Index Fund vs. Savings Account Calculator
Should you invest or keep your money in savings? Enter your starting balance, monthly contribution, and time horizon to see both projections side by side — and exactly how much the higher return is worth over the years.
Index Fund vs. Savings Account — Side-by-Side Calculator
Same starting balance, same monthly contribution, same time horizon. Only the rate of return differs.
High-yield savings rates float — this assumes today's rate holds for the full horizon.
7% is a common estimate for the long-run S&P 500 return after inflation. Actual years vary widely.
Savings account at 5%
$232,643
after 20 years
Index fund at 7%
$300,851
after 20 years
You contribute $130,000 either way. Investing instead of saving projects to $68,207 more — the opportunity cost of leaving it in the savings account.
When to keep money in savings vs. invest
The calculator shows the index fund pulling ahead, but that doesn't mean every dollar belongs in the market. The right home for money depends almost entirely on when you'll need it.
Keep it in savings
- Your emergency fund (3–6 months of expenses).
- Money you'll need within the next 1–3 years.
- A house down payment, wedding, or tuition with a known date.
- Any balance you can't afford to see drop 20–30% at the wrong moment.
Invest it in index funds
- Retirement savings 10+ years out.
- Long-horizon wealth you won't touch for a decade.
- Money already growing in a 401(k), IRA, or brokerage account.
- Contributions you can leave alone through a down market.
The mistake runs both ways: parking 20-year retirement money in a savings account gives up enormous compounding, while putting next year's rent in an index fund exposes it to a drawdown you can't wait out. Match the vehicle to the horizon and you get the best of both.
Historical index fund returns
The 7% default in the calculator isn't a promise — it's a planning estimate rooted in the long-run history of the broad US stock market. Two numbers are worth keeping straight:
- ~10% nominal.Over many decades, a broad S&P 500 index fund has returned roughly 10% per year before inflation, including reinvested dividends.
- ~7% real.Subtract long-run inflation of around 3% and the purchasing-power return lands near 7%. That's the figure most planners use, because it reflects what the money actually buys later.
The catch is that the average hides enormous year-to-year swings. Individual calendar years have ranged from gains above 30% to losses near 40%. A savings account never has a losing year; an index fund earns its higher average precisely by making you sit through the bad ones. The long-run number only shows up for investors who stay invested — which is why the assumption fits 10+ year money and not next year's expenses.
Past performance doesn't guarantee future results. If you want a more conservative projection, lower the expected return in the calculator above.
The opportunity cost of not investing
“Opportunity cost” is the return you give up by leaving long-term money in a low-yield account. It's invisible — nothing is lost on paper — but over decades it's often the single largest number in a saver's financial life.
Take $10,000 plus $500 a month for 30 years. In a 5% savings account that projects to roughly $461,000. In a 7% index fund it projects to roughly $691,000 — a difference of nearly $230,000for the exact same contributions. You didn't save a dollar more; the entire gap is the rate working on a growing base, year after year.
The lesson isn't “never use savings.” It's that money with a decade-plus horizon has a real, compounding cost when it sits in cash. Keep what you need safe and liquid — then make sure the long-term money is actually invested, where the higher return has time to do its work.
Further reading
If you're weighing whether to invest or keep saving, a few investing classics make the long-horizon case far better than any single calculator can — the behavioral discipline to stay invested matters as much as the math:
- The Simple Path to Wealth by JL Collins — a plain-English case for low-cost index funds.
- The Little Book of Common Sense Investing by John C. Bogle — the Vanguard founder on why fees and staying invested decide your outcome.
- The Psychology of Money by Morgan Housel — why patience and consistency beat raw intelligence.
You'll find these titles, with links, in the Recommended reading panel alongside this page.
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Frequently Asked Questions
Is it better to invest in an index fund or keep money in savings?
It depends on when you'll need the money. For anything you might spend in the next 1–3 years — an emergency fund, a near-term purchase — a savings account is better because the balance can't drop. For money you won't touch for 10+ years, an index fund's higher expected return (roughly 7% after inflation vs. ~4–5% in a savings account) usually wins by a wide margin, at the cost of accepting market swings along the way.
How much more could I earn investing instead of saving?
Over long horizons the gap is large because the rate difference compounds. $10,000 plus $500/month for 20 years grows to roughly $233,000 in a 5% savings account and roughly $301,000 in a 7% index fund — about $68,000 more for the same contributions. Stretch it to 30 years and the gap widens to well over $200,000. Use the calculator above to run your own numbers.
Why not just keep everything in a high-yield savings account?
High-yield savings rates near 5% look attractive today, but they float — when the Fed cuts, online-bank rates usually follow within weeks. A savings account also rarely outpaces inflation by much, so long-term money parked there loses purchasing power. The trade-off is safety: savings balances are FDIC-insured and never have a losing year, which is exactly what short-term money needs.
What return should I assume for an index fund?
A common planning assumption is about 7% per year after inflation, based on the long-run historical average of the broad US stock market. Nominal (before inflation) returns have averaged closer to 10%. Neither is guaranteed — individual years have ranged from roughly +30% to −40%. The calculator defaults to 7%; lower it if you want a more conservative projection.
Do I have to choose one or the other?
No — most people use both. Keep 3–6 months of expenses plus any money you'll need within a couple of years in a savings account, and invest the rest in low-cost index funds for the long haul. The two aren't competitors; they cover different time horizons.