Compound Interest Calculator with Regular Deposits and Yearly Table
Enter a starting amount, a yearly rate, a number of years and how often the interest is compounded to see the final balance and the interest earned.
Maintained by Hirantha Sandaruwan Lakshan. Updated 13 October 2026.
About this tool
Add a regular deposit, at the start or the end of each period, and get a table of the balance at the end of every year. The sums run in your browser and nothing is sent anywhere.
How it works
Compound interest means interest earns interest. With a nominal yearly rate r compounded m times a year, each period earns r / m. After n periods a lump sum P becomes P x (1 + r / m) to the power n. Rs. 100,000 at 12% compounded monthly earns 1% a month, so after 12 months it is 100,000 x 1.01 to the power 12 = Rs. 112,682.50, which is growth of 12.68% for the year.
A regular deposit D added every period grows too. Paid at the end of each period, the deposits are worth D x ((1 + i) to the power n - 1) / i, where i is the rate for one period. Paid at the start of each period, every deposit earns one more period of interest, so the total is that amount times (1 + i). At a rate of 0 the deposits just add up. The deposit is added every compounding period, so choose monthly compounding for a monthly deposit.
The effective annual rate shows what the yearly growth really is once compounding is counted: (1 + r / m) to the power m, minus 1. At a nominal 12%, annual compounding gives 12.00%, monthly gives 12.68% and daily gives 12.75%. More frequent compounding always helps a little, but the step from monthly to daily is small.
Interest is not rounded at each step, only in the results, so a bank that rounds every period can differ by a few cents. Tax, fees and inflation are not included.
The term can be any length. The whole compounding periods compound as above. If the term ends part way through a period, such as 18 months with annual compounding, the leftover part of the period earns simple interest at the same yearly rate, pro rata, on the balance at that point. That is how interest credited at the end of each period works. A regular deposit goes in on every period date inside the term: with deposits at the start of each period, one also goes in at the start of a final part period; with deposits at the end, none goes in for a part period that never finishes.
Worked examples
Rs. 100,000 at 12% compounded monthly for 1 year
The balance is Rs. 112,682.50, so the interest is Rs. 12,682.50 and the effective annual rate is 12.68%.
Rs. 10,000 at 5% compounded annually for 10 years
The balance is Rs. 16,288.95, which is Rs. 6,288.95 of interest.
Rs. 1,000 a month at 6% for 10 years, starting from nothing
With monthly compounding and each deposit at the end of the month, you reach Rs. 163,879.35: Rs. 120,000.00 of deposits and Rs. 43,879.35 of interest. If each deposit goes in at the start of the month, you reach Rs. 164,698.74.
Rs. 100,000 at 10% compounded daily for 1 year
The balance is Rs. 110,515.58 and the effective annual rate is 10.52%, a little above the 10% nominal rate.
Rs. 100,000 at 12% compounded annually for 18 months
After the first year the balance is Rs. 112,000.00. The other 6 months are half a period of simple interest: 112,000 x 12% x 0.5 = Rs. 6,720.00. The final balance is Rs. 118,720.00, which is Rs. 18,720.00 of interest.
What a 12% nominal rate really earns, by how often it is compounded
| Compounding | Times a year | Effective annual rate | Rs. 100,000 after 1 year |
|---|---|---|---|
| Annually | 1 | 12.00% | Rs. 112,000.00 |
| Semi-annually | 2 | 12.36% | Rs. 112,360.00 |
| Quarterly | 4 | 12.55% | Rs. 112,550.88 |
| Monthly | 12 | 12.68% | Rs. 112,682.50 |
| Daily | 365 | 12.75% | Rs. 112,747.46 |
Frequently asked questions
What is the compound interest formula?
A = P x (1 + r / n) to the power (n x t), where P is the starting amount, r the nominal yearly rate as a fraction, n the number of compounding periods a year and t the years. The interest is A minus P. With regular deposits, add the annuity term described above.
What is the difference between simple and compound interest?
Simple interest is always worked out on the original amount. Compound interest is worked out on the original amount plus the interest already added, so it grows faster the longer you leave it. The simple interest calculator on this site shows the first kind.
How do I add monthly deposits?
Choose monthly compounding and type the monthly amount in the regular deposit box. Pick whether each deposit goes in at the start or the end of the month. Deposits at the start earn one extra month of interest each.
Does compounding more often make a big difference?
Not a big one. At 12% a year, compounding monthly instead of annually adds 0.68 percentage points, and going from monthly to daily adds less than a tenth of a point more. The rate and the number of years matter far more.
How long does money take to double?
The rule of 72 says about 72 divided by the yearly rate in years, so about 6 years at 12%. The exact figure with annual compounding is 6.12 years. The rule is a quick estimate that gives a good indication for rates up to about 10%.
Can I use a term that is not a whole number of periods, like 18 months with annual compounding?
Yes. The whole periods compound as usual and the leftover part of a period earns simple interest at the same yearly rate. For Rs. 100,000 at 12% compounded annually for 18 months that gives Rs. 118,720.00. A formula that raises 1.12 to the power 1.5 gives Rs. 118,529.66 instead, because it lets interest build up between the dates when a bank would credit it. Your bank's rules decide which one applies to a real account.
Does this include tax on the interest?
No, the figures are before tax. In Sri Lanka, banks normally deduct 10% advance income tax from deposit interest unless you have given a self-declaration (we checked the Inland Revenue Department's circular on 13 October 2026; confirm with your bank). The fixed deposit calculator on this site has a tax box for that.
Sources & references
Updated Oct 13, 2026