound at the equivalent monthly rate, (1 + rate)^(1/12) − 1, and the balance earns exactly the rate entered over twelve months. The options switch to monthly or daily compounding, where the rate is divided by 12 or by 365.
Enter an assumed average annual return as the rate, and the fund's annual fees in the options. A stock index like the S&P 500 alternates strong years and down years: the calculator's constant rate gives an average path, not a forecast.
Yes: enter a negative monthly contribution. The calculator takes that amount out every month and tells you when the balance runs out, if it does. $200,000 invested at 4% with $1,000 withdrawn a month lasts 27 years and 2 months.
Yes, free and no sign-up. It works just as well as a cumulative interest calculator, a compound investment calculator or a savings calculator. The “Copy the link to this projection” button puts all your inputs in the address, so you can come back to it or share it.
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