The conversion most CD calculators get wrong
Banks advertise APY — annual percentage yield — which is the return after a year including compounding. But to project a balance month by month you need the nominal rate, and those are not the same number.
The relationship is APY = (1 + r/n)n − 1, so recovering the
nominal rate means inverting it: r = n · ((1+APY)1/n − 1). A
calculator that feeds the advertised APY straight in as the nominal rate is compounding a
number that already includes compounding, and it overstates your return. The error is
small on a one-year CD and grows with the term.
A useful consequence: once a bank quotes APY, compounding frequency stops mattering. Daily and annual compounding at the same APY produce the same maturity balance, by definition. Frequency only changes the answer when you are comparing quoted interest rates.
The number that actually matters is after tax
CD interest is taxed as ordinary income at your marginal rate — not at the lower long-term capital gains rate that applies to stocks held over a year. At a 22% marginal rate, a 4.50% APY is really about 3.51% in your pocket. At 32%, it is 3.06%.
There is a second wrinkle that catches people on multi-year CDs: interest is taxable in the year it is credited, not the year you withdraw it. On a five-year CD you can owe tax annually on money you are contractually unable to touch, which means finding the cash elsewhere. That alone is a reason many people prefer terms of a year or less.
Early withdrawal is where CDs bite
The penalty is quoted as a number of months of interest — commonly three months on short terms and six to twelve on longer ones. The trap is how it is computed: on the principal at the CD's stated rate, not on the interest you have actually accrued.
So if you close a 12-month CD after two months with a six-month penalty, you have earned roughly two months of interest and owe six. The bank takes the shortfall out of your principal, and you walk away with less than you deposited. This is legal, disclosed, and routinely surprising. The calculator shows a red warning at exactly the point where that threshold is crossed.
When a CD is and is not the right instrument
- Money with a known date. A house deposit in 18 months, a tax bill next April. You gain a guaranteed rate for accepting an illiquidity you were going to accept anyway.
- Not an emergency fund. The whole point of an emergency fund is immediate access. A high-yield savings account usually pays close enough to a short CD to make the lockup pointless — the comparison panel above shows the actual gap.
- Not a long-term growth vehicle. Over a decade, a CD reliably loses to inflation-adjusted equity returns. Its job is capital preservation with a known date, not growth.
- Watch for auto-renewal. Most CDs roll over automatically at whatever rate is current on the maturity date, and there is typically a grace window of only seven to ten days to opt out. Diary the maturity date.
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Frequently asked questions
What is the difference between APY and interest rate?
The interest rate is the nominal rate before compounding. APY is what you actually earn over a year once compounding is included, which is why it is always equal to or higher than the rate. Banks advertise APY because it is the bigger number — and because it is the one that lets you compare two CDs with different compounding schedules honestly.
Does compounding frequency change the result much?
Far less than people expect, because APY already accounts for it. At a fixed 4.50% APY, daily and annual compounding produce the same maturity balance by definition. Frequency only matters when you are comparing a quoted interest rate rather than a quoted APY.
How is CD interest taxed?
As ordinary income at your marginal rate, not at the lower capital-gains rate. It is also taxed in the year it is credited, even on a multi-year CD you have not cashed out — so you can owe tax on money you cannot yet touch. Enter your marginal rate above to see the after-tax figure, which is the only number worth comparing against other investments.
How does the early withdrawal penalty work?
It is quoted in months of interest, calculated on the principal at the CD's rate — not on the interest you have actually accrued. That distinction matters: if you withdraw before you have earned as much as the penalty costs, the bank takes the difference out of your principal, and you get back less than you deposited. The calculator flags exactly when that happens.
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