a little game about the future! type in your numbers and find out the age you could stop working. it all runs right here in your browser: nothing you type is saved or sent anywhere. 
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just for fun, not financial advice! real life has taxes, fees, and surprises (and no extra lives).
how this works
today's dollars: every number is in today's money, so $50,000 means "what $50,000 buys right now." that way you don't have to guess at prices in 2056. the "future dollars" number just shows how big that will look on a price tag later, using the inflation setting.
real returns: the investment styles are growth after inflation (about 3%, 4.5% or 5.5% a year). that's why the math can stay in today's dollars.
how the age is found: the game plays out your life one year at a time. while you work, your savings grow and you add to them. once you retire, they still grow, but you take out your spending (minus social security once it starts). your retirement age is the earliest age where the money never runs out before your plan-to age.
the luck range: markets have good and bad stretches, so the game runs again with 2% less growth a year (bad luck) and 2% more (good luck). the range shows how much that can move your answer.
the rule of thumb: a common guess is that you can take out about 4% of your savings a year and have it last. it's a quick double-check, not a promise.
what's left out: taxes (that's why your spending should include them), fees, health surprises, and everything else life throws at you.