How does the calculator turn a rate and term into a maturity balance?
The calculator starts with the stated annual rate, sometimes called the nominal rate. It splits that rate into equal pieces, one for each compounding period: twelve pieces for monthly compounding, four for quarterly, 365 for daily. Each period, the balance grows by one piece, and the new interest becomes part of the base for the next period.
The term you enter in months is converted into a count of compounding periods. An 18-month CD with monthly compounding has 18 periods; with quarterly compounding it has six. The deposit is multiplied by the growth factor for that many periods to reach the ending balance.
APY is computed separately by running the same periodic rate for exactly one year. It answers the question a stated rate cannot: how much does one dollar actually earn in a year once compounding is counted? Federal Truth in Savings rules require banks to disclose APY so you can compare accounts on equal footing.
B = P × (1 + r ÷ n)^(n × t) Interest = B − P APY = (1 + r ÷ n)^n − 1
- B
- balance at maturity
- P
- amount deposited
- r
- stated annual interest rate as a decimal
- n
- compounding periods per year (365, 12, 4 or 1)
- t
- term in years (months ÷ 12)
Example: a $25,000 CD at 4.25% for 18 months
Scenario: Dana moves $25,000 from a bonus into an 18-month CD with a stated rate of 4.25%, compounded monthly. She plans to leave the interest in the CD until it matures.
- Monthly rate4.25% ÷ 12 = 0.354167% (0.00354167)
- Number of periods18 months × 1 compounding per month = 18 periods
- Growth factor(1.00354167)^18 = 1.065706
- Ending balance$25,000 × 1.065706 = $26,642.65
- Interest earned$26,642.65 − $25,000 = $1,642.65
- APY(1.00354167)^12 − 1 = 4.334%
Dana's CD would be worth $26,642.65 at maturity, including $1,642.65 of interest, for an APY of about 4.33%.
This is a pre-tax figure that assumes no early withdrawal; a penalty or interest paid out to checking would lower it.
When is a CD calculator most useful?
Parking money with a known spending date
If you need cash for a tuition bill or a home purchase on a fixed date, matching a CD's term to that date gives you a predictable balance. The calculator tells you exactly what will be there when the term ends.
Choosing between term lengths
Banks often pay different rates for 6, 12, 18 and 24 months. Running each term through the calculator turns rate quotes into dollars, which makes it easier to judge whether a longer lock-up is worth the extra interest.
Checking an advertised APY
Enter the stated rate and compounding from the account disclosure and compare the APY result with the one advertised. A mismatch is a cue to ask the bank how interest is credited.
Estimating the tax you will owe
Interest earned in each calendar year is generally reported to you and to the IRS. Knowing the dollar amount ahead of time helps you set aside money for the tax bill.
What mistakes do people make when estimating CD earnings?
Entering APY where the stated rate belongs
If you type an APY into the rate field and also choose monthly compounding, the calculator compounds an already-compounded number and overstates earnings slightly. Use the stated rate with its compounding, or enter the APY with annual compounding.
Forgetting the early-withdrawal penalty
The projection assumes you hold to maturity. If there is a real chance you will need the money early, look up the penalty in the account disclosure and subtract it from the interest in your planning.
Ignoring automatic renewal
Many CDs roll over into a new term at whatever rate the bank offers on the maturity date. If you miss the short grace window, your money may be locked up again at a rate you did not choose.
Exceeding deposit insurance limits
Federal deposit insurance covers $250,000 per depositor, per insured bank, per ownership category, and credit union share insurance works the same way. A large CD plus interest can cross that line, so it is worth checking how your accounts are titled.
Treating interest as tax-free until maturity
On many CDs, interest is credited every month and is taxable in the year it is credited, not only when the CD matures. A multi-year CD can create a small tax bill each year.
How much does compounding frequency change a CD's payout?
Less than most people expect. On Dana's $25,000, 18-month CD at 4.25%, switching from annual to daily compounding adds about $35 over the whole term. The rate and the term drive the result; compounding is a fine adjustment on top.
That is why APY exists. When two banks quote the same stated rate with different compounding, the APY shows the small difference. When they quote different rates, the rate gap usually matters far more than the compounding schedule.
| Compounding | Ending balance | Interest earned | APY |
|---|---|---|---|
| Daily (365) | $26,645.55 | $1,645.55 | 4.341% |
| Monthly | $26,642.65 | $1,642.65 | 4.334% |
| Quarterly | $26,636.69 | $1,636.69 | 4.318% |
| Annually | $26,610.57 | $1,610.57 | 4.250% |
What happens if you break a CD early?
You usually get your money back minus a penalty, and on a young CD the penalty can reach into principal. Penalties are set by each bank and are often stated as a number of months or days of interest.
Suppose Dana's CD charged six months of simple interest, which is $25,000 × 4.25% ÷ 2 = $531.25. If she closed it after four months, she would have earned only $356.05, so she would walk away with $24,824.80, about $175 less than she deposited. Held a full 18 months, the same penalty would never apply.
If a penalty is charged, the IRS generally lets you deduct it as an adjustment to income; it is reported on the Form 1099-INT you receive. The interest itself is still taxable.
Terms used on this page
- Certificate of Deposit (CD)
- A certificate of deposit is a savings account that pays a fixed rate for a set term, such as six months or five years, in exchange for leaving your money untouched until it matures. CDs at insured banks and credit unions are covered by federal deposit insurance up to the limit, and early withdrawals usually trigger a penalty.
- APY (Annual Percentage Yield)
- APY is the amount you actually earn on savings in one year, expressed as a percentage, after accounting for how often interest compounds. A higher compounding frequency pushes APY above the stated interest rate. Banks must disclose APY under federal Truth in Savings rules, so it is the fairest way to compare accounts.
- Compounding Frequency
- Compounding frequency is how often interest is calculated and added to a balance, such as yearly, quarterly, monthly or daily. More frequent compounding means interest starts earning interest sooner, so the effective yearly yield rises slightly. The effect is real but modest, and it shrinks as rates fall.
- Early Withdrawal Penalty
- An early withdrawal penalty is a charge for taking money out of a certificate of deposit before it matures. It is usually expressed as a number of days' or months' worth of interest, such as 90 days or six months. If you have not yet earned enough interest, the penalty can reduce your original deposit.
- Deposit Insurance
- Deposit insurance is federal protection for money held at insured banks and credit unions. The FDIC covers bank deposits and the NCUA covers credit union shares, each up to $250,000 per depositor, per insured institution, per ownership category. If an insured institution fails, covered deposits are repaid, including interest accrued up to the limit.
CD Calculator: common questions
How much interest does a $10,000 CD earn in a year?
It depends on the APY: a $10,000 CD earns roughly its APY times $10,000 over one year. At an illustrative 4.00% APY, that is about $400. Enter $10,000, the bank's stated rate, 12 months and the compounding schedule in the calculator above for an exact figure. Remember that the interest is taxable and that an early withdrawal would usually cost part of it.
Is the APY on a CD the same as its interest rate?
No, the APY is usually a little higher than the stated rate. The stated rate is the yearly rate before compounding; APY includes the effect of interest earning interest during the year. They are equal only when interest compounds once a year. Compare CDs by APY, because it puts different compounding schedules on the same footing.
Do CDs compound daily or monthly?
It varies by bank; daily and monthly compounding are both common, and some CDs compound quarterly or pay simple interest. The account disclosure states the compounding method and how often interest is credited. Choose the matching option in the calculator. The difference between daily and monthly compounding on a typical CD is usually only a few dollars.
Can you lose money in a CD?
You can, but generally only by withdrawing early or by holding more than insurance covers at a bank that fails. An early-withdrawal penalty can exceed the interest earned on a young CD and cut into principal. Deposits at federally insured banks and credit unions are protected up to $250,000 per depositor, per institution, per ownership category. Inflation can also erode what the money buys.
Is CD interest taxable if I do not withdraw it?
Yes, generally. CD interest is taxable as ordinary income in the year it is credited or made available to you, even if it stays in the CD. Banks typically send Form 1099-INT when you earn $10 or more in a year, but the interest is taxable even without the form. Tax-advantaged accounts such as an IRA holding a CD follow different rules.
What happens when a CD matures?
When a CD matures, you can withdraw the money and interest without penalty during a grace period set by the bank. If you do nothing, many CDs renew automatically for a similar term at the bank's current rate. Mark the maturity date and decide in advance whether to cash out, move the money or let it renew.
How do I calculate CD interest for a term in months?
Convert the term to years by dividing the months by 12, then use balance = deposit × (1 + rate ÷ n)^(n × years). For a 9-month CD with monthly compounding, that is nine monthly periods. The calculator above handles the conversion when you enter the term in months, so you can test odd terms like 7 or 13 months.
Is a longer CD always better?
No, a longer CD only pays off if its rate is higher and you will not need the money before it matures. Longer terms often pay more, but they lock your money up longer and leave you stuck if rates rise. When long and short rates are close, the extra years may add little interest for a lot less flexibility.
Are CDs insured like savings accounts?
Yes, CDs at insured banks are covered by FDIC deposit insurance, and CDs at federally insured credit unions are covered by NCUA share insurance. Coverage is $250,000 per depositor, per institution, per ownership category, and CDs are combined with your other deposits in the same category at that institution. Accrued interest counts toward the limit too.
Does the calculator include taxes or penalties?
No, the results are pre-tax and assume the CD is held to maturity. To estimate after-tax interest, multiply the interest earned by one minus your tax rate. To see the effect of a penalty, subtract the bank's penalty amount from the interest. The deep-dive section above walks through one penalty example.
Sources
- Certificates of deposit (CDs) — U.S. Securities and Exchange Commission (Investor.gov). Accessed October 17, 2026.
- Deposit Insurance — Federal Deposit Insurance Corporation. Accessed October 17, 2026.
- Regulation DD (Truth in Savings), 12 CFR Part 1030 — eCFR / National Archives. Accessed October 17, 2026.
- Topic no. 403, Interest received — Internal Revenue Service. Accessed October 17, 2026.
We cite primary sources: regulators, central-bank data and federal agencies. Worked examples on this page are our own calculations, last checked October 17, 2026. See our methodology.