Annuity Formula. Annuities are used in retirement accounts, where the goal is to make a starting balance pay a fixed annual amount over a given number of years.. See How Finance Works for the annuity formula. Free annuity calculator to forecast the growth of an annuity with optional annual or monthly additions using either annuity due or immediate annuity. Experiment with other retirement planning calculators, or explore hundreds of individual calculators addressing other topics such as math, fitness, health, and many more. Future Value Calculator. The future value calculator can be used to calculate the future value (FV) of an investment with given inputs of compounding periods (N), interest/yield rate (I/Y), starting amount, and periodic deposit/annuity payment per period (PMT).