Free financial tool
Compound Interest Calculator
Watch your money grow in real time. Adjust your starting amount, monthly contribution, rate and time horizon — and see exactly what compounding does, year by year.
Rule of 72: at 7% per year, your money doubles roughly every 10.3 years.
Growth year by year
- You contributed
- Interest earned
See the yearly breakdown
| Year | Contributed | Interest | Balance |
|---|---|---|---|
| 1 | $2,200 | $112 | $2,312 |
| 2 | $3,400 | $318 | $3,718 |
| 3 | $4,600 | $626 | $5,226 |
| 4 | $5,800 | $1,043 | $6,843 |
| 5 | $7,000 | $1,577 | $8,577 |
| 6 | $8,200 | $2,236 | $10,436 |
| 7 | $9,400 | $3,030 | $12,430 |
| 8 | $10,600 | $3,968 | $14,568 |
| 9 | $11,800 | $5,060 | $16,860 |
| 10 | $13,000 | $6,318 | $19,318 |
| 11 | $14,200 | $7,754 | $21,954 |
| 12 | $15,400 | $9,380 | $24,780 |
| 13 | $16,600 | $11,211 | $27,811 |
| 14 | $17,800 | $13,261 | $31,061 |
| 15 | $19,000 | $15,545 | $34,545 |
| 16 | $20,200 | $18,082 | $38,282 |
| 17 | $21,400 | $20,888 | $42,288 |
| 18 | $22,600 | $23,985 | $46,585 |
| 19 | $23,800 | $27,392 | $51,192 |
| 20 | $25,000 | $31,131 | $56,131 |
What is compound interest?
Compound interest is interest that earns interest. When your money grows, the growth itself starts growing: the interest you earned last year is added to your balance, and next year's interest is calculated on that bigger amount. Over short periods the effect looks tiny — over decades it becomes the main engine of your savings.
That is why the two most powerful levers in the calculator above are not the rate: they are time and consistency. A modest monthly contribution started early routinely beats a much larger deposit made years later, simply because every extra year lets the interest compound one more time.
Move the sliders and watch the gold area of the chart. Early on, most of your balance is money you put in. Push the years far enough and the gold — the interest — takes over. That crossover is compound interest working.
The formula behind this calculator
A = P (1 + r/n)^(n·t)
P is your starting amount, r the annual rate as a decimal, n how many times per year interest compounds, and t the number of years. That classic formula covers a single deposit.
Monthly contributions grow the same way, but each deposit compounds only from the month it enters. This calculator simulates exactly that: it walks month by month, applies the effective monthly rate to your balance, then adds your contribution at the end of each month — the same method banks and financial planners use.
How to use this calculator
Start with your real numbers: what you already have saved, and what you can realistically add every month. Then pick a rate that matches where the money actually sits. High-yield savings accounts and term deposits typically pay somewhere between 1% and 5% per year; diversified stock index funds have historically averaged around 7–10% per year before inflation over long periods — with real ups and downs along the way, and never as a guarantee.
The compounding setting controls how often interest is added to your balance. Monthly compounding is the most common for savings products. The difference between monthly and yearly compounding is real but modest — time and contributions matter far more.
Everything recalculates instantly as you move a slider, so the fastest way to build intuition is to change one input at a time: double the years, then double the monthly contribution, then the rate, and compare which one moves the final balance most.
The Rule of 72
The Rule of 72 is the classic mental shortcut for compounding: divide 72 by your annual rate to estimate how many years your money needs to double. At 7% per year, 72 ÷ 7 ≈ 10.3 years per doubling. At 3%, it takes about 24 years.
Doublings stack: at 7%, money left alone for 30 years doubles almost three times — roughly 8× the original amount. It is an approximation, but a remarkably good one for rates in the normal savings-and-investing range, and a quick way to sanity-check any projection this calculator gives you.
Frequently asked questions
Does this calculator account for inflation or taxes?
No. Results are nominal: they show the balance before inflation, taxes, or fees. Inflation typically erodes 2–4% of purchasing power per year, so for long horizons consider using a rate net of inflation (for example 4–7% instead of 7–10%) to think in today's money.
What interest rate should I use?
Match the rate to the product. Savings accounts and term deposits commonly pay 1–5% per year. Diversified stock index funds have historically averaged 7–10% per year before inflation over long periods, but returns vary widely year to year and are never guaranteed. When in doubt, run a conservative and an optimistic scenario.
When are contributions added, and how is interest compounded?
The simulation moves month by month: interest accrues on your balance at the effective monthly rate derived from your chosen compounding frequency, and your monthly contribution is deposited at the end of each month.
Is anything I type saved or sent anywhere?
No. The calculator runs entirely in your browser. Nothing you enter is stored, tracked, or sent to any server.
What is the difference between simple and compound interest?
Simple interest is always calculated on the original amount only, so growth is a straight line. Compound interest is calculated on the original amount plus all interest already earned, so growth accelerates over time — that curve you see in the chart is the difference.