30 and 90 Day CD Calculator: See What a Short-Term Certificate Actually Pays
Punch in your deposit and rate, pick 30 or 90 days, and watch the exact dollar amount your money earns before you ever walk into a bank.
The principal you're locking away for the term.
Purely for display, the math works the same in any currency.
The yearly rate your bank quotes, as a percentage.
Pick any date, past or future, to see your exact maturity date.
Why Term Length Changes More Than You'd Expect
People often assume a 90 day CD simply pays triple what a 30 day one does, and mathematically that's roughly true if the rate stays identical. But banks rarely offer the same rate across every term. Sometimes a 30 day CD carries a promotional rate higher than the 90 day option to attract quick deposits, and sometimes it's the opposite. The only honest way to know which term wins for your money is running both through the same formula side by side, which is exactly what the comparison mode here does.
The Formula Behind Every Number You See
Interest = Principal multiplied by the annual rate, multiplied by the number of days in the term, divided by your chosen day count basis. If you deposit 10,000 at a 4.5 percent APY for 90 days on a 365 day basis, that works out to roughly 111 in interest. Swap to a 360 day basis and the same deposit earns slightly more, since each day represents a marginally larger fraction of the year. That small difference is exactly why the day basis toggle matters and why two banks quoting the same headline rate can pay you different amounts.
Reading Your Results Like a Banker Would
Beyond the headline interest figure, pay attention to the maturity date and, if you toggled it on, the penalty estimate. A CD that looks great on paper can lose its shine fast if life throws a surprise expense at you before it matures. Running the penalty numbers upfront, rather than discovering them after the fact, is the difference between a pleasant surprise and an unpleasant one.
Certificates of Deposit Around the World
Short-term time deposits go by different names depending on where you bank, term deposits in Australia and the UK, fixed deposits across much of Asia, and CDs in the United States and Canada, but the underlying math is identical everywhere. This calculator works regardless of currency or country since it only needs your deposit, your rate, and your term.
Standards This Tool Follows
| Reference | How it's applied here |
|---|---|
| ISO 8601 | Dates are entered and calculated in the unambiguous YYYY-MM-DD format |
| ISO 4217 | Currency selector uses standard three-letter international currency codes |
| WCAG 2.1 AA | All inputs are labeled, keyboard-navigable, and contrast-checked |
Frequently Asked Questions
Is a 30-day or 90-day CD better?
It comes down to your actual quoted rate and how soon you might need the cash back. A 90 day term usually earns more total interest purely from running longer, but only a direct comparison at your real rates tells you for certain.
How is interest on a short-term CD actually calculated?
Banks multiply your principal by the annual rate and the number of days in the term, then divide by either 365 or 360 depending on their internal convention, which this calculator lets you toggle between.
What happens if I withdraw from a CD early?
Most institutions apply a penalty worth a set number of days of interest, which on a very short CD can wipe out most or all of what you've earned so far, so it's worth estimating before committing funds.
Do I need to pay tax on CD interest?
In most places, yes, interest is generally treated as taxable income in the year it's paid, though the exact treatment depends on your local tax rules, so this tool only shows the gross amount earned.