DigiKitCompound Interest Calculator
Enter a starting amount, an annual rate, a time period and an optional monthly contribution to see the future value and the interest earned.
How compound interest works
With compound interest, the interest you earn is added to the balance and then earns interest itself. Without contributions the balance after t years is A = P × (1 + r ÷ m)m × t, where P is the starting amount, r the annual rate as a decimal and m the number of compounding periods per year.
With monthly contributions the calculator works month by month. It converts your rate and compounding frequency into an equivalent monthly rate, grows the balance by that rate each month and then adds the contribution at the end of the month. The number of months is the years you enter × 12, rounded to the nearest whole month.
Worked examples
Starting amount only. 10,000 at 7% for 10 years grows to 19,671.51 when compounded annually and 20,096.61 when compounded monthly. The 425.10 difference comes only from how often interest is added.
| Compounding (10,000 at 7%, 10 years) | Future value |
|---|---|
| Annually | 19,671.51 |
| Twice a year | 19,897.89 |
| Quarterly | 20,015.97 |
| Monthly | 20,096.61 |
| Daily | 20,136.18 |
With contributions. Start with 5,000, add 200 a month, earn 6% compounded monthly and leave it for 20 years. You put in 5,000 + 200 × 240 = 53,000 and the balance reaches 108,959.20, of which 55,959.20 is interest.
Contributions only. Saving 300 a month for 30 years at 6% (starting from 0) gives 301,354.51 from a total of 108,000 contributed.
How to use the calculator
- Enter the amount you are starting with. It can be 0 if you only plan to contribute monthly.
- Enter the expected annual interest rate and the number of years.
- Choose how often interest compounds. Monthly is the default.
- Add a monthly contribution if you plan to save regularly, then press Run.
Things to keep in mind
- The result is a projection at a constant rate. Real investments move up and down, and savings rates change, so treat the figures as an illustration rather than a forecast.
- Taxes, fees and inflation are not included. Inflation reduces what the future amount can buy.
- Contributions are assumed to arrive at the end of each month. Contributing at the start of the month would give a slightly higher result.
- Compounding more often helps, but the gain flattens: going from monthly to daily adds far less than going from annual to monthly.
- As a rough check, the rule of 72 says money doubles in about 72 ÷ rate years; at 7% that is about 10.3 years.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is calculated only on the original amount. Compound interest is calculated on the original amount plus the interest already earned, so growth speeds up over time.
Does the calculator include taxes or inflation?
No. It shows the nominal growth at the rate you enter. Taxes, account fees and inflation are not modelled.
When are the monthly contributions added?
At the end of each month, after that month's interest has been applied.
Can I use a fraction of a year?
Yes. The calculator rounds the period to whole months, so 1.5 years is treated as 18 months.
Is my data stored?
No. The calculation runs in your browser and nothing is sent to a server.