This is the question that starts every pre-retiree’s planning, ignites every Financial Independence Retire Early (FIRE) aspirant’s passion, and begins every “some-day-I-might-stop-working-maybe” conversation. Finances also cause friction in couples, so finding this answer is a source of marital discord as well as being anxiety-inducing.
The easy answer – SWR
How do you answer this question? Simple. Pull out your basic calculator, enter your monthly expenses, multiply by 12 to get annual and then multiply again by 25. That’s your number. That’s the 4% rule. This exercise is called finding your “Safe Withdrawal Rate” or SWR. A SWR is the amount that you can withdraw reliably until your death without running out of money in the worst case scenario.
Incidentally, even the creator of the 4% rule, Bill Bengen, revised his number upward to 4.7% in 2020 — meaning the multiplier is closer to 21x, not 25x. But that’s a footnote on a flawed foundation.
If you’re reading this, you probably know the “easy answer” is overly simplistic to the point of being almost useless for many. It’s also probably a bit scary. After all, a monthly budget of $8,000 (US median expenses in your 50s) requires a retirement nest egg of $2.4 million. That’s only achieved by an estimated 2% of retirees. Gulp.
But why is the easy answer not helpful? Why does the 4% rule just… not make sense for most people?
The harder answer — “It’s complicated”
The more I dug into retirement planning, the more variables surfaced that the 4% rule simply doesn’t account for. Here are the ones that affect most people:
- Income offsets. Social Security, pensions, and part-time work after retirement all reduce the portfolio you actually need — sometimes dramatically. Two people with identical savings can have very different retirement outlooks depending on these.
- Spending isn’t flat. There’s something called the retirement spending smile — expenses start higher in the go-go years (travel, hobbies, bucket list), dip in the slow-go middle years, then rise again at end of life with healthcare and care costs. A flat 4% assumption misses all of that.
- Timeline uncertainty. The 4% rule was built on a 30-year horizon. Retire at 55 and you may need 35-40 years of runway. Retire at 67 and the math looks different. No one knows their end date — which is exactly why a probability-based approach beats a single fixed number.
- Taxes and healthcare. Withdrawals from a traditional 401(k) are taxable as ordinary income — your $2M isn’t $2M spendable. And if you retire before 65, you’re on your own for healthcare until Medicare kicks in. These two factors alone can shift your number by hundreds of thousands of dollars.
So yeah — (waves at all the bulleted items) — just factor all that in, m’kay? Good luck.
Banging your head against the wall — of math
At some point, if you’re like me, you try to run these numbers using whatever combination of free calculators, paid tools, and Excel spreadsheets you can find. It only gets you so far.
If you’re like me you get so spun up about it you give up. For a time. Then you come back — because you do need these answers, and after all you’re a sensible person who should be able to figure this out. So you work through a couple more variables. Then give up again. It’s just too mathy, right? Until recently, the tools were too generic and inflexible to model anything close to a customized plan.
Do we really need PhDs in finance to retire with any sense of peace about the numbers?
I hope not.
The good news is that DIY tools are advancing. We’re getting there. No tool is perfect — no one knows the future of market returns. But the more seriously we model the inputs, the more confidence we gain in the outputs.
Reframing the question
When you ask “how much do I need to retire,” you’re asking a question that can’t be answered with certainty — not without a crystal ball and perfect knowledge of every variable between now and the end of your life.
So can I propose a reframe?
What you’re actually looking for is confidence that you’re modeling your retirement correctly and that you have a high probability of success given your specific situation. The honest answer to “how much do I need to retire” is that it’s not a single number — it’s a probability. And you want that probability to be as high as you can make it. Many financial planners target the 75-80% range as adequate. Personally, I want to be higher than that.
That’s the question worth asking: what’s my probability of success?
Your probability of success
The fastest way to answer that question is to model your specific situation. The 25x rule — or 21x if you take Bengen’s revision seriously — gives you a rough starting point. A probability-based model gives you something you can actually plan around.
The DIY Retiree Retirement Probability Calculator lets you enter your actual savings, Social Security estimates, spending pattern, and timeline as inputs and returns your probability of success across 2,000 historical market scenarios. Use it to customize your plan and build confidence. Hopefully as you work through it, your answer to “what’s my probability of success?” gets closer and closer to 100%.
Tony Markey is an MBA and the founder of DIY Retiree. Read more about his story here