How the sums work
I take your take-home income and split it by the savings rate. What you save goes into the pot each year; what you spend sets the target, using the common 25x rule: a pot of 25 times your annual spending can plausibly fund that spending indefinitely. Then I grow the pot from zero at your real return, adding a year's savings each time, and count the years until it crosses the target.
The reason the rate dominates is that it controls both sides. Save more and the pot grows faster, but you're also funding a smaller lifestyle, so the target itself drops. A better return only helps one side, and it's the side you control least.
Worked example
With the defaults, on $60,000 take-home, a 30% savings rate means saving $18,000 and spending $42,000, a $1,050,000 target, reached from zero in 28 years at 5% real returns; at 10% it takes 52 years, at 50% it takes 17, and at 64% just 11, which is why the savings rate matters more than the return.
Frequently asked questions
Why does the savings rate beat the return?
The savings rate works on both sides of the sum at once. Every extra percent saved grows the pot faster and shrinks the lifestyle the pot has to fund, so the 25x target falls at the same time as your savings rise to meet it. A better return only helps one side.
Does this include my existing savings?
No, this version starts from zero so the rates compare fairly. If you already have a pot, treat the result as an upper bound and knock a few years off mentally, or use the coast FIRE calculator, which starts from your current balance.
Should I use gross or net income?
Net. Use your take-home pay after tax, because that is the money you actually split between saving and spending. Using gross income would understate your true savings rate and overstate your spending.