Required minimum distributions (RMDs) are unusual among tax obligations in one specific way: you can calculate, decades in advance, almost exactly when they'll start and roughly how large they'll be — assuming reasonable account growth. That predictability makes RMDs one of the more plannable tax events in retirement, yet a lot of retirees still get surprised by the size of the bill when it finally arrives.
When RMDs start, and why the age depends on your birth year
Under the SECURE 2.0 Act, RMDs begin at age 73 for anyone born 1951 through 1959, and at age 75 for anyone born 1960 or later — a five-year-wide birth window (1951-1959) all sharing the same starting age, then a clean jump to 75 for everyone born after. Your very first RMD gets a grace period: it can be delayed until April 1 of the year AFTER you reach the applicable age, but doing so means you'll owe TWO RMDs in that following calendar year (the delayed one plus that year's own), which can push you into a higher tax bracket for that single year — a detail worth planning around rather than defaulting into.
How the size of the bill is calculated
Your RMD amount is your account balance as of December 31 of the PRIOR year, divided by a "distribution period" pulled from the IRS Uniform Lifetime Table — a number that gets smaller every year as you age. A 75-year-old's distribution period might be around 24.6, while an 85-year-old's might be around 16.0. Because the divisor shrinks over time, the REQUIRED PERCENTAGE of your balance you must withdraw increases every single year, even if your account balance never changes — a $500,000 balance at a distribution period of 24.6 requires about $20,325 (roughly 4.1%); the same $500,000 balance at a distribution period of 16.0 would require about $31,250 (roughly 6.25%).
Why it's worth planning for decades in advance
Unlike most tax events, the RMD start age and the account balance driving the calculation are both knowable well ahead of time. That predictability is exactly what makes strategies like Roth conversions attractive in the years before RMDs begin: converting Traditional balances to a Roth account in a lower-income year (say, between retirement and the RMD start age) means paying tax on that converted amount NOW, at a potentially lower rate than you'd face on a forced RMD withdrawal later — and once converted, that money is never subject to RMDs (or future income tax) again.
Common mistakes
Ignoring RMD planning until the year they actually start is the most common missed opportunity — the years between retirement and RMD age are often the best window for strategies like Roth conversions, precisely because income (and tax rate) tends to be lower then than once RMDs and Social Security are both flowing. The second is delaying the very first RMD to the following April without realizing it creates a double-RMD tax year. The third is forgetting the steep 25% penalty for a missed or shortfall RMD — set up automatic distributions with your account custodian well before your RMD age arrives, rather than relying on remembering to do it manually each December.