Reference formulas and worked examples below keep their stated units. Monetary examples use USD; dimensional examples use US customary units unless labeled otherwise. Metric calculator inputs are converted to these reference units before calculation. Choosing another currency does not convert example amounts or exchange money.
How to use this compound interest calculator
Enter a starting balance, contribution amount and duration. Select nominal-rate compounding or an effective annual return, then set deposit frequency and timing.
How the calculation works
For nominal rate r with n compounds per year, annual growth factor G = (1 + r/n)^n. For continuous compounding, G = exp(r). An effective annual input instead uses G = 1 + r.
Money grows by G raised to elapsed years. A deposit at time t contributes deposit × G^(ending time − t). Contributions and compounding use separate frequencies. Fractional compounding periods are modeled by this exponential growth interpolation.
End deposits occur after each full deposit interval through the end date. Start deposits occur at the beginning of each interval strictly before the end. A partial final interval therefore can contain a start deposit but no end deposit.
The yearly table lists modeled balances, actual deposits included by each date, and growth above those contributions. No monthly cent rounding, fees, taxes or inflation are added.
Worked example
With $1,000, no deposits and a 12% nominal rate, annual compounding gives $1,120 after a year; monthly gives about $1,126.83. At 0% return, $1,000 plus twelve $100 deposits gives $2,200. With a half-month horizon, beginning monthly deposits include one payment while ending deposits include none.
Limits and assumptions
Constant hypothetical return, not a guarantee. Negative growth is supported, but no random market path is simulated. Fractional periods use exponential interpolation and days/weeks are regular fractions of a year, not bank day-count rules. Taxes, fees, withdrawals and inflation are excluded.
Frequently asked questions
Can I compound daily but deposit monthly?
Yes. The two frequencies are independent. The model uses the effective annual growth implied by daily compounding and applies it over each deposit’s time invested.
What if I enter APY?
Choose Effective annual return / APY. It already includes compounding effects; the nominal compounding selector is then excluded.
Do deposits change when I change their frequency?
The per-period amount stays the same. Review it for the new frequency: $100 per week contributes more than $100 per month.
Are start and end deposits the same in a partial period?
No. A start deposit can occur before a partial ending interval finishes; an end deposit is counted only after a full interval.