Compound Interest Calculator

This compound interest calculator has two tabs. Basic takes a single lump sum and rolls it forward over the number of periods and rate you enter. Recurring Investment adds a fixed monthly contribution on top of the initial principal and shows a year-by-year or month-by-month breakdown. Amounts are always shown with a dollar sign and there is no currency conversion. It is a pure math projection that assumes a constant rate. It ignores taxes, fees and inflation, and it is not investment advice; real returns fluctuate and can be negative. All calculations run in your browser. The exact rules and worked examples are below, and the Limits section lists the restrictions that still apply.

How to use Basic mode

1. Open the Basic tab and fill in 'Initial principal ($)' (default 10000), 'Number of periods (n)' (default 20) and 'Rate of return per period (%)' (default 5).
2. Click Calculate. If the principal or periods are zero or less, or the rate is negative, you see 'Please enter valid values (principal and periods must be greater than 0)'.
3. Two cards show Total interest and Final amount. The 'Calculation details' table lists interest, total and cumulative rate for each period, but only the first 10 periods are displayed.
4. Share opens the system share sheet (or copies the text and shows it in an alert if sharing is unavailable). 'Download result' saves a text file that includes all periods.

How to use Recurring Investment mode

1. Open the Recurring Investment tab and enter 'Initial principal ($)' and 'Monthly contribution ($)'.
2. Under 'Investment term', choose Years or Months and enter the number. Under 'Rate of return', choose Annual or Monthly and enter the percentage.
3. Pick 'Annually' or 'Monthly' in the Compounding dropdown and click Calculate.
4. The page shows Total interest, Final amount and Total invested. Use the Yearly / Monthly radio buttons to switch the breakdown table (at most the first 20 years or the first 60 months are shown). If the term is not a whole number of years, the last row's year label has one decimal, such as 1.5.

How it calculates, with examples (run through the tool's own algorithm)

In Basic mode, 'Rate of return per period (%)' is the rate for each single period: the per-period rate is rate / 100, and the balance compounds n times with no further division by n. With the defaults of 10000, 20 periods and 5% per period, the final amount is 26532.98 (total interest 16532.98, cumulative rate 165.33% on the last row). With 1 period and 5%, the final amount is exactly 10500. With 10000, 10 periods and 10% per period, it is 25937.42. In Recurring mode, the monthly rate is annual rate / 100 / 12 when you pick Annual, or the monthly rate used directly when you pick Monthly. With 'Monthly' compounding, each month's interest is merged into the balance straight away. With 'Annually' compounding, each month's interest is first recorded as pending interest and merged into the principal only every 12 months; in between, the displayed balance = principal + pending interest. In both cases interest is applied first and the month's contribution is added at the end of the month. Take an initial 1000, 1000 per month, 3 years and a 5% annual rate. With 'Monthly' compounding, the total invested is 37000, interest is about 2915 and the final amount is about 39915. With 'Annually' compounding, the total invested is again 37000, interest is about 2855 and the final amount is about 39855. For another case, 1000 principal, no contributions, 12% annual rate and 1 year gives about 1127 with Monthly compounding (consistent with 1.01 to the power 12) and exactly 1120 with Annually.

Limits and known quirks

• Basic mode has no year/month choice: 'Rate of return per period (%)' applies to whatever you define as one period, so periods and rate must use the same time unit. For example, '5% per year for 20 years' is entered as 20 periods and 5%, giving about 26533. If you only have an annualized rate but want monthly compounding, convert it to a monthly rate yourself, or use Recurring Investment mode with the monthly contribution set to 0.
• 'Annually' compounding in Recurring mode works as 'accumulate monthly, merge every 12 months': interest earned within a year does not earn further interest (contributions already made do earn interest each month) until the 12th month, when it is merged into the principal. If the term is not a multiple of 12 months, the pending interest after the last full year still counts in the displayed balance but earns nothing further.
• The helper text under the monthly contribution says 'Added at the end of each month; each contribution earns interest from the next month', which matches the calculation: the first contribution is added at the end of month 1. Total invested is therefore initial principal + monthly amount x number of months (in the 3-year example, 1000 + 1000 x 36 = 37000).
• Each month interest is applied first and the contribution is added afterwards, so a contribution earns nothing in the month it is made, as if deposited at the end of the month.
• The rate is a constant. You cannot vary it by year, add inflation, fees, taxes or dividend reinvestment, or make withdrawals.
• The Basic table shows only the first 10 periods, and Recurring shows at most 20 years or 60 months. The full Basic table is in the downloaded file. The Recurring download contains every row of whichever breakdown (Yearly or Monthly) is currently selected.
• Displayed amounts are rounded to whole numbers and use a dollar sign. The numbers in the share alert and the text file are rounded the same way.
• The rate cannot be negative, so you cannot simulate losses.

What happens to Basic results as periods increase?

Because the rate per period is fixed, each extra period multiplies the balance by (1 + rate per period) once more. With 10000 and 5% per period, 1 period gives 10500, 10 periods give 16288.95 and 20 periods give 26532.98, so growth speeds up. It also means that with the same rate per period, more periods always give a larger result.

Can I use this to decide which fund to buy?

No. It is a mathematical illustration of how a principal and regular contributions would grow under an assumed fixed return. Real funds, stocks and savings products fluctuate, are not guaranteed and carry costs. Use it to understand the effect of compounding and long-term saving, not to predict actual results.

How should I choose between annual and monthly rate?

If a product quotes an annualized return, choose Annual and the tool divides it by 12 to get the monthly rate. If you have a monthly rate, choose Monthly and it is used directly. Whether you pick Annually or Monthly compounding then depends on how often interest is merged into the principal: every 12 months with Annually, every month with Monthly.

Where do Share and Download send my data?

Share uses your browser's built-in share feature, so you decide where it goes in the system sheet, and this site does not receive it. Download generates a text file locally on your device.

Privacy and processing

All numbers are computed in your browser and are not uploaded to this site's servers or saved. Reloading the page returns inputs and results to their defaults.