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計算方法

balance(next) = (balance - withdrawal) x (1 + return)
withdrawal(next) = withdrawal x (1 + inflation)

withdrawal

amount taken at the start of each year

return

assumed annual return, applied to what remains after the draw

inflation

annual increase applied to the withdrawal, if indexing is on

計算例

A $1,000,000 balance with $40,000 withdrawn each year, growing 5% and indexed to 3% inflation, reaches zero in year 34. Without inflation indexing the same inputs never deplete, because the 5% return exceeds the flat 4% draw.

よくある質問

What is the 4% rule?

A convention from retirement research in the 1990s, notably William Bengen 1994 study and the Trinity study that followed, which examined historical US market data and discussed withdrawing 4% of a starting balance, adjusted for inflation. It is a research finding about the past, not a rule that promises any particular outcome, and its assumptions are debated.

Why does inflation change the answer so much?

Indexing withdrawals to inflation means the amount you take grows every year while the balance is shrinking, so the two effects compound against each other. At 3% inflation a $40,000 withdrawal is about $54,000 after fifteen years, which pulls the depletion year materially forward.

Can this tell me whether I can retire?

No. It runs one fixed rate of return, which real markets never provide, and the order in which good and bad years arrive changes the outcome substantially even at the same average. Treat it as arithmetic on assumptions you chose, and consider speaking to a licensed financial adviser about your own circumstances.

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