Estimate the future value of savings with this free compound interest calculator. Enter a starting balance, nominal annual interest rate, time horizon, and optional monthly deposits. Choose annual, semiannual, quarterly, monthly, or daily interest compounding, then calculate a year-by-year projection. The result separates the money you put in from interest earned, so you can distinguish growth from your own savings contributions. The what-if comparison shows the effect of increasing the monthly deposit.
## How compound interest works. Interest is earned on your initial balance and on interest already credited in earlier periods. A single $10,000 deposit growing at a hypothetical fixed nominal rate of 7% compounded monthly for 20 years becomes approximately $40,387.39. The calculation is $10,000 × (1 + 0.07/12)^(12 × 20). The annual percentage rate, compounding frequency and time period must use matching units; the 7% is a nominal annual rate, not an annual percentage yield.
## Monthly contributions change the equation. When you also deposit $100 at the end of every month, the same 20-year scenario ends with approximately $92,480.05. You deposited $10,000 at the beginning and another $24,000 across 240 months, for a total $34,000 of your own money. The remaining approximately $58,480.05 is projected interest. These are hypothetical amounts based on a constant rate; actual investment performance or bank interest can vary, and taxes and fees reduce the money you keep.
## Why compounding frequency matters. At a given positive nominal annual rate, compounding interest more often produces a somewhat higher annual effective yield. For example, at 7% nominal, monthly compounding implies an effective annual rate of (1 + 0.07/12)^12 − 1, approximately 7.23%. The calculator supports five frequencies. To keep end-of-month contributions consistent across all options, it converts the selected compounding rate into an equivalent effective monthly rate before modeling each deposit. That is an annual-equivalent projection and need not match the exact crediting dates of an individual bank account.
This calculator is most useful when making decisions where time horizon matters — choosing between paying off a low-rate debt versus investing the money, deciding how many years to extend an investment before withdrawing, or comparing two savings accounts with different compounding frequencies. It is also the right tool for visualising the cost of starting late: run the same monthly contribution starting at age 25 versus 35 and see how significant a 10-year delay really is.
The most pervasive mistake is overweighting compounding frequency relative to rate and time. Daily compounding on a 3% account does not remotely approach monthly compounding on a 4% account — rate dominates frequency in every realistic scenario. A second error is ignoring inflation when projecting long-term savings: $100,000 in 30 years is not $100,000 in today's purchasing power. Use the real return rate (nominal rate minus inflation) for meaningful long-term projections.
Someone invests $500/month from age 30 to 65 at a 6% annual return. Total contributions: $210,000. Final balance: approximately $591,000 — nearly $381,000 is interest earned on contributions alone. Now delay the start by 10 years: same $500/month from age 40, same rate. Total contributions: $150,000. Final balance: approximately $309,000. A $60,000 difference in contributions produces a $282,000 difference in outcome — that is the cost of waiting, and it is invisible without a compound interest calculator.
Compound interest with an end-of-month deposit
i = (1 + r/f)^(f/12) − 1
A = P(1+i)^n + M × ((1+i)^n − 1)/i (i ≠ 0)
A = P + M × n (i = 0)
P = starting amount; M = amount added at each month-end; r = nominal annual rate as a decimal; f = interest compounding periods per year; n = years × 12, required to be a whole number of months. The formula is an equivalent-monthly-rate model, not a promise of future interest.
Invest $10,000 today at a constant hypothetical 7% nominal interest, compounded monthly, for 20 years with no extra deposits.
Result: $10,000 becomes approximately $40,387.39 before taxes and fees, assuming a constant 7% nominal rate compounded monthly.
For a one-time deposit, use P(1+r/f)^(ft), where P is principal, r is the nominal annual rate, f is compounding frequency and t is years. If you also add money monthly, include the future value of those contributions. The calculator handles both.
With a positive nominal annual rate and the same initial deposit, daily compounding typically creates a slightly larger effective annual yield. This calculator converts each selected frequency into an equivalent monthly growth rate while keeping monthly deposits at month-end.
Yes. Enter a nonnegative monthly contribution. It is added at the end of each month after that month's modeled interest is calculated.
At a constant hypothetical 7% nominal annual rate compounded monthly, without additional deposits, $10,000 grows to approximately $40,387.39. Actual returns, taxes, fees and inflation may change your result.
The ending balance includes your starting amount, all additional deposits and interest accumulated over the term. For the default $100/month illustration, $34,000 is deposited and approximately $58,480.05 is modeled as interest.
No interest is earned; the ending amount is the initial balance plus all monthly contributions. The calculator explicitly handles this case without division by zero.
No. It is a nominal projection under a constant assumed rate. Account fees, tax, inflation and investment losses are not included.
They are added at the end of each month. Depositing earlier would cause the contributions to accrue more interest over the same term.