Future Value Annuity Calculator: Your Money's Best Friend

What an annuity's future value means

An annuity here just means a series of equal payments made at regular intervals — $500 every month is the classic case. "Future value" is what that whole stream of deposits, plus the interest each deposit earns along the way, adds up to by the end. Unlike a lump-sum compound interest calculation, every payment compounds for a different length of time: the first monthly deposit earns interest for nearly the full term, while the last deposit earns almost none.

Walking through the built-in example

Depositing $500 every month for 10 years at a 5% nominal annual rate, compounded monthly, produces a future value of $77,641.14. Here's how the calculator gets there. First it converts the nominal 5% rate into an effective annual rate using the compounding frequency: (1 + 0.05/12)^12 - 1 ≈ 5.116%. Because deposits and compounding are both monthly here, the rate per payment period works out to essentially the simple monthly rate, 0.05/12 ≈ 0.4167%. Over 120 monthly payments (10 years × 12), the standard future-value-of-an-ordinary-annuity formula, payment × [((1 + i)^n - 1) / i], gives 500 × [((1.004167)^120 - 1) / 0.004167] ≈ 500 × 155.28 ≈ $77,641.14. Total contributions are just 500 × 120 = $60,000, so the remaining $17,641.14 is interest earned purely from compounding.

Why payment and compounding frequency can differ

The calculator lets you set payment frequency and compounding frequency independently — for example, monthly deposits into an account that only compounds quarterly. When they differ, it first converts the nominal rate to an effective annual rate based on the compounding frequency, then converts that effective annual rate back down to a rate per payment period. This two-step conversion is what keeps the math correct even when deposits and compounding don't land on the same schedule.

Ordinary annuity vs. annuity due

"End of period" (ordinary annuity) assumes each deposit is made after that period's interest would have accrued — the standard assumption for things like 401(k) contributions taken from a paycheck. "Beginning of period" (annuity due) assumes the deposit happens first and then earns interest for that period too, which the calculator handles by multiplying the ordinary-annuity result by (1 + rate per period) — a small but real difference that grows with more payments and higher rates.