How Much Do You Need to Retire? The 4% Rule, Explained

“How much do I need to retire?” is one of those questions everyone eventually asks and almost nobody answers with real numbers. The honest answer depends on your current savings, how much you add each month, the return you earn along the way, and how many years you have left before you stop working. Once you have those four inputs, there’s a standard way to turn them into both a projected balance and a rough monthly income figure, and it’s simpler than it looks.

How compounding grows your contributions

Every dollar you save today has more time to grow than a dollar you save next year. That’s the entire idea behind compounding: your existing balance earns a return, that return gets added to the balance, and the next period’s return is calculated on the larger total. Do this every month for a decade or three, and the gap between “money you put in” and “money you end up with” gets surprisingly large.

Retirement projections almost always compound monthly rather than annually, because that matches how most people actually save: a fixed contribution comes out of each paycheck or bank account every month, not once a year. To do that math correctly, an annual return rate has to be converted into a monthly one first, and the number of years has to become a number of months.

The 4% rule: turning a balance into income

A projected balance on its own doesn’t tell you much until you translate it into spending money. That’s where the “4% rule” comes in. It’s a rule of thumb, originally based on historical US market returns, suggesting that withdrawing about 4% of a portfolio’s value in the first year of retirement, and adjusting for inflation after that, has historically had a good chance of lasting roughly 30 years without running out.

It’s a starting point for a conversation, not a law of physics. But it’s a useful, widely cited way to translate a lump sum into “roughly how much could this pay me per month,” which is usually the number people actually care about.

The formula

Monthly Rate = Annual Return Rate ÷ 12
Months = Years to Retirement × 12

Projected Balance = Current Savings × (1 + Monthly Rate)^Months
                   + Monthly Contribution × [((1 + Monthly Rate)^Months − 1) ÷ Monthly Rate]

Estimated Monthly Income = Projected Balance × 4% ÷ 12

The first line inside “Projected Balance” is your current savings compounding on its own. The second line is the growth of your ongoing monthly contributions, each one compounding for a different number of remaining months depending on when it was made. Add them together and you get the total projected balance at retirement. Multiply that balance by 4% for an annual withdrawal amount, then divide by 12 for a monthly figure.

Worked example: starting with $30,000

Say you have $30,000 saved today, add $500 a month, expect a 7% annual return, and plan to retire in 30 years.

• Monthly rate: 7% ÷ 12, and months: 30 × 12 = 360
• Projected balance at retirement: $853,480.42
• Estimated monthly income (4% rule): $2,844.93

Notice how much of that balance is growth rather than deposits: $30,000 plus $500 a month for 360 months is $210,000 contributed out of pocket. The rest, over $640,000, comes purely from investment returns compounding over three decades. That’s the practical argument for starting early: time in the market does more of the work than the size of any single contribution.

Worked example: starting with $100,000

Now say you start with more but have less time: $100,000 saved, $1,000 a month, a 6% annual return, and 20 years until retirement.

• Monthly rate: 6% ÷ 12, and months: 20 × 12 = 240
• Projected balance at retirement: $793,061.34
• Estimated monthly income (4% rule): $2,643.54

Even with a larger starting balance and a bigger monthly contribution, a shorter time horizon and a lower return produce a somewhat smaller ending balance than the first example. Years in the market and the rate of return matter as much as, sometimes more than, how much you’re able to save each month.

What the 4% rule doesn’t account for

Two caveats are worth keeping in mind before you treat any projection like this as a promise.

First, the 4% rule isn’t guaranteed. It’s a historical observation about past US market performance, not a rate that’s mathematically certain to hold up in the future. Sequence of returns (whether the market happens to drop early in your retirement) matters a lot, and some financial planners now recommend a more conservative 3% to 3.5% withdrawal rate instead, especially for longer retirements.

Second, this kind of projection models nominal growth, meaning it doesn’t subtract inflation. A dollar 30 years from now won’t buy what a dollar buys today, even if the balance on paper looks large. If you want a more conservative, inflation-adjusted estimate, use a “real” return rate instead of a nominal one: subtract your expected inflation rate from your expected annual return before running the numbers. Doing that will lower the projected balance, but it’ll better reflect actual future purchasing power.

None of this means the math is wrong or not worth doing. It means a projected balance and estimated income are a planning tool, a way to sanity-check whether you’re on track, not a guarantee of what your bank balance will read on your retirement date.

FAQ

Is the 4% rule guaranteed to work? No. It’s a historical rule of thumb based on past US market returns, not a guarantee. Actual safe withdrawal rates depend on market performance during your specific retirement years, how your money is invested, and how long your retirement lasts. Some retirees use a more conservative 3% to 3.5% instead.

Why does the calculator compound monthly instead of yearly? Because most people contribute monthly, not annually. Compounding monthly matches how the money actually gets deposited and grows, and it produces a more accurate projected balance than assuming one lump contribution per year.

Does this projection account for inflation? No, it projects nominal dollar growth. If you want to account for inflation eroding purchasing power over time, use a lower “real” return rate (your expected return minus expected inflation) instead of the nominal rate you’d otherwise enter.

Use the Retirement Calculator to run your own numbers.