1 Month CD Calculator

What a one month CD pays

A one month CD is a time deposit with a maturity date about 30 days out. You hand the bank a lump sum, the bank quotes an annual percentage yield, and at maturity you get the principal back plus roughly one month of interest. The Consumer Financial Protection Bureau describes the trade in plain terms: the bank keeps the money for the term, and you accept an early withdrawal penalty in exchange for a fixed rate.

Because the term is short, the dollar amounts are small. Ten thousand dollars at 4.00 percent APY earns 32.88 dollars in 30 days. That is not a reason to skip the product. It is a reason to treat a one month CD as a parking spot for money that already has a date attached, such as a quarterly tax payment, a closing date, or a bonus you have to leave untouched for a month.

Bank CDs are insured by the FDIC up to 250,000 dollars per depositor, per insured bank, per ownership category, and credit union versions carry the same limit through the NCUA. The coverage includes interest posted to the account, so a 30 day CD under the limit carries no credit risk.

Short term CDs are mostly an online bank product. Minimums of 1,000 to 2,500 dollars are common, some banks open at 500, and a few sell no penalty versions that behave like a savings account with a fixed end date.

The 30 day interest formula

For a term under a year, the bank accrues simple interest on the deposit and prorates it by days. The formula is I equals P times r times d divided by 365, where P is the deposit, r is the annual rate as a decimal, and d is the number of days the money sits in the account. At 4.00 percent on 10,000 dollars for 30 days, I equals 10,000 times 0.04 times 30 divided by 365, which is 32.88 dollars.

Divide that by 30 and the account earns 1.10 dollars a day. Keep the daily figure in mind when you weigh an early exit, because penalties are quoted in days of interest too, and the comparison is day against day.

The APY on the disclosure is the number to compare across banks. Over 30 days the gap between a nominal rate and an APY that compounds daily is a few cents, smaller than the spread between two banks, so the headline yield decides the shopping trip, not the compounding schedule.

Ask which day count the bank uses. Most consumer CDs accrue on an actual/365 basis, a few use actual/360, and a 360 day basis pays about 1.39 percent more interest for the same stated rate. On a 10,000 dollar deposit that is 46 cents, trivial for one month, worth knowing if you plan to roll the CD over every month for a year.

Worked example: 10,000 dollars at 4.00 percent

Here is the same 10,000 dollar deposit at four APYs a one month CD might quote. Every row uses a 30 day term and a 365 day year, so the only variable is the rate.

Doubling the deposit doubles every figure in the table. A 25,000 dollar deposit at 4.50 percent earns 92.47 dollars in the same 30 days, which is 3.08 dollars a day, and the maturity value is 25,092.47 dollars.

The spread between the best and worst row is 12.33 dollars on 10,000 dollars. A quarter point of APY is worth about 2.06 dollars over the month, so chasing a rate across town costs more in gas than it returns unless the deposit is large or you can open the account online in a few minutes.

Early withdrawal penalties on a 30 day term

Regulation DD, at 12 CFR 1030.4(b)(6)(ii), requires the bank to state in the account disclosure whether an early withdrawal penalty applies, how it is calculated, and when it is charged. That paragraph in your agreement is the authority for your CD, not a blog table. Structures differ by bank and by term, and a one month CD usually lands in the shortest, harshest bucket, because the bank priced the deposit assuming 30 days of funding.

Four structures show up often at US banks. Each row below is a 10,000 dollar deposit at 4.00 percent closed on day 12, when 13.15 dollars of interest has accrued.

The 90 day bucket is the one that catches people. A penalty longer than the term means the CD can never earn enough to cover an early exit, so a 30 day commitment is only safe if you can truly leave the money alone for about 90 days. Wells Fargo, for example, publishes a one month interest penalty for terms under 90 days and a three month penalty for terms of 90 days through a year, while other large banks use a 90 day bucket for terms under six months. Read your own disclosure, then enter the penalty days in the calculator above to see your number.

What an early exit costs, day by day

A penalty quoted in days is easy to compare against interest quoted in days. With a 90 day penalty on a 30 day CD, the charge is fixed at 98.63 dollars on a 10,000 dollar deposit at 4.00 percent, while the interest you keep grows by 1.10 dollars for every day you wait. This table runs the exit day forward.

Every early day loses money against holding to maturity, and days 5 through 29 also land below the original deposit. The penalty shrinks the balance instead of just cancelling interest, which is the difference between a penalty that costs you yield and one that costs you principal.

Two habits avoid the problem. First, keep the emergency fund in a savings account and only put money with a real date into a CD. Second, if there is any chance you need the cash, look for a no penalty CD, which typically allows withdrawal after the first six days with no charge in exchange for a slightly lower APY.

Interest is taxable in the year the bank credits it, and the bank reports it on Form 1099 INT. The IRS Publication 550 covers the reporting rules. A penalty charged in a later year does not automatically reverse a form you already received, so ask the bank how it will report the correction.

A 4 week Treasury bill next to a one month CD

TreasuryDirect auctions 4 week bills every week and publishes every result. The 4 week bill auction that closed on November 2, 2026 cleared at a 3.910 percent discount rate, an investment rate of 3.976 percent, and a price of 99.695889 per 100 dollars of face value. A 10,000 dollar bill cost 9,969.59 dollars and paid back 10,000 dollars 28 days later, a gain of 30.41 dollars.

The two products are not interchangeable. Interest on a Treasury bill is exempt from state and local income tax, while CD interest is taxed at every level. A bill has no early withdrawal penalty, but it also has no early redemption: you sell it in the secondary market at whatever price buyers offer, and if rates rose after you bought, that price sits below what you paid. The CD has a disclosed penalty and deposit insurance, and it cannot lose principal to a rate move.

For a month of parking, the two land within a few dollars of each other when a bank's one month APY tracks the bill rate. The bill wins on tax in a state with an income tax, and the CD wins on certainty, on a known penalty, and on the fact that you can open it at your own bank in minutes. TreasuryDirect lists the auction calendar and sells in 100 dollar increments.

What to check before you open a 30 day CD

Six things decide whether a 30 day CD is the right home for a balance, and five of them are printed in the account disclosure the bank must give you before the account opens.

The APY with the exact term, not a range. The penalty formula and the day count behind it. The grace period after maturity, which is often 7 to 10 days and is your only free exit. The minimum deposit and whether interest is paid out or compounded into the balance. The FDIC ownership category, which matters once your balances at one bank pass 250,000 dollars. And the renewal terms, because many CDs renew automatically at whatever rate the bank offers that day, which is frequently below the rate you originally locked.

Run your own numbers above before you sign. Set the deposit, the APY from the disclosure, the term in days, the penalty days, and the day you might need the money, and read the net proceeds line. If that line sits below your deposit, the CD is not a good fit for that money, whatever the headline APY says.