Compounding needs both a rate and a frequency
A quoted annual number is incomplete until you know what it means. This tool treats the entered rate as nominal annual interest divided into twelve equal monthly rates. It does not accept an effective annual yield as though the two definitions were interchangeable.
For example, a nominal 12% rate means 1% per month in this model. Twelve monthly growth factors produce an effective annual increase of (1.01^12 − 1), about 12.68%. The compound interest calculator shows this derived rate in its details so the assumption can be checked.
Separate saving from growth
The balance includes money you supply and growth attributed to the assumed rate. With 1,000 initially and 100 a month for two months, contributions total 1,200. At a nominal 12% rate with end-of-month payments, the projected balance is 1,221.10; only 21.10 is modeled growth.
A large ending balance is therefore not evidence that the same amount was earned as interest. Keep the contribution total beside the ending value when comparing scenarios.
Contribution timing changes the result
A beginning-of-month contribution is available for one more compounding period than the corresponding end-of-month contribution. In the two-month example, moving contributions to the beginning raises the balance to 1,223.11.
Use the timing that matches the situation you intend to model. The selector does not reproduce exact transaction dates, irregular deposits or daily accrual. Real account statements may round balances or interest differently.
Treat the projection as a scenario
A fixed-rate curve is useful for understanding sensitivity to contribution size, time and rate. It is not a forecast of market returns. Fees, taxation, inflation and variable performance can change the eventual purchasing power or cash available.
Use the percentage calculator for simple relative comparisons. For a loan, the loan payment calculator models payments that reduce a balance instead of recurring deposits that build it.