CD Calculator Guide: APY, Compounding & Early Withdrawal定期存款(CD)计算器指南：年化收益率、复利与提前支取
A certificate of deposit is the simplest interest-bearing product your bank sells: you lock up a fixed sum for a fixed term at a fixed rate, and the bank promises to give it back with interest on a specific date. The math is straightforward — but the difference between APY and APR, the impact of daily vs monthly compounding, and the real cost of early withdrawal are where people get surprised.
What Is a CD?
A CD is a time deposit. You give the bank (or credit union) a lump sum — typically $500 to $100,000 — for a fixed term: 3 months, 6 months, 1 year, 5 years. In return, the bank pays you a fixed interest rate, usually higher than a savings account because your money is locked up. Pull the money out early and you pay a penalty, often several months of interest.
CDs are FDIC insured up to $250,000 per depositor, per bank, per ownership category — meaning you cannot lose the principal as long as you stay within the limits.
APY vs APR — Why It Matters
Banks advertise two numbers, and they're not the same:
- APR (Annual Percentage Rate) — the simple annual interest rate, ignoring compounding.
- APY (Annual Percentage Yield) — what you actually earn after compounding is folded in. This is the only number that lets you compare products fairly.
A CD advertised at "5.00% APR with daily compounding" actually yields about 5.13% APY. Banks are required to show APY prominently because the truth-in-savings act makes them — and because the difference becomes meaningful over multi-year terms.
The Compounding Formula
For a CD with principal P, annual rate r, compounding n times per year, held for t years:
- A — final balance (principal + interest)
- P — initial deposit
- r — annual interest rate (decimal)
- n — compounding periods per year (12 for monthly, 365 for daily)
- t — years the money is on deposit
Effective APY = (1 + r/n)n − 1
Worked example: $10,000 in a 5-year CD at 4.5% APR, compounded monthly:
A = 10,000 × (1 + 0.045/12)60 = 10,000 × (1.00375)60 ≈ $12,515
That's $2,515 in interest — or about 25% return on your principal over 5 years. Not bad for doing literally nothing.
Daily vs Monthly vs Continuous
How often interest compounds changes the APY meaningfully, even at the same nominal rate:
| Compounding | Effective APY at 5% | $10,000 grows to (5 years) |
|---|---|---|
| Annual (n=1) | 5.000% | $12,763 |
| Monthly (n=12) | 5.116% | $12,834 |
| Daily (n=365) | 5.127% | $12,840 |
| Continuous | 5.127% | $12,841 |
Once you cross monthly compounding, the gains from more frequent compounding are essentially noise — under $10 over five years on $10,000. Choose based on the rate, not the compounding style.
Early Withdrawal Penalty Math
If you cash out before maturity, the bank forfeits some of your interest — typically a number of months of interest, depending on term:
- 3-month CD: often no penalty, or 1 month
- 6-month to 1-year CD: usually 3 months of interest
- 2-year to 5-year CD: usually 6 months to 12 months of interest
Worse: if your CD hasn't yet earned enough interest to cover the penalty, the bank will dip into your principal. A 6-month CD that has only earned 2 months of interest will forfeit both months — and you'll get less than you deposited.
Some banks advertise "no-penalty CDs" but offer a lower APY to compensate. Compare the rate against the penalty you'd actually pay. For most savers, the no-penalty version is a worse deal.
CD Laddering — A Better Strategy
Instead of putting $50,000 into one 5-year CD, split it across five CDs with staggered maturities: $10,000 each in 1-year, 2-year, 3-year, 4-year, and 5-year CDs. Every year, one CD matures and you can either spend it or roll it into a new 5-year CD at whatever the current rate is.
Laddering gives you three wins: penalty-free access to a portion of your money each year; the higher average rate of long-term CDs; and automatic rebalancing into higher rates if rates rise.
Try It
Use the CD Calculator to:
- Compare two CDs with the same nominal rate but different compounding — see the actual difference over 1, 3, and 5 years.
- Model a CD ladder — 5 staggered CDs and watch the renewal schedule.
- See the dollar cost of an early withdrawal at various points in the term.
The calculator supports monthly, quarterly, daily, and continuous compounding and shows effective APY alongside nominal rate.