The future value of an annuity formula is used to calculate what the value at a future date would be for a series of periodic payments.The future value of an annuity formula assumes that1. The rate does not change2. The first payment is one period away3. The periodic payment does not changeIf the rate or periodic payment does change, then the sum of the future value of each individual cash flow would need to be calculated to determine the future value of the annuity. If the first cash flow, or payment, is made immediately, the future value of annuity due formula would be used.