What a retirement calculator is trying to answer
Most retirement planning starts with a simple question: how much will I have in retirement? A retirement calculator answers that by projecting the future value of today’s savings plus regular contributions, using an assumed annual rate of return over the years remaining until retirement age.
A second question follows immediately: how much income could that balance support? This is where a retirement income calculator or FIRE calculator often introduces a withdrawal rule so the projected pot can be translated into annual or monthly spending power.