Retirement Income From Index Funds: The 3-Step Math

Michael Chen

Michael Chen writes about personal finance fundamentals. Bay Area-based · finance enthusiast for 15 years.

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Quick answer: Project your index fund balance at retirement with a cautious real return (3% to 4%), multiply it by a withdrawal rate of 3% to 4%, and add Social Security. $455,500 at 3.5% gives about $1,329 a month before taxes. Treat it as an estimate and redo it every year.↗ Share on X

To estimate retirement income from index funds, do three steps: project what your savings could grow to by retirement using a cautious return assumption, multiply that total by a withdrawal rate (many planners use 3% to 4%), and add Social Security or any pension. For example, $400,000 in index funds times 4% gives about $16,000 a year, or roughly $1,333 a month, before taxes. That number is an estimate, not a promise, because markets move up and down.

Below is the full method, with worked numbers you can copy into a spreadsheet.

What numbers do you need before you start?

READ ALSOTraditional vs Roth IRA: How Beginners Can Pick the Right One →How to Read a Fund’s Prospectus Without Falling Asleep →Fees Beginners Must Watch When Investing in Index Funds →

Gather five numbers. You can find most of them in 15 minutes.

1. Current balance in your index funds (401(k), IRA, brokerage account).

2. Yearly contribution you add now, including any employer match.

3. Years until retirement. If you are 40 and plan to stop at 67, that is 27 years.

4. Expected return after inflation. This is the hardest one. We cover it next.

5. Other income in retirement: your Social Security estimate (free at ssa.gov with a my Social Security account), a pension, or rent from a property.

Write them down. Everything else is simple arithmetic.

What return should you assume for index funds?

Nobody knows future returns. What you can do is pick a cautious number and test a few others.

Use a real return. "Real" means the return after inflation. If your money grows 7% in a year but prices rise 3%, your real gain is about 4%. Real returns let you think in today's dollars, so $1,000 in your answer buys roughly what $1,000 buys now. That makes the result much easier to understand.

A practical way to set it:

These are planning assumptions, not forecasts. Stock-heavy portfolios have historically grown more over long periods, but they also drop harder in bad years. If your plan only works at the optimistic number, it is not a safe plan yet.

How do you project your balance at retirement?

READ ALSOMonthly Investment for a Comfortable Retirement →Crafting Sustainable Retirement Income with Index Funds Wisely →How to Open a Roth IRA in 5 Simple Steps →

You need two pieces: growth on what you already have, and growth on what you will add.

Piece 1 — growth of your current balance:

Future value = current balance × (1 + r)^years

Piece 2 — growth of your future contributions:

Future value = yearly contribution × [((1 + r)^years − 1) ÷ r]

Here "r" is your yearly real return written as a decimal (4% = 0.04). This version treats contributions as one deposit per year, which is close enough for planning.

A worked example

Maria is 40. She has $60,000 in a total market index fund. She adds $500 a month ($6,000 a year), including her employer match. She plans to retire at 67, so she has 27 years. She uses a 4% real return.

• Growth factor: 1.04^27 ≈ 2.88
• Piece 1: $60,000 × 2.88 ≈ $173,000
• Piece 2: $6,000 × [(2.88 − 1) ÷ 0.04] ≈ $6,000 × 47.1 ≈ $282,500
• Total at 67: about $455,500 in today's dollars

You don't have to do exponents by hand. In Google Sheets or Excel, type:

`=FV(0.04, 27, -6000, -60000)`

It returns about 455,500. The minus signs are just how the spreadsheet marks money you put in.

How do you turn that balance into yearly income?

This is where the withdrawal rate comes in. The withdrawal rate is the percent of your starting balance you take out in your first year of retirement. After that, you raise the dollar amount each year to keep up with inflation.

A common planning guideline is the "4% rule": take 4% in year one, then adjust for inflation. Many planners suggest starting lower, between 3% and 4%, if you retire early, expect a long retirement, or want more room for bad markets.

Maria's numbers at a 4% real return:

Now add Social Security. Say her statement shows about $1,800 a month at 67. Her total estimate at a 3.5% withdrawal rate: $1,329 + $1,800 = about $3,129 a month, before taxes, in today's dollars.

Why should you run three scenarios instead of one?

One number gives false comfort. Run a low, middle, and high case, then plan your life around the low one.

Here is Maria with three return assumptions and a 3.5% withdrawal rate:

The gap between the low and high case is about $500 a month. That gap is the real uncertainty in your plan. If you can live on the low case, you are in a much stronger spot.

What reduces the income you actually get to spend?

The estimate above comes before several real-life costs. Subtract these:

1. Taxes. Money from a traditional 401(k) or traditional IRA is taxed as ordinary income when you take it out. Roth accounts follow different rules, and qualified withdrawals are generally tax-free. A regular brokerage account is taxed on gains and dividends.

2. Fund fees. An expense ratio (the yearly fee a fund charges) of 0.05% barely matters. An expense ratio of 1% can eat a large share of your future income. Check yours on the fund's page.

3. Health costs. Medicare starts at 65, but premiums and out-of-pocket costs remain. If you retire earlier, you need other coverage until then.

4. Bad early years. A big market drop in your first years of retirement hurts more than the same drop later, because you are selling shares while prices are low. This is called "sequence risk." Keeping one to two years of spending in cash or short-term bonds can help you avoid selling stocks at the bottom.

What if the number is too low?

Most people who run this math for the first time find a gap. Here is how three common changes affect Maria's 4% case:

Pick the change you can actually keep doing. A $100 increase you keep for 25 years beats a $500 increase you drop after six months. Moving to a low-cost index fund is often the easiest win, because you do it once.

How often should you redo this calculation?

Once a year is enough for most people. A good moment is when your year-end statement arrives. Update:

• your real balance (not last year's projection),
• your contribution amount,
• your Social Security estimate,
• the years you have left.

Also redo it after a big life change: a raise, job loss, marriage, divorce, or an inheritance. Don't redo it after every market dip. Checking daily only makes it harder to stay invested.

When should you talk to a professional?

This method is good for a first estimate. It is not personal financial advice, and it cannot see your full situation. Talk to a fee-only fiduciary financial planner (someone legally required to act in your interest and paid by you, not by product commissions) or a tax professional if:

• you are within 10 years of retiring,
• you have a pension with several payout options,
• you have money in several account types and want to plan the order of withdrawals to lower taxes,
• someone is offering you an annuity,
• you have health issues that change how long you expect retirement to last.

A single planning session is often cheaper than one year of high fund fees.

Your next step

Open a blank spreadsheet today and enter your five numbers. Type `=FV(0.03, years, -yearly_contribution, -current_balance)` using your own figures, then multiply the result by 0.035 and divide by 12. Add your Social Security estimate from ssa.gov. That is your cautious monthly income number. Write it down, set a calendar reminder for one year from today, and compare the new number with this one.

FAQ

Is the 4% rule still safe to use?

It is a planning guideline, not a promise. Many planners suggest starting between 3% and 4%, especially if you retire early or expect a long retirement. Run your numbers at both rates and plan around the lower one.

Should I use a 7% or 10% return for index funds?

Those figures usually ignore inflation. For planning in today's dollars, use a real return after inflation, such as 3% to 4% for a stock-heavy portfolio, and test a higher number only as a best case.

Does this estimate include taxes?

No. Withdrawals from traditional 401(k)s and IRAs are taxed as ordinary income, while qualified Roth withdrawals are generally tax-free. Subtract your expected taxes, and ask a tax professional if you have several account types.

Educational content, not personalized financial advice. Sources cited where applicable.