Daily Interest Calculator
Daily interest is the dollar amount a balance earns, or a loan charges, for a single day. It looks tiny on its own, $0.41 a day on a $5,000 savings balance at 3%, and that is exactly why it gets underestimated: lenders, card issuers, and closing statements all work in per day figures, and those figures decide real money over weeks and months.
This calculator shows both versions of the number: simple daily interest for an exact number of days, with a 365 or 360 day-count choice, and a daily compounding tab for balances where each day’s interest starts earning interest of its own.
Estimates only. Banks and lenders round per day amounts and may use tiered rates or a different day-count convention, so confirm payoff figures with the institution that holds the balance.
Quick Answer
- Quick Answer
- How to use this daily interest calculator
- The simple daily interest formula
- Worked example: $5,000 at 3% for 30 days
- What one day of interest costs at borrowing rates
- 365 or 360: the banker's year, priced
- Where a per day figure actually gets used
- Simple or compound: pick the tab that matches your balance
- What competing calculators miss
- Frequently Asked Questions
How to use this daily interest calculator
Start on the Simple daily interest tab unless your balance compounds. Enter the principal, which is whatever balance the interest runs on: a savings balance, a loan payoff amount, or an unpaid invoice. Enter the annual rate exactly as your statement or loan note states it, then the exact number of days, counting calendar days rather than business days. Leave the day-count basis on 365 unless your paperwork specifies a 360 day year, and press Calculate.
Read the headline figure first. In simple mode it is the interest for one day, the per diem, and the rows underneath scale it to your full period: total interest, and principal plus interest. Switch to the Daily compounding tab when interest is added to the balance as it accrues, as most savings accounts do. In that tab the headline becomes the future value, the basis selector disappears because compounding here always runs on a 365 day year, and an extra row shows the effective annual rate, the APY.
The simple daily interest formula
Simple daily interest splits the annual rate into a daily rate, then charges or pays that rate on the original principal for each day in the period. Nothing compounds, so every day costs or earns the same amount.
The day-count basis is the only judgment call in the formula, and it is not yours to choose freely: it comes from the account or loan agreement. The United States government uses the same structure for money it owes late payees, and the Bureau of the Fiscal Service simple daily interest method prices it as principal x (rate / 360 x days). If you hold a monthly rate or a monthly interest figure instead of an annual one, the annual interest to monthly calculator converts between the two views before you come back here for the daily figure.
Worked example: $5,000 at 3% for 30 days
The calculator is pre-filled with this case. Each row below is a figure it returns, in the order it computes them.
| Step | Calculation | Result |
|---|---|---|
| Daily rate | (3 / 100) / 365 | 0.00822% |
| Interest per day | $5,000 x 0.00008219 | $0.41 |
| Total interest, 30 days | $0.41 per day x 30, unrounded | $12.33 |
| Total amount | $5,000 + $12.33 | $5,012.33 |
| Same balance, 360 day basis | $5,000 x (0.03 / 360) x 30 | $12.50 |
One rounding point is worth knowing before you check the math by hand: the tool multiplies the unrounded daily figure, $0.4109589, by the number of days. If you round the per day figure to $0.41 first and then multiply, you get $12.30 for 30 days, three cents less than the $12.33 shown. The unrounded method is the correct one, and it is how lenders compute per diem amounts.
What one day of interest costs at borrowing rates
On savings balances the per day figure is small enough to ignore. At borrowing rates it is not, which is why the same formula matters more on the cost side.
Read those bars as a delay cost. Paying a 20% balance ten days late adds about $54.79 of interest on $10,000, before any late fee. Paying five days early saves about $27.40. None of that is visible in a monthly statement total, and it is the main reason a per day calculator exists.
365 or 360: the banker's year, priced
A 360 day basis divides the same annual rate by a smaller number, so every day costs slightly more. Lenders who use it are not breaking a rule; the convention is disclosed in the loan documents, and some federal formulas use it too. Your job is only to match the selector to your paperwork.
| $10,000 at 5% | 365 day basis | 360 day basis | Difference |
|---|---|---|---|
| Interest per day | $1.37 | $1.39 | $0.02 |
| Interest over 30 days | $41.10 | $41.67 | $0.57 |
| Interest over 365 days | $500.00 | $506.94 | $6.94 |
The gap is small on consumer balances and grows with the balance, which is why the convention shows up most in commercial and bridging loans. On a $500,000 balance at the same rate, the 360 day basis adds about $347 over a year. If a payoff quote looks a little higher than your own estimate, a 360 day basis is the first explanation to check.
Where a per day figure actually gets used
Mortgage closings. When a loan funds mid month, the lender collects the interest from the closing date to the end of that month up front. According to CFPB guidance on prepaid interest charges, that daily interest appears on page 2, section F of the Closing Disclosure, and it is exactly a per diem from this formula multiplied by the remaining days.
Loan payoffs and daily simple interest loans. Most auto loans and many personal loans accrue interest daily on the outstanding balance, so the payoff amount changes every day and a quote is only good for a set number of days. When the balance itself falls with each payment, the daily reducing balance calculator follows the balance down day by day instead of holding the principal fixed.
Late payments and invoices. Contracts and government prompt payment rules price lateness by the day, using this same simple formula. Savings accounts sit on the other side: interest accrues daily even when it is credited monthly, and Regulation DD requires banks to work it out on the daily balance or the average daily balance. For rate shopping, the companion APR per month calculator breaks an annual rate into its monthly slice.
Simple or compound: pick the tab that matches your balance
The two tabs diverge slowly, then all at once. Over 30 days on $5,000 at 3% they differ by a single cent. Over a year on a larger balance the compounding tab pulls ahead, because interest starts earning interest.
Simple daily interest fits when
- A payoff, invoice, or late charge is priced per day on a fixed balance
- The period is short: days or a few weeks
- Your paperwork states a 360 or 365 day basis
- You need the per diem itself, not a future balance
Daily compounding fits when
- A savings or money market balance accrues and keeps its interest
- The period runs to months or years
- You are comparing accounts and need the APY
- The balance grows untouched: $10,000 at 5% for 365 days reaches $10,512.67, not $10,500.00
If compounding is the whole question rather than one tab of it, including regular daily deposits or a target you are working backwards from, the daily compound calculator is the fuller tool. This page stays deliberately focused on the per day figure and exact short periods.
What competing calculators miss
Most daily interest tools ranking for this term do one slice of the job well and leave the rest to the reader.
Miss 1: no day-count choice. Several tools hard-code 365 and never mention 360, so a borrower checking a payoff priced on a banker's year gets a quietly wrong total. The selector here takes one click and the worked example above shows what it is worth.
Miss 2: the daily rate without the daily dollars. Some results lead with a percentage like 0.00822% and bury the amount that actually matters. This tool headlines the dollars per day, because $0.41 a day is the figure people budget, negotiate, and check statements against.
Miss 3: time entered in years. Tools built on the textbook I = P x r x t formula ask for time in years, so 30 days becomes 0.0822 years typed by hand, with rounding errors invited in. Here time is entered in days, the unit every per diem question is actually asked in.
Miss 4: no compounding check. A simple interest result is often presented as the final answer even for savings balances that compound. The second tab here answers that in the same screen and shows the APY, so you can see the gap instead of assuming it away.
Disclaimer: this calculator is an educational tool for estimating interest. It is not financial advice, and results are estimates that may differ from bank or lender calculations due to rounding, tiered rates, and day-count conventions. For decisions about borrowing or saving, consult a qualified financial professional.
Frequently Asked Questions
No. APR is the annual rate. The daily interest rate is that annual rate divided by the number of days in the year, usually 365, so a 5% APR becomes a daily rate of 0.01370%. The daily interest amount is that rate applied to your balance: $1.37 a day on $10,000 at 5%.
Use 365 for savings accounts and most personal finance math. Use 360 when your loan paperwork specifies it: the 360 day banker's year divides the rate by a smaller number, so each day costs slightly more. On $10,000 at 5%, the 360 basis gives $1.39 a day against $1.37 on a 365 basis.
Yes. Interest accrues on calendar days, not business days, so Saturdays, Sundays, and holidays all earn or cost interest. That is why the day-count basis is 365 rather than roughly 260 working days, and why a payoff quote keeps changing over a weekend.
Multiply the unrounded daily amount by the number of days in the month. The worked example earns $0.4109589 a day before rounding, so a 30 day month gives $12.33 and a 31 day month gives $12.74. Rounding the daily figure to $0.41 first understates the month by a few cents.
No. In simple mode every day is charged on the original principal only, so the daily amount never changes during the period. If your interest is added to the balance as it accrues, as in most savings accounts, use the Daily compounding tab, where $10,000 at 5% for 365 days grows to $10,512.67 instead of the $10,500.00 simple interest gives.
It is the daily interest collected up front for the partial month between your closing date and the end of that month, because mortgage interest is paid in arrears and your first regular payment does not cover those days. Closing later in the month means fewer per diem days to prepay, though the same interest is owed over the loan either way.
Banks round per diem amounts to many decimal places and may use a different day-count convention, leap year handling, an average daily balance, or tiered rates that change with the balance. This calculator gives an accurate estimate from a single rate; for a dispute, ask the bank for the daily rate and balance method on your statement and match those settings here.
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References
- U.S. Department of the Treasury, Bureau of the Fiscal Service, "Simple Daily Interest" (Prompt Payment) - the federal formula for daily interest owed, principal x (rate / 360 x days), with a worked late payment example. Bureau of the Fiscal Service: Simple Daily Interest. Accessed October 2026.
- Consumer Financial Protection Bureau, "What are prepaid interest charges?" - the CFPB's explanation of daily interest collected at mortgage closing, listed on page 2, section F of the Closing Disclosure. CFPB: What Are Prepaid Interest Charges?. Accessed October 2026.
- Consumer Financial Protection Bureau, Regulation DD (Truth in Savings), 12 CFR Part 1030 - the federal rule governing how banks calculate and disclose interest and APY on deposit accounts. CFPB: Regulation DD, 12 CFR Part 1030. Accessed October 2026.
- Consumer Financial Protection Bureau, Appendix A to Part 1030, "Annual Percentage Yield Calculation" - the official APY formula used when comparing the compounding tab's effective rate. CFPB: Appendix A to Part 1030, APY Calculation. Accessed October 2026.
- Electronic Code of Federal Regulations, Title 12, Part 1030, "Truth in Savings (Regulation DD)" - current regulatory text covering the daily balance and average daily balance methods. eCFR: 12 CFR Part 1030, Truth in Savings. Accessed October 2026.
- Bankers Online, Regulation DD Section 1030.7, "Payment of interest" - the rule text requiring interest on the full amount of principal for each day, at a daily rate of at least 1/365 of the interest rate. Bankers Online: Regulation DD 1030.7, Payment of Interest. Accessed October 2026.
- U.S. Securities and Exchange Commission, Investor.gov, "Compound Interest Calculator" - the SEC's growth calculator, the reference point for the compounding tab's future value and APY figures. SEC Investor.gov: Compound Interest Calculator. Accessed October 2026.
- Capital One, "APR vs. APY: What's the Difference?" - a plain language comparison of the annual rate charged on borrowing and the effective yield paid on deposits. Capital One: APR vs. APY. Accessed October 2026.
- Federal Deposit Insurance Corporation, "National Rates and Rate Caps" - the FDIC's published national average deposit rates, for judging whether a quoted daily accrual rate is competitive. FDIC: National Rates and Rate Caps. Accessed October 2026.
- Federal Reserve Bank of St. Louis, FRASER archive, "Final Regulation DD (Truth in Savings)" - the original rulemaking record behind standardized yield and daily balance disclosure. FRASER: Final Regulation DD (Truth in Savings). Accessed October 2026.