To calculate daily simple interest, multiply your loan’s principal balance by the annual interest rate, divide by the number of days in the year, and then multiply by the number of days since your last payment. On a $10,000 balance at 5% APR, that works out to about $1.37 a day. The math is quick once you have the three inputs, but the divisor your lender uses and the exact day your payment posts can shift the result in ways most borrowers never catch.
The Formula and What Goes Into It
One formula does the work:
Interest = Principal × (Annual Rate ÷ Days in Year) × Number of Days
It’s easier in two steps. First, convert the annual rate to a daily rate by dividing it by the days in the year. Second, multiply that daily rate by the principal and then by the number of days you want to cover. The dollar figure you get is the interest that accrued during that window.
Simple interest charges only on the current principal, never on interest that has already accrued. That single feature is what separates it from compound interest, and it’s why extra or early payments on a simple interest loan actually save you money.
Three numbers drive everything, and all three sit in your loan documents or your servicer’s online portal:
- Principal balance. The current amount you owe, not counting fees or penalties. On a brand-new loan it’s the amount borrowed; on an older loan it’s whatever is left after prior payments reduced it.
- Annual percentage rate. The yearly cost of borrowing as a percentage. Federal law requires your lender to disclose the APR and the total finance charge in dollars before closing.1Office of the Law Revision Counsel. 15 USC 1638 – Transactions Other Than Under an Open End Credit Plan
- Number of days. The time period you’re calculating for, usually the days in a billing cycle or the days between two payments.
If any of these aren’t visible in your account, call your servicer and ask for the current principal balance and the contract interest rate.
Pick the Right Divisor: 365, 360, 365.25, or Actual
Lenders don’t all divide by the same number when they convert an annual rate to a daily rate. The convention in your promissory note quietly changes what you pay.
- Actual/365. Divide the annual rate by 365 whether or not it’s a leap year. This is the most common convention for consumer loans like auto loans and personal loans. In a leap year, the divisor doesn’t adjust, so borrowers pay one extra day of interest.
- Actual/Actual. Divide by 365 in a normal year and 366 in a leap year. The most precise method, and generally the fairest to borrowers.
- Actual/360. Divide by 360, which produces a slightly higher daily rate. The U.S. Treasury uses this for its Prompt Payment interest, and some commercial lenders use it as well. Borrowers effectively pay interest on five extra days per year, six in a leap year.2Fiscal.Treasury.gov. Prompt Payment – Simple Daily Interest
- 365.25. Federal student loan servicers divide by 365.25, which smooths the leap year difference over a four-year cycle.3Nelnet – Federal Student Aid. FAQs – Interest and Fees
Check your loan agreement to see which one applies. If it says “actual/360” or “360-day year,” your daily rate is higher than it would be on a 365-day divisor. On a large balance held for years, the gap becomes real money.
A Worked Example
Say you owe $10,000 on a personal loan at 5% APR, and you want to know how much interest accrues in a 30-day billing cycle.
Step 1. Find the daily rate. Divide 0.05 by 365:
0.05 ÷ 365 = 0.000136986
Step 2. Multiply by the principal.
$10,000 × 0.000136986 = $1.37 per day
Step 3. Multiply by the number of days.
$1.37 × 30 = $41.10 in interest for the cycle
That $41.10 is the interest portion of your monthly payment. Anything you pay above it reduces your principal.
The Same Numbers on a 360-Day Divisor
If your lender uses a 360-day year instead, the daily rate becomes 0.05 ÷ 360 = 0.000138889. Multiply by $10,000 and you get $1.39 a day. Over 30 days, that’s $41.67. The single-month difference is only $0.57, but across a five-year term on $10,000 of principal it adds roughly $34 in extra interest. Scale the same math to a $300,000 mortgage and the convention choice matters a great deal.
Why the Day Count Isn’t Just an Input
On a simple interest loan, interest accrues every single day, so the date your payment posts changes how much of it kills interest versus principal.
Take a $10,000 balance at 8.5% APR. The per diem is $2.33. If 33 days pass between payments, you owe $76.85 in interest. If you pay four days earlier, so only 29 days elapse, the interest drops to $67.53. Four days early on one payment saves $9.32.4Bank of America. Explanation of Simple Interest Calculation
The reverse works the same way. Paying a few days late every cycle, even inside a grace period that spares you late fees, means more days of accrual on each cycle. Your monthly payment doesn’t change, but a bigger slice goes to interest and a smaller slice reduces principal. Year after year, those extra days quietly inflate what the loan costs you.
The practical move: pay a few days before your due date rather than after. Every day shaved off the cycle pushes more of the payment into principal, and a lower principal drops the daily interest on every cycle after that. One extra payment a year has the same compounding effect in reverse.
Where This Calculation Applies
Auto Loans
Most auto loans run on daily simple interest. Payments credit and reduce the balance on the day they’re received, so the actual payment date drives the interest cost.5Federal Reserve. Leasing vs. Buying – Example – Daily Simple Interest Method Extra principal payments during the term lower the balance and cut the interest portion of every payment after that.
Federal Student Loans
The formula is the same, but the divisor is 365.25: (Current Principal Balance × Interest Rate) ÷ 365.25.3Nelnet – Federal Student Aid. FAQs – Interest and Fees One catch is capitalization. During deferment or forbearance, interest still accrues daily even though you aren’t paying. When the period ends, the unpaid interest is added to principal, and daily interest from then on is calculated on the larger balance. Paying even small amounts during deferment can keep capitalization from inflating what you owe.
Some Mortgage Structures
Certain mortgage products, adjustable-rate mortgages in particular, use daily simple interest between payment dates. The day-count convention varies, so check your note. Most fixed-rate conventional mortgages run on a standard amortization schedule instead, though the underlying math is closely related.
A Boundary Worth Knowing
Not every loan works this way. On a precomputed interest loan, the lender calculates all the interest for the full term upfront and bakes it into the payment schedule. Extra payments don’t reduce the interest, because it was set before you made your first payment.6Consumer Financial Protection Bureau. What’s the Difference Between a Simple Interest Rate and Precomputed Interest on an Auto Loan Simple interest is far more common today, but precomputed loans still exist. If you plan to pay extra or pay off early, confirm the loan is simple interest before you sign.
Calculating a Payoff
You can’t pay off a simple interest loan by writing a check for the current balance. Interest keeps accruing between your last payment and the day the payoff arrives, so the number moves daily. A payoff statement from your lender quotes a specific dollar amount good through a specific date: current principal plus the per diem interest times the number of days until that date.
Most lenders add a few days of cushion so the quote stays valid while your payment is in transit. If the payment lands after the quoted date, you owe extra per diem for each additional day. Ask your servicer for the per diem when you request a payoff, and you can work out the exact figure yourself if your timing slips.