Compound Interest Calculator
Project what a starting balance plus monthly contributions becomes, in both nominal dollars and what it will actually buy.
Your plan
Everything recalculates as you type.
What you already have invested today.
Added at the end of each month.
Broad stock index assumptions usually sit between 7% and 10%.
Used only for the purchasing-power figure.
Raise your deposit each year in line with your raises.
Balance after 30 years
$854,537
$407,394 in today's purchasing power at 2.5% inflation
Total contributed
$190,000
Starting balance plus every deposit.
Growth
$664,537
78% of the final balance was never your money.
Crossover year
Year 8
The first year returns beat your own deposits.
Compounding is doing the heavy lifting: 78% of the final balance is growth, not deposits. From year 8 onward the portfolio adds more each year than you do.
What actually moves the number
The same projection, re-run with one input nudged. Compare the size of the two levers before deciding which one to chase.
Harder to control than it looks, and usually bought with more risk.
Entirely within your control, starting this month.
At your current inputs the extra percentage point wins, because the balance is already large enough that a percentage point of it exceeds $1,200 a year of new money. This is the point where fees and expense ratios start to matter more than saving harder.
Growth over time
Grey is money you put in. Blue is money the market added.
Year by year
| Year | Contributed | Growth | Balance |
|---|---|---|---|
| Year 1 | $6,000 | $1,055 | $17,055 |
| Year 2 | $6,000 | $1,641 | $24,695 |
| Year 3 | $6,000 | $2,275 | $32,970 |
| Year 4 | $6,000 | $2,961 | $41,932 |
| Year 5 | $6,000 | $3,705 | $51,637 |
| Year 6 | $6,000 | $4,511 | $62,148 |
| Year 7 | $6,000 | $5,383 | $73,531 |
| Year 8 | $6,000 | $6,328 | $85,859 |
| Year 9 | $6,000 | $7,351 | $99,210 |
| Year 10 | $6,000 | $8,459 | $113,669 |
| Year 11 | $6,000 | $9,659 | $129,329 |
| Year 12 | $6,000 | $10,959 | $146,288 |
| Year 13 | $6,000 | $12,367 | $164,655 |
| Year 14 | $6,000 | $13,891 | $184,546 |
| Year 15 | $6,000 | $15,542 | $206,088 |
| Year 16 | $6,000 | $17,330 | $229,419 |
| Year 17 | $6,000 | $19,267 | $254,685 |
| Year 18 | $6,000 | $21,364 | $282,049 |
| Year 19 | $6,000 | $23,635 | $311,684 |
| Year 20 | $6,000 | $26,095 | $343,778 |
| Year 21 | $6,000 | $28,758 | $378,537 |
| Year 22 | $6,000 | $31,643 | $416,180 |
| Year 23 | $6,000 | $34,768 | $456,948 |
| Year 24 | $6,000 | $38,151 | $501,099 |
| Year 25 | $6,000 | $41,816 | $548,915 |
| Year 26 | $6,000 | $45,785 | $600,700 |
| Year 27 | $6,000 | $50,083 | $656,782 |
| Year 28 | $6,000 | $54,738 | $717,520 |
| Year 29 | $6,000 | $59,779 | $783,299 |
| Year 30 | $6,000 | $65,238 | $854,537 |
Returns are modelled as a smooth annual rate. Real markets deliver the same average through a sequence of very different years, and the order those years arrive in matters a great deal once you start withdrawing.
Key takeaways
- Compound interest is calculated monthly here: each month the balance grows by one twelfth of the annual return, then that month's contribution is added.
- The crossover year is the first year your portfolio earns more from returns than you add from contributions, and it is the single most useful number in any compounding projection.
- A nominal projection overstates what you will actually be able to buy; dividing by (1 + inflation) raised to the number of years converts it to today's purchasing power.
- At an 8% annual return, $500 invested monthly for 30 years grows to roughly $745,000, of which about $565,000 is growth rather than deposits.
- Raising your contribution by a few percent a year, in line with raises, moves the final balance more than chasing a slightly higher return does.
Frequently asked questions
- How is compound interest with monthly contributions calculated?
- Each month the running balance is multiplied by (1 + annual rate ÷ 12), and then the monthly contribution is added on top. Repeating that for every month of the time horizon gives the final balance. This calculator adds contributions at the end of each month, which is the conservative convention and matches how most payroll deductions and automatic transfers actually land.
- What annual return should I assume?
- A long-run assumption of 7% to 10% is standard for a broad stock index, because the S&P 500 has averaged roughly 10% nominal and roughly 7% after inflation over multi-decade periods. Use the lower end if you want a projection you are unlikely to fall short of. Bonds and cash belong far lower, in the 2% to 5% range.
- Why is the inflation-adjusted number so much lower?
- Because inflation compounds against you at the same time your money compounds for you. At 2.5% inflation, prices roughly double every 28 years, so a balance 28 years out buys about half of what the same headline number buys today. The real figure is the honest one to plan against.
- What is the crossover year?
- The crossover year is the first year in which your investment returns exceed the amount you contributed that year. Before it, you are the main engine of the portfolio. After it, the portfolio is. It usually arrives somewhere between year 8 and year 15 at typical return and contribution levels, and reaching it is the reason early contributions matter far more than late ones.
- Does this account for taxes or fees?
- No, it models a gross return. To approximate a taxable account or a fund with a meaningful expense ratio, subtract the drag from the return you enter. A 0.6% expense ratio on an 8% assumption becomes 7.4%, and over 30 years that difference alone costs a mid-six-figure portfolio tens of thousands of dollars.