Solve for present value, future value, rate, term, or payment — the same five-key time-value-of-money tool used by financial calculators like the BA II Plus or HP 12C.
Every compound-interest situation — a loan, a savings account, a bond, a retirement account — is really the same equation wearing a different outfit. It relates five quantities: how much you start with (PV), what you add or withdraw each period (PMT), how many periods pass (N), the interest rate (I/Y), and what you end up with (FV). Know any four, and the fifth is fully determined — that's the whole idea behind this calculator's five tabs.
Cash flowing toward you is positive; cash flowing away from you is negative. A savings deposit is a positive PV (money you have) with negative PMTs (money you keep adding) building toward a positive FV (money you'll have later). A loan flips this: positive PV (money the bank gives you) with positive PMTs you pay back, heading toward zero. Getting the signs right is the difference between a sensible answer and a confusing one.
That's the sign convention at work, not an error — a negative result usually just means that value flows in the opposite direction from the ones you entered as positive.
It changes how many periods each payment has to earn interest. A payment made at the start of a period earns interest for that whole period; one made at the end doesn't start earning until the next period begins — which is why annuities-due (beginning-of-period) always produce a slightly larger future value than ordinary annuities (end-of-period), all else equal.
Solving for N answers "how long until I reach my goal?" Solving for I/Y answers "what rate of return am I actually getting?" — both common questions once you already know your starting amount, contributions, and target.
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