The U.S. financial advice landscape converges on a few uncontroversial points: contribute to your 401(k) at least to the employer match, max out a Roth IRA if your income permits, and invest in low-cost index funds for the long term. What’s less commonly explained is the actual math behind why “long-term investing” beats “high-yield savings” so dramatically. Here’s a concrete example: take $100,000 today, leave it for 10 years. In a 5% HYSA, you’ll have about $163,000. In an S&P 500 index fund averaging 10% over those 10 years, you’ll have about $260,000. That $97,000 difference is the difference between a year of college tuition and a down payment on a house. The math depends on time horizon, but the gap reveals why “park it in savings” is rarely the right long-term strategy for serious money.
$100K, 10 years, three scenarios
Same starting capital, three different homes:
| Scenario | Return | Year 10 Balance | After-tax (CA) |
|---|---|---|---|
| A. HYSA at 5% APY | 5.0% | $162,889 | $158,196 (state+federal 33%) |
| B. S&P 500 index at 10% (historical) | 10.0% | $259,374 | $233,437 (15% LTCG) |
| C. 50/50 mix | 7.5% (weighted) | $206,103 | $193,328 |
The gap between A and B is $96,485 — and that’s after-tax. The 5% APY assumption is current (2026); the 10% S&P 500 is the long-term historical average that includes dividends reinvested.
Run these numbers in the interest tool compare panel and you can see year-by-year accumulation, including the effect of regular contributions or different starting amounts.
Why is 10% S&P 500 a “fair” assumption?
The S&P 500’s compound annual growth rate (CAGR) including reinvested dividends:
- 1928-2024 (97 years): approximately 10% nominal, 7% real (after inflation)
- 1995-2024 (30 years): approximately 11.5%
- 2015-2024 (10 years): approximately 12.7% (above average; recent strength)
- 2000-2010 (worst 10-year period): approximately 0% nominal (bubble + financial crisis)
The 10% figure is conservative compared to recent decades but reflects the long-term truth. Critically: any individual 10-year period can deviate substantially from 10%. Investing in March 2000 produced 0% over the next 10 years. Investing in March 2009 produced 16%+ over the next 10. Time horizon and entry point both matter.
Why does the gap matter?
The $97K difference at 10 years is meaningful but not transformative. The gap really matters at 30 years.
| Time horizon | HYSA (5%) | S&P 500 (10%) | Gap |
|---|---|---|---|
| 5 years | $127,628 | $161,051 | $33,423 |
| 10 years | $162,889 | $259,374 | $96,485 |
| 20 years | $265,330 | $672,750 | $407,420 |
| 30 years | $432,194 | $1,744,940 | $1,312,746 |
Over 30 years, the difference is $1.3 million on the same $100K starting amount. This is why retirement accounts (401k, IRA, Roth) almost always recommend stock-heavy allocations — the time horizon makes the math overwhelming.
When HYSA wins
Despite the long-term math, HYSA wins for specific situations:
- Money needed within 3-5 years. Stock market 5-year returns have been negative in some periods (e.g., 2000-2005). Don’t risk a house down payment or wedding fund on equities.
- Emergency fund. The whole point is liquid, accessible, principal-protected. Stocks fail this on principal protection.
- Inflation hedge for the next 1-2 years. HYSA at 5% beats inflation at 2.5% on a real basis short-term.
- Behavioral discipline issue. If you’d panic-sell at -30% drawdown, a HYSA is better than equities even with lower returns. Bad behavior costs more than low returns.
The “100 minus age” allocation rule
A common rule of thumb for stock allocation is “100 minus your age” or “120 minus your age” for stocks:
| Age | ”100 minus” Stocks | ”120 minus” Stocks |
|---|---|---|
| 30 | 70% | 90% |
| 40 | 60% | 80% |
| 50 | 50% | 70% |
| 60 | 40% | 60% |
| 70 | 30% | 50% |
The “120 minus” version reflects increased life expectancy and the need for portfolios to last 30+ years post-retirement. Target-date funds (e.g., Vanguard 2055) automatically follow a glide path roughly along these lines.
Cost of fees over 30 years
A 1% expense ratio sounds small but compounds devastatingly over 30 years.
| Expense ratio | $100K at 10% gross, 30 years |
|---|---|
| 0.03% (VTI, VTSAX) | $1,710,000 |
| 0.50% (typical mutual fund) | $1,490,000 |
| 1.00% (managed fund) | $1,290,000 |
| 1.50% (active manager) | $1,120,000 |
Going from 0.03% to 1% costs you $420K over 30 years — about 25% of the final balance. Choose low-cost index funds. VTI, VTSAX, FXAIX, and similar broad market index funds in the 0.03-0.05% range are standard.
How U.S. compares globally
| Country | HYSA equivalent | Stock index | 10-year gap |
|---|---|---|---|
| 🇺🇸 U.S. | HYSA 5% | S&P 500 10% | $97K |
| 🇰🇷 Korea | Deposit 4% | KOSPI 7% | $50K equiv |
| 🇯🇵 Japan | Deposit 0.30% | TOPIX 5% | $60K equiv |
U.S. has the highest savings yields AND the highest stock returns, with the largest absolute gap. Tax-advantaged wrappers (401k, IRA, Roth, HSA) significantly amplify after-tax returns vs. taxable accounts in all three markets, but the U.S. has the most generous annual contribution limits.
Tool — model your 10-year math
The interest tool accepts your starting balance, term, and rate. Compare panel lets you stack HYSA, mortgage, and (with monthly contributions) regular investing scenarios side-by-side. The diff line shows the gap dollar-for-dollar so the math behind “stocks for the long term” is concrete rather than abstract.
The choice between savings and stocks isn’t about picking one — it’s about allocating between them based on your time horizon. Money you’ll need in 1-2 years stays in HYSA. Money you won’t touch for 10+ years generally belongs in low-cost index funds. The $97K gap on a 10-year, $100K timeline is real, but only if you can stomach the volatility along the way. The smart move is rarely picking sides; it’s deciding how much of your portfolio belongs in which bucket.
