How the sums work
I convert your annual return to a monthly rate:
then run the pot forward month by month: apply a month's growth, take the withdrawal, repeat until the pot hits zero. I also work out the withdrawal your pot could sustain forever, which is simply the pot times the monthly rate. Withdraw only the growth and the balance never falls.
One honest caveat. A flat average return flatters reality, because real markets deliver returns in a lumpy order and bad early years while you're withdrawing do lasting damage. That's sequence-of-returns risk, and it means you should treat these results as optimistic rather than a promise.
Worked example
With the defaults, a $500,000 pot with $3,000 a month withdrawn and 5% growth lasts 280 months, which is 23 years and 4 months; the withdrawal the pot could sustain forever at 5% is about $2,037 a month.
Frequently asked questions
What is sequence-of-returns risk?
It's the risk that bad market years land early in retirement, while you're withdrawing. Selling into a downturn locks in losses the later recovery can't repair, so two retirees with the same average return can end up in very different places. A flat average, like the one used here, hides that entirely.
Why doesn't this adjust for inflation?
To keep the sums honest in one step. Enter a real return, something like 4 to 5% instead of a nominal 7 to 8%, and every figure comes out in today's money, including the withdrawals. If you enter a nominal return instead, remember the later withdrawals will buy less than they do now.
How does the 4% rule relate to this?
The 4% rule takes the same question from the other end: it starts from a withdrawal rate historically likely to survive 30 years and tells you the pot you'd need. This tool starts from your pot and withdrawal and tells you the years. The retirement calculator covers the pot-building side.