Compound Interest Calculator
See how savings grow with compound interest and regular monthly deposits. The table below breaks down contributions, interest and balance for every year.
Final balance
$109,333
After 20 years
Total contributions
$58,000
Total interest earned
$51,333
Effective annual rate
5.116%
Interest share of balance
47.0%
Year-by-year breakdown
| Year | Deposits | Interest | Total deposits | Total interest | Balance |
|---|---|---|---|---|---|
| 1 | $2,400 | $567 | $12,400 | $567 | $12,967 |
| 2 | $2,400 | $719 | $14,800 | $1,287 | $16,087 |
| 3 | $2,400 | $879 | $17,200 | $2,165 | $19,365 |
| 4 | $2,400 | $1,047 | $19,600 | $3,212 | $22,812 |
| 5 | $2,400 | $1,223 | $22,000 | $4,435 | $26,435 |
| 6 | $2,400 | $1,408 | $24,400 | $5,843 | $30,243 |
| 7 | $2,400 | $1,603 | $26,800 | $7,446 | $34,246 |
| 8 | $2,400 | $1,808 | $29,200 | $9,254 | $38,454 |
| 9 | $2,400 | $2,023 | $31,600 | $11,277 | $42,877 |
| 10 | $2,400 | $2,249 | $34,000 | $13,527 | $47,527 |
| 11 | $2,400 | $2,487 | $36,400 | $16,014 | $52,414 |
| 12 | $2,400 | $2,737 | $38,800 | $18,751 | $57,551 |
| 13 | $2,400 | $3,000 | $41,200 | $21,751 | $62,951 |
| 14 | $2,400 | $3,276 | $43,600 | $25,028 | $68,628 |
| 15 | $2,400 | $3,567 | $46,000 | $28,595 | $74,595 |
| 16 | $2,400 | $3,872 | $48,400 | $32,467 | $80,867 |
| 17 | $2,400 | $4,193 | $50,800 | $36,660 | $87,460 |
| 18 | $2,400 | $4,530 | $53,200 | $41,190 | $94,390 |
| 19 | $2,400 | $4,885 | $55,600 | $46,075 | $101,675 |
| 20 | $2,400 | $5,258 | $58,000 | $51,333 | $109,333 |
Total deposits include the initial deposit.
Compound interest formula
Without regular deposits, the balance after t years is:
A = P × (1 + r / n)^(n × t)
where P is the initial deposit, r the annual interest rate as a decimal and n the number of compounding periods per year.
With monthly deposits, the calculator simulates every month. It converts the annual rate into an equivalent monthly rate, (1 + r/n)^(n/12) − 1, adds that month's interest to the balance, then adds your deposit at the end of the month. With no deposits this gives exactly the same result as the formula above.
Worked example
You invest $10,000 at 5 % a year with yearly compounding and leave it for 10 years:
- Year 1: $10,000 × 1.05 = $10,500
- Year 2: $10,500 × 1.05 = $11,025
- Year 10: $10,000 × 1.05^10 ≈ $16,288.95
With monthly compounding the same deposit grows to about $16,470.09. Adding $100 every month on top (monthly compounding) gives about $15,528 from the deposits alone, of which $12,000 is your money and around $3,528 is interest. The longer the money stays invested, the larger the interest share becomes.
Frequently asked questions
What is compound interest? +
Compound interest is interest earned on both your original money and the interest it has already earned. Each period the balance grows a little, and the next interest payment is calculated on that larger balance.
How much difference does compounding frequency make? +
Less than most people expect. At 5 % a year, monthly compounding gives an effective annual rate of about 5.12 % and daily compounding about 5.13 %. The interest rate, the amount you deposit and the time you stay invested matter far more.
Does the calculator include inflation and taxes? +
No. It shows nominal growth before tax. To see growth in today's money, you can enter a rate reduced by your expected inflation, for example 5 % − 2 % = 3 %.
When are the monthly contributions added? +
At the end of each month. The first deposit starts earning interest the following month. Deposits made at the start of each month would end up slightly higher.
What is the rule of 72? +
A quick mental shortcut: divide 72 by the annual interest rate to estimate how many years it takes to double your money. At 6 % that is about 12 years. Use the calculator for precise numbers.
For information only. This calculator gives estimates based on the numbers you enter. It is not financial, tax or investment advice. Check important decisions with a qualified professional.
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