Certificate of deposit basics
How is CD interest calculated?
A standard CD estimate grows one opening deposit at the disclosed annual percentage yield for the length of the term. The balance at maturity equals the original deposit plus accumulated interest.
Deposit, APY, and term drive the estimate
A basic CD calculation needs three inputs:
- Opening deposit: the principal placed in the CD.
- APY: the effective annual yield under the account's stated assumptions.
- Term: the time from opening to maturity.
Plain Finance Tools uses the APY directly and assumes interest remains in the CD. For deposit P, APY y, and a term of m months, the model is:
Estimate CD interest and maturity value using the same method.
Worked example
Compare six months with one year
Inputs: $10,000 opening deposit and 5% fixed APY, with all interest retained.
| Term | Maturity value | Interest earned |
|---|---|---|
| 6 months | $10,246.95 | $246.95 |
| 12 months | $10,500.00 | $500.00 |
The six-month result is not exactly $250 because a 5% APY describes the effective growth over a full year. The half-year growth factor is the square root of 1.05.
Reproduce both terms →Why APY is useful
APY places deposit growth on an annual basis and incorporates the effect assumed from compounding. That makes it more useful for comparing deposit accounts than a nominal rate alone. The detailed relationship is covered in CD APY versus interest rate.
Federal Regulation DD includes standardized APY calculations and examples for time accounts. The exact bank disclosure controls the real account, especially for stepped rates, variable rates, or products that pay interest out instead of retaining it.
What can change the actual result
The estimate assumes one fixed APY, no additional deposits, no withdrawals, and holding the CD to maturity. Actual results can differ because of day-count rules, interest-crediting dates, required interest payouts, taxes, and rounding.
The Consumer Financial Protection Bureau recommends comparing the term, rate, and early-withdrawal penalty. The FDIC's CD shopping guidance also advises checking the deposit agreement and confirming that the institution is insured.
A higher maturity value is not automatically a better fit: a longer term also restricts access to the deposit for longer. Review the site methodology before treating the result as comparable to a specific account.
Common questions
Frequently asked questions
What determines how much interest a CD earns?
The opening deposit, APY, length of the term, and whether interest remains in the account determine the estimated maturity value.
Why does a six-month CD not earn the full annual APY?
APY is an annual measure. A six-month term covers only part of a year, so the deposit receives roughly half a year of growth under a fixed-yield assumption.
Does the maturity value include my original deposit?
Yes. Maturity value is the opening deposit plus accumulated interest. Interest earned is the difference between those amounts.
Does the calculation include an early-withdrawal penalty?
No. The estimate assumes the CD is held to maturity. Penalties and withdrawal rules differ by product and institution.