The formula behind the curve
Compound interest means each period's interest is added to the balance, and the next period's interest is calculated on the larger number. The formula is A = P x (1 + r/n)^(n x t), where P is the starting amount, r the annual rate, n the number of compounding periods per year and t the years.
A deposit of 10,000 at 6 percent for 10 years compounded yearly grows to about 17,908. With simple interest, you would only reach 16,000, because the interest never earns interest of its own. The 1,908 difference is the compounding effect.
The rate you enter should be the annual rate. The calculator divides it by the number of periods for you, so do not convert it to monthly first. If your account states a monthly rate, multiply by 12 for the annual figure.
Does compounding frequency matter?
A little. The same 10,000 at 6 percent for 10 years ends at about 17,908 with yearly compounding, 18,194 with monthly and about 18,220 with daily. The jump from yearly to monthly is visible, but from monthly to daily it is small.
Time and rate matter far more. Check how a bank or fund states its rate: a nominal annual rate compounded monthly is lower than the effective annual rate it produces, and comparing the two directly would be misleading.
Loans work the same way in reverse. A credit card that compounds daily at 24 percent costs noticeably more than the headline rate suggests, which is why carrying a balance gets expensive so quickly.
The power of regular contributions
Adding money every month often beats a big lump sum, because each contribution starts compounding as soon as it goes in. Saving 200 per month for 20 years at 7 percent builds a balance of about 104,000, from 48,000 of your own money. The remaining 56,000 or so is growth.
The chart shows two lines: what you put in and what the balance becomes. The gap between them widens as the years pass, which is the main message of compounding. Starting early beats starting big.
Try changing only the monthly amount and watch the chart. Adding 50 per month often moves the final balance more than a full percentage point of extra return, and you control it completely.
Using the numbers sensibly
The calculator assumes a fixed rate for the whole period. Real investments move up and down, and savings account rates change. Inflation also reduces what the future amount can buy, so for long horizons try a lower rate to see a cautious scenario.
Fees, taxes and withdrawals are not included. Treat the output as an estimate for planning, not as a promise or financial advice.