I’m a little surprised no one has mentioned FIRECalc It was created to answer the question “With what you have today, and what it costs you to live, can you retire and maintain the same lifestyle?”
At its simplest it’s just another generic retirement asset calculator. Its intended use is to take advantage of the many details of holding a portfolio over time. The home page explains it as follows.
“Without any other information, FIRECalc will assume you want to keep your annual spending about the same for as many years as you specify, you aren’t planning on receiving any Social Security or pension, and your retirement portfolio is invested in a “couch potato” portfolio of 75% stock index and 25% bond funds, with a 0.18% fee to the fund. But you can change any or all of those assumptions. You can refine the spending and investment assumptions, explore the impact of some of your decisions, factor in all sorts of exceptions, and/or let FIRECalc search for savings or spending levels that will let you meet your goals. Just click on any of the tabs at the top to enter the optional information.”
“Each tab will take you to a description as well as the place to enter the optional information, but in short: Start Here is this page, where you enter spending, portfolio balance, and years. Other Income/Spending is where you enter Social Security, pensions, and other yearly adjustments. Not Retired? is where you specify when the plan is to start, and how much you’ll add to your portfolio until then. Spending Models lets you select from several approaches to spending during retirement. Your Portfolio allows selection of different investments. Portfolio Changes lets you make lump sum changes to your portfolio at some future year. Investigate allows you to investigate the consequences of some of the other choices you can make in planning your retirement.”
There’s an active support forum.