In late 2023, when the Fed funds rate hit 5.25-5.50%, you could suddenly get 5% APY on a certificate of deposit. People who hadn't thought about CDs in a decade were opening them. And most of them made one of two mistakes: they either locked up too much money for too long, or they didn't lock up enough because they thought rates would keep climbing.
Both mistakes stem from the same gap: not understanding how CD math works and what you're actually optimizing for.
What a CD actually is
A certificate of deposit is a time deposit at a bank. You agree to leave your money untouched for a fixed term (3 months to 5 years, typically), and the bank agrees to pay you a fixed interest rate for that term. If you withdraw early, you pay a penalty, usually 3-12 months of interest depending on the term.
The rate is fixed at purchase. This is the key difference from a savings account, where the rate can change at any time. When you buy a 12-month CD at 4.75%, you're guaranteed 4.75% for those 12 months regardless of what rates do.
This guarantee is both the appeal and the risk.
APY vs. APR on CDs
Banks quote CD rates as APY (Annual Percentage Yield), which includes the effect of compounding. The APR (Annual Percentage Rate) is the simple interest rate before compounding.
The relationship:
APY = (1 + APR/n)^n - 1
Where n is the number of compounding periods per year. A CD with a 4.75% APR compounding monthly:
APY = (1 + 0.0475/12)^12 - 1
APY = (1.003958)^12 - 1
APY = 1.04856 - 1
APY = 4.856%
The difference between APR and APY is small for typical CD rates but adds up on larger deposits. On $100,000, the difference between 4.75% APR and 4.856% APY is $106 per year.
Daily compounding produces a slightly higher APY than monthly. A 4.75% APR compounded daily yields 4.864% APY. The difference from monthly is negligible ($8 per year on $100,000), but if two CDs offer the same APR, pick the one with more frequent compounding.
The reinvestment risk problem
Here's the scenario that catches people. You lock in a 5% CD for 12 months. When it matures, rates have dropped to 3.5%. You now have to reinvest at a lower rate. The 5% is gone.
This is reinvestment risk: the risk that when your CD matures, prevailing rates are lower than your locked-in rate. It's the mirror image of opportunity cost, where you lock in and rates rise.
You can't eliminate both risks simultaneously. But you can manage them with a CD ladder.
The CD ladder strategy
A CD ladder splits your deposit across multiple CDs with staggered maturity dates. Instead of putting $50,000 into a single 5-year CD, you put $10,000 each into 1-year, 2-year, 3-year, 4-year, and 5-year CDs.
After year one, the 1-year CD matures. You reinvest it into a new 5-year CD. After year two, the original 2-year CD matures, and you reinvest into another 5-year CD. After five years, you have five 5-year CDs, one maturing each year.
The benefits:
Liquidity. You always have a CD maturing within the next 12 months. If you need the money, you wait for the next maturity instead of paying an early withdrawal penalty.
Rate averaging. In a rising rate environment, each reinvestment captures the higher rate. In a falling rate environment, your longer-term CDs still earn the higher rates they locked in. You never catch the top or bottom of the rate cycle, but you capture the average, which is a reasonable outcome.
Higher average yield. Longer-term CDs generally pay higher rates (the yield curve is usually upward-sloping). A ladder gives you exposure to those higher rates while maintaining periodic liquidity.
Calculating early withdrawal penalties
The early withdrawal penalty is the real cost of a CD's illiquidity. Typical penalties:
- 3-month CD: 1-2 months of interest
- 12-month CD: 3-6 months of interest
- 5-year CD: 6-12 months of interest
For a $25,000, 12-month CD at 4.75% with a 6-month interest penalty:
Annual interest: $25,000 * 0.0475 = $1,187.50
Monthly interest: $1,187.50 / 12 = $98.96
6-month penalty: $98.96 * 6 = $593.75
If you need to withdraw after 8 months, you've earned $791.67 in interest but pay $593.75 in penalties. Your net earnings are $197.92, equivalent to roughly a 1.2% annualized return. You didn't lose principal, but you gave up most of the interest.
In some cases, breaking a CD early is still the right move. If you locked in at 3% and rates have risen to 5.5%, breaking the old CD and opening a new one at the higher rate can be net positive, especially if you have significant time left on the term. Run the numbers both ways.
CDs vs. alternatives
High-yield savings accounts currently offer 4-5% APY with no lock-up period. If CD rates aren't significantly higher than HYSA rates, the CD's illiquidity premium isn't worth it. CDs make sense when the term premium (the extra yield for locking up) is at least 0.25-0.50% above what a savings account offers.
Treasury bills are another fixed-income alternative with some advantages. T-bill interest is exempt from state income tax, which effectively boosts the yield for people in high-tax states. A 4.5% T-bill in California (13.3% top state rate) is equivalent to a 5.19% CD on a state-tax-adjusted basis.
I Bonds offer inflation protection but have a $10,000 annual purchase limit and a 1-year minimum holding period. They're complementary to CDs, not a replacement.
Three mistakes with CDs
Chasing the highest rate without reading the terms. Some banks offer promotional rates that require a large minimum deposit, revert to a lower rate after an initial period, or have unusually harsh early withdrawal penalties. Read the fine print.
Putting emergency fund money in long-term CDs. Emergency funds need to be accessible immediately. A CD with a 6-month early withdrawal penalty defeats the purpose. Keep 3-6 months of expenses in a savings account. Use CDs for money you genuinely won't need for the term.
Ignoring FDIC limits. FDIC insurance covers $250,000 per depositor, per institution, per ownership category. If you're depositing more than $250,000, spread it across multiple banks.
For calculating CD returns across different rates, terms, and compounding frequencies, and for modeling ladder strategies, I built a CD calculator at zovo.one/free-tools/cd-calculator.
The CD decision is fundamentally a bet on the direction of interest rates. If you think rates are going down, lock in now. If you think they're going up, stay short-term and wait. If you're not sure -- and nobody is -- build a ladder and capture the average. The math is certain even when the rate environment isn't.
I'm Michael Lip. I build free developer tools at zovo.one. 350+ tools, all private, all free.
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