How much interest could your savings earn this month, including one deposit or withdrawal? Estimate it from your balance, earning days, and nominal annual interest rate, not APY; your account’s rules determine whether the estimate fits.
Savings interest for a chosen period
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What can this savings interest calculator do?
The model uses one constant nominal rate, a starting principal, 1–366 earning days, and optionally one balance change. It does not compound interest within the period; defaults are hypothetical, not current bank offers.
Enter a positive change for a deposit, a negative change for a withdrawal, or zero for no transaction. A change effective on day 16 means the old balance earns on days 1–15 and the new balance earns from day 16 onward.
Use the day the change starts earning interest, not necessarily the day you initiate a transfer. Enter money in whole cents and the rate as a plain number, such as 3.6 for 3.6%; the notes under the rounding section list every input limit.
Which account rules do you need?
Start with the account disclosure, not just its advertised yield. For covered banks, Regulation DD section 1030.4 requires disclosures covering the interest rate and APY, variable-rate terms, compounding and crediting frequency, balance calculation, minimum balances, fees, and when noncash deposits begin earning interest.
- This CFPB rule excludes credit unions; check your credit union’s applicable account disclosures.
- Use the nominal annual interest rate, separately from APY. A deposit’s nominal rate is not a borrowing APR.
- Enter actual earning days, not automatically 30.
- Choose 365 or 366 only according to the account’s method. The calculator does not infer a year or confirm that 366 applies.
Example: $5,000 becomes $6,000 on day 16
Hypothetical inputs: assume a 3.60% nominal annual rate, 30 earning days, and a 365 divisor. A $1,000 deposit starts earning on day 16, increasing principal from $5,000 to $6,000.
No interest compounds within the period. For this example, combined interest is credited once at the end, with no fees, taxes, or other balance changes.
| Earning days | Principal | Calculation | Interest |
|---|---|---|---|
| 1–15 (15 days) | $5,000 | 5,000 × 0.036 × 15 ÷ 365 | About $7.39726027 |
| 16–30 (15 days) | $6,000 | 6,000 × 0.036 × 15 ÷ 365 | About $8.87671233 |
| Combined interest | Not an ending balance | 5,940 ÷ 365 | About $16.27397260, rounded once to $16.27 |
Result: ending principal is $6,000. Crediting the rounded $16.27 would produce an assumed balance of $6,016.27, not confirmation that the bank has credited or made that money available.
What to check: use the deposit’s interest-effective date and keep principal, earned interest, and credited balance separate. Applying $6,000 to all 30 days would count the deposit before it began earning.
What changes if you withdraw instead?
Start with $2,000 at a hypothetical 4.00% nominal rate for 31 days, using 365. A $500 withdrawal effective on day 11 leaves $1,500 for the final 21 days.
Interest is $2,000 × 0.04 × 10 ÷ 365 + $1,500 × 0.04 × 21 ÷ 365, or about $5.64383562. Rounded once, it is $5.64; ending principal is $1,500 and assumed credited balance is $1,505.64.
Do not apply $1,500 to all 31 days. That would ignore the higher balance during the first ten days.
How does the formula handle rounding?
Segment interest = principal × (nominal annual rate ÷ 100) × segment days ÷ divisor. Add the unrounded segment amounts, then round combined interest once to the nearest cent; exact half-cent amounts round upward.
For example, $35 at 3.65% for 30 days with a 365 divisor earns exactly $0.105 in this model. That rounds to $0.11, giving an assumed credited balance of $35.11.
How the calculator handles inputs and rounding
- Money: whole cents only. Starting and ending principal cannot exceed $10 million.
- Withdrawals: cannot exceed starting principal. Uncredited interest cannot fund a withdrawal.
- Rate: 0%–100%, a mathematical input range, not a realistic forecast.
- Number format: ordinary decimal notation, with no commas, % signs, or exponents. Enter 3.6 for a 3.6% nominal rate.
Even with a zero change, the tool requires a valid change day within the period.
The model combines whole-cent principal and earning days with an exact decimal fraction for the nominal rate. It uses integer arithmetic for the final rounding decision, not a floating-point adjustment; it does not round daily or segment interest to cents first.
Displayed segment and combined values before cent rounding are ordinary numeric approximations. They are not used to decide the final cent rounding.
The rate’s exact decimal fraction comes from the parsed number’s internal representation. Scientific notation generated internally by JavaScript is an implementation detail, not accepted input syntax.
Browser number parsing can limit precision before the model receives an input. Your bank’s rounding and account rules may differ, so exact model rounding does not guarantee statement-matching cents.
What if you only know APY?
The tool does not accept APY, and APY divided by 365 is not the account’s daily interest rate. Get the stated interest rate from the disclosure; the APY, APR, and interest-rate guide explains the distinction.
For context only, a separate constant-balance illustration uses $5,000 × ((1.04)^(30/365) − 1). With a hypothetical 4.00% APY, that is about $16.14.
This assumes uniform daily-equivalent growth, retained interest, unchanged yield, a 365-day year, and no fees or transactions. It does not establish a bank’s actual daily rate, compounding schedule, or accrued interest.
The equivalent daily growth factor is (1.04)^(1/365), not 1 + 0.04/365. This is neither a second calculator mode nor the same account as the nominal-rate example.
Why might your statement show another amount?
For covered institutions, Regulation DD section 1030.7 permits daily-balance or average-daily-balance methods and does not require a particular compounding or crediting frequency. It specifies a daily rate of at least 1/365 of the interest rate and permits 1/366 in a leap year; banks do not all automatically switch to 366.
Accrual timing is linked to funds-availability law. Initiating a transfer, posting it, earning interest, and having funds available to spend are not necessarily the same event.
Daily compounding, multiple transactions, changing rates, tier rules, fees, taxes, closing before interest is credited, or different rounding can change the result. This calculator does not determine those rules or guarantee a match to your statement.
For a firm deadline, make a contribution plan without relying on uncertain interest to close the gap. When the statement arrives, reconcile the rate, earning dates, and credit; ask the bank about differences instead of forcing the estimate to match.
When shouldn’t you rely on this estimate?
- Tiered or changing terms: check for rate tiers, minimum balances, fees, or a midperiod rate change that could make this model unsuitable.
- More than one deposit or withdrawal: the model handles one balance change per period.
- Only an APY is available: the tool needs the stated nominal interest rate from the disclosure.
- Comparing a longer savings deadline: for money needed in about a year, see HYSA vs. CD on the same deadline.
Privacy: the calculator model runs locally and does not store or transmit inputs. That limited claim does not cover the site’s analytics, cookies, or network behavior; do not enter account identifiers or other sensitive information.
Quick decision checklist
- Take the nominal interest rate, not the APY, from your account disclosure.
- Count actual earning days and the day your deposit or withdrawal starts earning.
- Use a 365 or 366 divisor only as your account’s method specifies.
- Keep principal, earned interest, and credited balance as separate figures.
- Reconcile the estimate with your statement and ask the bank about any gap.
Sources and notes
- CFPB Regulation DD § 1030.1: Scope
- CFPB Regulation DD § 1030.4: Account disclosures
- CFPB Regulation DD § 1030.7: Payment of interest
AI-assisted article, not independently reviewed by a human financial professional. Examples recalculated and layout revised: 2026-09-28 (KST). Original source check: 2026-09-25 (UTC); selected official sources rechecked on 2026-09-28 (KST). Read our editorial policy or report a correction.
