Compound interest calculator with monthly contributions
See how a starting balance and regular savings could grow. Compare contributions and interest, choose APY or nominal interest explicitly, and keep every assumption visible. All rates are hypothetical; no account is needed.
Your saving scenario
An editable example, not a return forecast. Amounts stay in this page and are not saved.
Correct the marked entry to calculate. Previous results are hidden.
Projected balance
Contributed capital
Modeled growth
Equivalent rates
Purchasing-power value
Scenario comparison
Yearly breakdown
| Years elapsed | Regular deposits | Contributed capital | Modeled growth | Projected balance |
|---|
Contributions and growth over time
Solid line: projected balance. Dashed line: contributed capital. A balance below contributed capital means a modeled loss. All values are also in the yearly table.
Hypothetical constant growth, before fees and taxes. Contributions use equal annual intervals, not a bank calendar. No investment recommendation or guaranteed return.
Try changing only the regular contribution or assumed rate in “Compare another scenario”. The baseline remains visible so you can see what caused the difference.
How it works
Separate your contributions from modeled growth
Enter a starting balance and an amount per contribution interval. The projected balance adds the grown starting capital to every contribution grown from its modeled payment time. Capital is shown separately from growth, including negative growth. The yearly table includes deposit counts and a final partial year, so the total is traceable to the assumptions.
Make the annual rate and payment convention explicit
US annual percentage yield (APY) already includes compounding. Enter it as an effective annual rate. For a nominal annual interest rate, choose its stated compounding frequency instead. This investment calculator models constant growth only: it does not select securities or forecast returns. End deposits are counted only at completed intervals; beginning deposits at their start. A zero horizon counts no regular deposits. Optional inflation changes the purchasing-power figure only, not the nominal balance. Compare one changed rate or contribution while preserving these conventions.
Common questions
Why do two calculators give different totals?
Check the rate basis, compounding frequency, deposit timing and counted payments. A 5% nominal rate compounded monthly is not 5% effective annual growth. Partial periods and intermediate rounding can also differ. This calculator keeps full internal precision and displays the actual payment count and equivalent rate.
Are contributions made at the beginning or end?
You choose the timing; the default is the end of each interval. Beginning deposits grow for longer. With a one-month horizon and quarterly payments, end timing counts zero deposits and beginning timing counts one. At zero duration neither mode counts a recurring deposit.
Is this a guaranteed return?
No. This is constant-rate hypothetical mathematics before fees and taxes. Real investment returns vary and can be negative. No product is recommended. Assumed inflation is also a scenario input, not a forecast. Editable example rates are not current achievable returns or financial advice.
Practical guides
- Nominal vs effective annual interest: compare like with like
Understand why the same displayed annual percentage can produce different balances, with independently checked contribution examples.
- Beginning vs end contributions and partial periods
See when deposits enter the calculation, how many are counted, and why timing changes a result without changing contributed capital.
Scope and limitations
Constant hypothetical growth before fees and taxes; no financial product selection, guaranteed return, tax or account eligibility calculation. Equal annual intervals do not reproduce bank dates, business-day accrual or account-specific terms. Entered amounts stay in this page’s memory and are not saved.