CD Calculator (Certificate of Deposit)

A certificate of deposit (CD) is a savings product where you agree to leave a deposit with a bank for a fixed period — anywhere from a few months to several years — in exchange for a guaranteed interest rate. CDs are FDIC-insured (up to $250,000 per depositor per bank), the rate doesn’t change once you open the CD, and the math is fully knowable up front. That last property is why a calculator like this one exists: given a principal, a rate, and a term, the maturity value is a one-line formula.

The formula is compound interest: balance = P × (1 + r/n)^(n × t), where P is principal, r is the nominal annual rate (as a decimal), n is the number of compounding periods per year, and t is the term in years. For a $10,000 deposit at a 4.4% nominal rate compounded monthly for 5 years, that’s 10000 × (1 + 0.044/12)^60 = $12,455.75. The interest is the difference: $2,455.75.

Banks rarely advertise the nominal rate — they advertise APY (annual percentage yield), which is the actual annual return after compounding. The relationship between the two is APY = (1 + r/n)^n − 1. A 4.4% nominal rate compounded monthly produces an APY of (1 + 0.044/12)^12 − 1 ≈ 4.491%. APY is the apples-to-apples number across compounding frequencies — two CDs at the same APY produce identical maturity values regardless of whether one compounds daily and the other monthly. This calculator’s APY mode lets you skip the compounding question entirely; the Nominal mode is there for the rare cases where your rate sheet specifies a nominal rate and frequency separately.

Compounding frequency itself almost doesn’t matter at consumer-CD rates. A 4.5% APY produces the same maturity value whether the bank compounds daily, monthly, or quarterly — that’s the definition of APY. Even comparing 4.5% nominal rates across frequencies, the spread is small: 4.5% nominal compounded daily yields 4.602% APY; compounded monthly it yields 4.594% APY. The difference at $10,000 over a year is about $0.78. The compounding choice is mostly cosmetic.

CD interest is taxed as ordinary income — federal, plus state if your state has an income tax. Your bank issues a 1099-INT for the year. For multi-year CDs, you owe tax each year on the interest accrued that year, even though you can’t withdraw it without penalty. This calculator’s optional tax field applies a flat marginal rate to the total interest at the end — a simple model that approximates the cumulative tax bill but doesn’t account for the slightly lower effective return when reinvested interest is post-tax. The simple model is what every consumer CD calculator uses; it’s accurate enough for comparing offers.

The most common CD pitfall is the early-withdrawal penalty. If you cash out before the maturity date, the bank charges a penalty — usually 3 to 12 months’ worth of interest, depending on the term. The penalty can erase several years of compounding, so locking up money for a CD only makes sense for a horizon you’re confident about. The calculator doesn’t model the penalty (terms vary too widely by bank) — check your CD’s disclosure document for the exact rule.

Privacy is the final detail worth stating explicitly. Every calculation in this tool runs locally in your browser. Your form inputs (principal, rate, term, tax rate) are saved to your own browser’s localStorage so a refresh doesn’t lose them, but the data never leaves your device — there’s no server, no API, no analytics on the input values. The storage key is cd:inputs:v1. To wipe it, use your browser’s “clear site data” tool.

This is not financial advice. This calculator is a math tool. CD rates, tax law, and your personal situation all matter for an actual decision — consult a tax professional or financial advisor before committing significant funds.