Compound Interest Calculator

Compound interest with monthly contributions: see the balance grow year by year with a breakdown of principal and interest.

Initial principal

Monthly contribution

Annual rate (%)

Period (years)

Final balance	31,998
Total contributed (principal + contributions)	22,000
Total interest earned	9,998

Year	Balance	Contributions so far	Interest so far
1	11,740	11,200	540
2	13,568	12,400	1,168
3	15,490	13,600	1,890
4	17,510	14,800	2,710
5	19,634	16,000	3,634
6	21,867	17,200	4,667
7	24,213	18,400	5,813
8	26,680	19,600	7,080
9	29,273	20,800	8,473
10	31,998	22,000	9,998

Projection only: monthly compounding is assumed, contributions are added at the end of each month, and taxes, fees and inflation are not included. Results are not a guarantee of future returns.

How to use

Enter an initial principal, a monthly contribution, an annual rate and a period in years. The balance is projected with monthly compounding. Each month:

balance = balance × (1 + annual rate ÷ 100 ÷ 12) + monthly contribution. Interest for the month is applied first, then the contribution is added.

Example: a principal of 10,000 with 100 per month at 5% for 10 years grows to about 32,000 — 22,000 contributed and about 10,000 of interest.

Notes

Calculations use exact decimal arithmetic (compoundInterest from @yunedia/core-math, compounded monthly) and are rounded to whole units for display. This is a projection, not a guarantee: taxes, fees and inflation are not included, and it is not financial advice. All calculations run in your browser.

FAQ

Are contributions added at the start or the end of the month?

At the end. Each month, interest is applied first and the contribution is added afterwards, so results are slightly lower than a start-of-month assumption.

Can I calculate with yearly compounding instead?

This tool always compounds monthly. Treat the result as a monthly-compounding projection; it will not exactly match a yearly-compounding calculation.