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Model a starting deposit and regular contributions using an entered rate assumption. See final balance, contributions, interest, and a yearly timeline.
| Year | Contributions to date | Interest to date | Balance |
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Compound interest differs from simple interest because each period's growth can itself grow later. With starting principal P, annual rate r, compounding periods per year n, and time t years, the principal-only standard formula is A = P(1 + r/n)^(nt). For an end-of-period contribution PMT made every compounding period, the contribution term is PMT[((1 + r/n)^(nt) − 1)/(r/n)]. Beginning-of-period contributions multiply that contribution term by (1 + r/n). The Investor.gov compound-interest calculator explains the standard variables and calculation.
This page keeps contribution frequency separate from compounding frequency. When they match, the period schedule is direct. When they differ, the calculator simulates the timeline period by period, adding contributions at the selected boundary and applying the chosen compounding factor. A zero rate is handled as principal plus contributions, avoiding division by zero. Interest is final balance minus starting deposit minus all contributions.
Enter $1,000 initial deposit, $100 monthly contribution, a 6% annual rate assumption, monthly compounding, end-of-month timing, and 2 years. The standard schedule gives a final balance of approximately $3,670.36. Total contributions are $3,400 ($1,000 starting deposit plus $100 × 24), and estimated interest is approximately $270.36. Display values are rounded to cents, while the calculation retains more precision. A second simple check is $10,000 at 5% compounded yearly for 10 years with no contributions: the result is approximately $16,288.95. Switch the same inputs to yearly compounding and the balance is about $3,662.86: slightly less, because each monthly deposit earns only for the months it has actually been in the account, at the monthly rate equivalent to 6% a year compounded once, (1.06)1/12 − 1.
Do not use this calculator as a guaranteed investment forecast, a retirement recommendation, a tax calculation, or a promise of what a bank or market will pay. It omits inflation, taxes, fees, contribution limits, withdrawals, defaults, changing rates, volatility, and sequence of returns. For a loan, use an amortization calculator; for a business price, use a margin or break-even calculator. For a real plan, compare the assumptions with official product documents and a qualified professional.
The currency symbol is cosmetic and no data leaves the browser. A negative contribution is allowed as a withdrawal-like scenario, but the calculator reports the arithmetic and does not silently invent an overdraft policy. The rate is an assumption, not a promised return. This is an educational estimate, not financial, tax, legal or investment advice. Not financial advice — arithmetic only.
Compound interest is interest calculated on the starting balance and on interest already added. This calculator also models regular contributions under your chosen timing assumption.
No. The annual rate is an assumption for arithmetic. Inflation, taxes, fees, volatility, deposits, and withdrawals can make actual results different.
End-of-period contributions are added after that period's growth. Beginning-of-period contributions are added before growth, so they receive an extra period of compounding.
Yes. Select the contribution frequency separately from compounding frequency. The calculator simulates the timeline rather than forcing a closed-form shortcut.
The standard future-value formula and definitions are explained by the SEC Investor.gov compound-interest calculator. This page adapts it for the entered contribution schedule.
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