A few years ago, opening a high-yield savings account felt like a small life upgrade. Rates had spent most of the prior decade close to 0.05% — five basis points, the kind of number you do not bother typing into a calculator — and suddenly the same banks were offering 4%. People who had ignored savings rates entirely started moving money in five-figure chunks.
Take a representative rate of 4.10%. On a $20,000 balance, that is $820 a year of interest. Is that a lot? It is more than a brokerage cash sweep typically pays. It is also less than the marketing makes it sound. The math is worth running.
The headline rate
APY — annual percentage yield — is the actual amount your balance grows over a year, assuming the rate holds and interest compounds. At 4.10% APY, $20,000 becomes $20,820 after twelve months if the rate never changes. The rate does change. Most high-yield savings accounts pay a "variable" rate, which means the bank adjusts it in response to broader interest-rate moves. If the Fed cuts rates 100 basis points, expect your APY to drop 70–90 basis points within a quarter.
What tax takes
Interest from a savings account is taxed as ordinary income in the US. At a marginal federal rate of 22% — which is where many middle-income filers sit — your $820 of interest becomes $640 after federal tax. Add state tax of, say, 5%, and you are at $599. (No state tax? Lucky. Add zero.)
This is the bit most marketing pages quietly omit. The headline APY is pre-tax. Your bank does not withhold the tax for you, but the IRS sees it via the 1099-INT the bank files in January.
What inflation takes
The other thing the headline number ignores is that your $20,000 in 2025 buys less than your $20,000 in 2024. If headline inflation runs at 2.8% — in the rough range the Bureau of Labor Statistics Consumer Price Index has shown recently — then in real terms your $20,000 has lost about $560 of purchasing power.
You still have $20,820 in nominal dollars. But the bundle of goods it buys has shrunk to about $20,260 in 2024 dollars. Net of inflation and tax, your $820 of nominal gain becomes around $40 of real gain.
| Step | Amount |
|---|---|
| Starting balance | $20,000 |
| Interest at 4.10% APY | +$820 |
| Federal tax at 22% | −$180 |
| State tax at 5% | −$41 |
| Inflation drag at 2.8% | −$560 |
| Real after-tax return | +$39 |
The real number
So: a year of leaving $20,000 in a high-yield savings account at the headline-impressive rate of 4.10% earns you, in real after-tax terms, about $39. That is roughly the price of two pizzas.
This is not a criticism of the account. It is a clarification of what the account is for. A high-yield savings account is not an investment. It is a place to store cash you cannot afford to lose, where the interest at least keeps pace with — and slightly outpaces — inflation. That is the whole job.
A high-yield savings account is doing its job if it merely keeps your money from shrinking. Anything beyond that is a small bonus.
When the math still says yes
For three things, the math still strongly says yes:
- The emergency fund. The whole point is you do not lose it. A real after-tax return of zero or slightly positive is exactly what you want.
- Sinking funds. Same logic. The pet fund needs to be there in March; small but positive real return is a free upgrade.
- Cash you genuinely need within 12 months. A down payment building toward a 2026 home purchase, a wedding next summer.
For everything else — money you do not need for five-plus years — a savings account is, in real terms, the slow loss it has always been when rates were low. Index funds do not eliminate risk, but they do, over decade-long horizons, beat inflation by enough to matter. Run the same table on a $20,000 brokerage balance with a 5% real return assumption and the bottom line is +$1,000, not +$39. That is the difference that compounds.
